nuclear fusion energy essay

The chase for fusion energy

An emerging industry of nuclear-fusion firms promises to have commercial reactors ready in the next decade.

By Philip Ball 17 November 2021

The ancient village of Culham, nestled in a bend of the River Thames west of London, seems an unlikely launching pad for the future. But next year, construction will start here on a gleaming building of glass and steel that could house what many people consider to be an essential technology to meet demand for clean energy in the twenty-first century and beyond.

Long derided as a prospect that is forever 30 years away, nuclear fusion seems finally to be approaching commercial viability. There are now more than 30 private fusion firms globally, according to an October survey by the Fusion Industry Association (FIA) in Washington DC , which represents companies in the sector; the 18 firms that have declared their funding say they have attracted more than US$2.4 billion in total, almost entirely from private investments (see ‘Fusion funding’). Key to these efforts are advances in materials research and computing that are enabling technologies other than the standard designs that national and international agencies have pursued for so long.

Fusion funding

Private fusion firms have disclosed more than $2.4 billion in funding.

TAE Technologies

880 US$ million

Helion Energy

Commonwealth Fusion Systems

General Fusion

Tokamak Energy

Other (12 firms)

TAE Technologies 880 US$ million

Helion Energy 578

Commonwealth Fusion Systems 250

General Fusion 200

Tokamak Energy 200

Other (12 firms) 302

The latest venture at Culham — the hub of UK fusion research for decades — is a demonstration plant for General Fusion (GF), a company based in Burnaby, Canada. It is scheduled to start operating in 2025, and the company aims to have reactors for sale in the early 2030s. It “will be the first power-plant-relevant large-scale demonstration”, says GF’s chief executive Chris Mowry — unless, that is, its competitors deliver sooner.

Designed by British architect Amanda Levete, GF’s prototype plant illustrates the way fusion research has shifted from gargantuan state- or internationally funded enterprises to sleek, image-conscious affairs driven by private companies, often with state support. (GF will receive some UK government funding; it has not disclosed how much.)

Artist’s impression of General Fusion’s planned plant at Culham, UK. Credit: AL_A for General Fusion. Lead image: The world's strongest high-temperature superconducting magnet will be used in a 2025 fusion reactor in Massachusetts. Credit: Gretchen Ertl, CFS/MIT-PSFC, 2021

In this respect, advocates of fusion technology say it has many parallels with the space industry. That, too, was once confined to government agencies but is now benefiting from the drive and imagination of nimble (albeit often state-assisted) private enterprise. This is “the SpaceX moment for fusion”, says Mowry, referring to Elon Musk’s space-flight company in Hawthorne, California.

“The mood has changed,” says Thomas Klinger, a fusion specialist at the Max Planck Institute for Plasma Physics (IPP) in Greifswald, Germany. “We can smell that we’re getting close.” Investors sense the real prospect of returns on their money: Google and the New York City-based investment bank Goldman Sachs, for instance, are among those funding the fusion company TAE Technologies, based in Foothill Ranch, California, which has raised around $880 million so far. “Companies are starting to build things at the level of what governments can build,” says Bob Mumgaard, chief executive of Commonwealth Fusion Systems (CFS), based in Cambridge, Massachusetts.

And just as private space travel is now materializing, many industry observers are forecasting that the same business model will give rise to commercial fusion — desperately needed to decarbonize the energy economy — within a decade. “There’s a very good shot to get there within less than ten years,” says Michl Binderbauer, chief executive of TAE Technologies. In the FIA report, a majority of respondents thought that fusion would power an electrical grid somewhere in the world in the 2030s.

A technician works inside TAE’s ‘Norman’ demonstration fusion reactor. Credit: TAE Technologies

Several fusion researchers who don’t work for private firms told Nature that, although prospects are undeniably exciting, commercial fusion in a decade is overly optimistic. “Private companies say they’ll have it working in ten years, but that’s just to attract funders,” says Tony Donné, programme manager of the Eurofusion consortium which conducts experiments at the state-run Joint European Torus, established at Culham in the late 1970s. “They all have stated constantly to be about ten years away from a working fusion reactor, and they still do.”

Timelines that companies project should be regarded not so much as promises but as motivational aspirations, says Melanie Windridge, a plasma physicist who is the FIA’s UK director of communications, and a communications consultant for the fusion firm Tokamak Energy, in Culham. “I think bold targets are necessary,” she says. State support is also likely to be needed to build a fusion power plant that actually feeds electricity into the grid, adds Ian Chapman, chief executive of the UK Atomic Energy Authority (UKAEA).

But whether it comes from small-scale private enterprise, huge national or international fusion projects, or a bit of both, practical nuclear fusion finally seems to be on the horizon. “I’m convinced that it’s going to happen”, says Chapman. Chris Kelsall, chief executive of Tokamak Energy, agrees. “Sooner or later this will be cracked,” he says. “And it will be transformative.”

Seventy-year dream

Nuclear fusion, says Klinger, is “the only primary energy source left in the Universe” that we have yet to exploit. Ever since the process that powers the stars was harnessed in the 1950s for hydrogen bombs, technologists have dreamt of unlocking it in a more controlled manner for energy generation.

Existing nuclear power plants use fission: the release of energy when heavy atoms such as uranium decay. Fusion, by contrast, produces energy by merging very light nuclei, typically hydrogen, which can happen only at very high temperatures and pressures. Most efforts to harness it in reactors involve heating the hydrogen isotopes deuterium (D) and tritium (T) until they form a plasma — a fluid state of matter containing ionized atoms and other charged particles — and then fuse (see ‘Fuel mix’). For these isotopes, fusion starts at lower temperatures and densities than for normal hydrogen.

D–T fusion generates some radiation in the form of short-lived neutrons, but no long-lived radioactive waste, unlike fission. It is also safer than fission because it can be switched off easily: if the plasma is brought below critical thresholds of temperature or density, the nuclear reactions stop.

Many reactors fuse deuterium (D) with tritium (T) to release energy. This mix ignites, or creates a self-sustaining fusion reaction,at around 100 million kelvin. It produces neutrons, which can make the chamber radioactive.

Helium-4 (α)

Other reactions, such as fusing protons (p) with boron-11 ( 11 B), don’t produce neutrons, but ignition requires higher temperatures.

p– 11 B

What makes it so difficult to conduct in a controlled manner, however, is the challenge of containing electrically charged plasma that is undergoing fusion at temperatures of around 100 million kelvin — much hotter than the centre of the Sun. Generally, researchers use magnetic fields to confine and levitate the plasma inside the reactor. But instabilities in this infernal fluid make containment very difficult, and have so far prevented fusion from being sustained for long enough to extract more energy than is put in to trigger it.

This is necessarily big science, and until this century, only state-run projects could muster the resources. The scale of the enterprise is reflected today in the world’s biggest fusion effort: ITER, a fusion reactor being constructed in southern France and supported by 35 nations, including China, European Union member states, the United States, Russia, South Korea and Japan, with a price tag of at least $22 billion.

A D-shaped magnetic coil (left) that will form part of the giant ITER fusion reactor in France.

A part of ITER’s vacuum vessel, inside which plasma will be held.

Credit: ITER Organization

Although the first test runs are scheduled for 2025, full D–T fusion is not scheduled until 2035 , ultimately with the goal of continuously extracting 500 MW of power — comparable to the output of a modest coal-fired power plant — while putting 50 MW into the reactor. (These numbers refer only to the energy put directly into and drawn out of the plasma; they don’t factor in other processes such as maintenance needs or the inefficiencies of converting the fusion heat output into electricity.)

A further series of big reactors might follow ITER: China, which has three fusion reactors feeding results into ITER, plans a China Fusion Engineering Testing Reactor (CFETR) in the 2030s, and both South Korea and the EU propose to build demonstration power plants that would follow on from ITER.

The big national and international efforts won’t succeed soon enough to enable the decarbonization needed to address climate change, although fusion is expected to become a key part of the energy economy in the second half of the century. But private companies hope to have working and affordable devices sooner (see ‘Fusion rush’).

Fusion rush

Firms and governments are developing many kinds of fusion reactor. They all heat gas to create a plasma, confined at such high temperatures that atomic nuclei fuse, releasing energy that can be harnessed for electricity. Here are five prominent designs.

Illustrations by Tomáš Müller

Tokamak (ITER and other facilities)

Superconducting magnetic coils — cooled by liquid helium — hold plasma in a toroidal vessel.

Mini-tokamak (Tokamak Energy, Commonwealth Fusion Systems and others)

Magnets made of high-temperature superconductors produce stronger fields and can be cooled more easily, allowing more compact, spherical tokamaks to be built.

Linear (Colliding beams) reactor (TAE Technologies)

Packets of plasma are fired into a central chamber and rotate rapidly inside a solenoid (coiled-wire electromagnet).

Magnetized Target Reactor (General Fusion)

A spinning ball of liquid metal confines plasma; pistons then rapidly compress it. The plasma is allowed to expand, then compressed again.

Stellarator (Wendelstein 7-X)

A complicated twisted loop of magnetic fields confines the plasma.

As with space exploration, one of the benefits of a private fusion sector is greater diversity of approaches than monolithic state enterprises can muster. ITER is using the most common approach to confining plasma, in a device called a tokamak, which uses powerful superconducting magnets to hold the plasma in a ring-shaped (toroidal) vessel. The flow of the electrically charged plasma particles themselves also generates a magnetic field that helps to confine the plasma.

But a tokamak isn’t the only option. In the early days of fusion, in the 1950s, US astrophysicist Lyman Spitzer showed that magnetic fields could be configured in a twisted loop, rather like a figure of eight, to make a ‘magnetic bottle’ that could be filled with plasma. This design was known as a stellarator. But solving the equations describing the plasma for this complex geometry was too computationally intensive, so the concept was mostly abandoned once tokamaks had been shown to work.

As supercomputers became available in the late 1980s, however, researchers revisited the idea. This led to a stellarator project at the IPP called the Wendelstein 7-X reactor. Costing more than €1 billion (US$1.15 billion) to build, staff and operate up to its first plasma testing in 2015, with construction costs of €370 million largely borne by the German government, Wendelstein 7-X will be completed by the end of this year. Then comes a long process of working out how to operate it routinely as a demonstration project.

An engineer works on the construction of the complex plasma vessel of Wendelstein 7-X, a reactor in Greifswald, Germany. Credit: Stefan Sauer/dpa via Alamy

Stellarators have the advantage that their plasma is more easily confined, with no need (as in tokamaks) to drive strong electric currents through it to keep a lid on instabilities, says fusion physicist Josefine Proll at Eindhoven University of Technology in the Netherlands. But it’s not clear whether it will be possible to implement stellarator technology in a reactor in 20–30 years. “It seems not all that likely at this moment,” she says. “We have a lot of basic questions still to answer,” says Klinger. “This is a first-of-a-kind machine, so one must be patient and go step by step.” Private companies set shorter-term goals because they have to satisfy their stakeholders, he says — but that doesn’t mean they can deliver.

Alternative designs

Some private fusion companies are sticking with the tokamak design, but scaled down. At Tokamak Energy, a team of around 165 employees is working on a spherical tokamak, shaped like an apple with its core removed. At 3.5 metres across, it will be many times smaller than the ITER tokamak, which, with surrounding cooling equipment, will be almost 30 metres wide and tall. Some state-funded schemes are considering the compact spherical design, too: the UKAEA, for example, has launched a project called STEP (Spherical Tokamak for Energy Production) that aims to create such a device in a prototype plant that would deliver at least 100 MW to the national grid by 2040. The UKAEA has shortlisted five sites to host the plant, and expects the final choice to be made next year.

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Hydrogen plasma in Tokamak Energy’s ST40 spherical tokamak. This video has no sound. Credit: Tokamak Energy Ltd

Key to these designs are new kinds of magnets made from ribbons of high-temperature superconducting materials, which should produce much stronger fields than the conventional superconducting magnets used by ITER. They are “a potential game-changer”, says Klinger — not just because of their higher fields, but also because conventional superconductors need liquid-helium cooling. That is an engineering nightmare: liquid helium’s viscosity is almost zero, allowing it to leak through any tiny cracks. High-temperature superconductors, by contrast, can be cooled with liquid nitrogen, which is abundant, cheap and easy to store.

Both Tokamak Energy (in collaboration with CERN, Europe’s particle-physics laboratory near Geneva, Switzerland) and CFS are banking on these new magnets. In August, CFS announced that it had made them in the form needed for its tokamaks — “on schedule and on budget”, Mumgaard says proudly.

In 2018, CFS was spun off from the Plasma Science and Fusion Center of the Massachusetts Institute of Technology (MIT) in Cambridge, and Klinger considers the firm “the most promising, most valuable and most thought-through private fusion initiative”. MIT and CFS together are preparing to build what Mumgaard calls “the first fusion machine that makes net energy” — producing more energy than goes into it. Named SPARC, it is being constructed in Devens, Massachusetts. Mumgaard says it will be running by the end of 2025, and will be “commercially relevant” because it will generate around 100 MW of power.

Researchers working on a magnet for the CFS/MIT ‘SPARC’ reactor. Credit: Gretchen Ertl, CFS/MIT-PSFC, 2021 

First Light Fusion, a company spun off from the University of Oxford, UK, in 2011, is pursuing a different strategy, called inertial confinement. Here, the fusion plasma isn’t held by magnetic fields: rather, a shock wave compresses it to the immense densities needed for fusion, and the plasma retains its shape just for a split second by inertia alone, before spreading out and dissipating its energy. The idea has been around since the 1950s, and is also being studied at the US National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in California, where pea-sized plastic capsules of D–T fuel are imploded by nanosecond pulses of laser light to ignite fusion. In August, NIF reported a laser shot that produced a fleeting energy output 8 times higher than it had ever before achieved — and amounted to 70% of the energy that had gone into the reaction. That has raised hopes of net gain from inertial-confinement laser fusion, although such an energy-intensive process might be more useful for fundamental research than for large-scale power generation.

At First Light, the compression shock wave is created not by energy-hungry lasers, but by using an electromagnetic projectile gun to fire a small piece of material into a target containing the hydrogen isotopes. The company is keeping details of the process secret, but has said that to achieve fusion, it will need to fire the material at 50 kilometres per second — twice as fast as is typically achieved in current shock-wave experiments.

GF is taking yet another approach, called magnetized target fusion. It involves the plasma being compressed more slowly — for instance, using pistons — but with the aid of magnetic confinement that prevents heat from dissipating as the plasma is squeezed. This idea, suggested in the early 1970s by researchers at the US Naval Research Laboratory in Washington DC, seeks an optimal compromise between the energy-intensive high magnetic fields needed to confine a tokamak plasma, and the energy-intensive shock waves, lasers or other methods used to rapidly compress plasma in inertial-confinement designs.

GF’s design for its Culham reactor uses a centrifuge to spin a chamber filled with molten lead and lithium. That motion opens a cavity in the liquid metal, where the plasma sits. A piston system pumps more liquid metal into the chamber, compressing the plasma over a few tens of milliseconds. Fusion begins; then the pressure is released and the process repeated in pulses, about once a second.

One especially neat aspect of this reactor is how it generates tritium fuel — a hugely expensive resource that can be made only in nuclear reactions, and decays rapidly. In ITER and other designs, tritium will be produced when neutrons escaping the reactor hit a lithium blanket lining the tokamak. In GF’s design, tritium is made when neutrons hit lithium within the liquid-metal compression system itself.

GF has cracked key challenges only in the past few years — making a plasma target that lasts for long enough to be compressed, and smoothly and rapidly collapsing the liquid- metal cavity. The firm says, however, that after it has its UK demonstration plant operating in 2025, it will “power homes, businesses and industry with clean, reliable and affordable fusion energy by the early 2030s”.

A General Fusion technician works on the plasma-injector system for one of the company’s reactors. Credit: General Fusion

TAE Technologies has, in some ways, an even more audacious concept. It plans to abandon D–T fuel altogether, instead fusing boron-11 atoms with hydrogen-1 nuclei (protons). This idea, championed by TAE’s co-founder, the Canadian plasma physicist Norman Rostoker, and dubbed p– 11 B fusion, requires temperatures ten times greater than for D–T fusion: about one billion kelvin. The advantage is that this reaction uses only abundantly available fuel, and generates no neutrons that could contaminate the reactor. Binderbauer says that the concept offers lowers maintenance costs and a much more sustainable end goal.

In TAE reactors, the plasma is confined inside a cylindrical magnetic field made by a solenoid — a design that draws on particle-accelerator technologies. The plasma rotates around the axis; that rotation, as in a spinning top, generates inherent stability. Confinement doesn’t require strong external magnetic fields; those are mostly generated by the spinning plasma itself. To keep it rotating, tangential beams of boron inject angular momentum, rather as a top is torqued by a whip.

The company has made prototypes to demonstrate this set-up; since 2017, it has been working with a test system called Norman, and it is now starting work on a device called Copernicus that will run with normal hydrogen (or other non-fusing) plasmas to avoid producing neutrons. Computer simulations will show what energy would be generated if real fusion fuel were used. If TAE achieves the conditions needed for D–T fusion — which it hopes to do by around the middle of this decade — the company plans to license the technology to others who are pursuing those fuels. Binderbauer calls Copernicus a “stepping stone” to the temperatures needed for p– 11 B fusion. “We’re convinced that we can go to the billion-degree level,” he says — and he hopes to see this towards the end of the decade.

TAE’s ‘Norman’ test reactor. Credit: TAE Technologies

Among the many other private fusion firms, Helion Energy, in Everett, Washington, has attracted the most interest from investors: this month, it announced a $500-million funding round, bringing its total to $578 million. Its aim is to generate electricity directly from fusion, rather than using the process to heat fluids and drive turbines. Helion’s technique involves firing pulses of plasma together inside a linear reactor, then rapidly compressing the merged plasma with magnetic fields. When fusion occurs, the plasma expands and its magnetic field interacts with that surrounding the reactor to induce an electric current. Helion hopes to fuse a mixture of deuterium and helium-3, which would not produce neutrons as a by-product. But helium-3 itself would need to be produced by D–D fusion. The company is building a demonstration reactor called Polaris, which it aims to have in operation by 2024.

How Helion’s technology will generate electricity. This video has no sound. Credit: Helion Energy

Cheaper reactors?

The reactors built by private companies, being smaller than ITER-scale projects, will be much more affordable. Tokamak Energy’s co-founder, David Kingham, envisages billion-dollar devices, and Binderbauer thinks TAE’s systems could be built for around $250 million.

The aim is to make small fusion reactors that are compatible with existing energy grids. Kelsall says they could also serve industries that are particularly energy-intensive, such as metal smelting — a sector that can’t be supplied by renewables. Mowry adds that shipping could be another important market: devices producing around 100 MW of power are “just the right size for a large container ship”.

Donné remains cautious about the prospects, however, saying that private companies “are working on aggressive time paths compared to publicly funded projects, but also have a much higher risk of potential failure”. All the same, TAE, for one, insists that it is still on the track that it promised in the mid-2010s, of having a fusion device ready for commercialization by around the end of this decade (see ‘Future promises’).

Future promises

Private firms are making bold promises about delivering commercial fusion reactors in the 2030s.

State sponsored

Helion: Net electricity (small amounts) from Polaris reactor.

Giant international effort ITER: test runs.

Commonwealth Fusion Systems (CFS): First fusion machine expected to generate more energy than it uses.

General Fusion: operate UK demonstration plant.

TAE Technologies: reactors ‘ready for commercialization’ by late 2020s.

CFS: aims to have 200 megawatt plant supplying electricity grid in early 2030s.

General Fusion: targets reactors for sale in early 2030s.

First Light Fusion: anticipates its first power plant in 2030s.

Tokamak Energy: fusion power plant (pilot) in 2030s.

ITER: to run fusion with deuterium–tritium fuel.

China Fusion Engineering Test Reactor might complete construction

UK Atomic Energy Authority hopes STEP fusion power plant can supply energy to national grid.

Helion: Net electricity

(small amounts)

from Polaris reactor.

Giant international

effort ITER: test runs.

TAE Technologies: reactors ‘ready for commercialization’

by late 2020s.

international

effort ITER:

Helion: Net

electricity (small

amounts) from

Polaris reactor.

Commonwealth

Fusion Systems

(CFS): First fusion

machine expected

to generate more

energy than it uses.

General Fusion:

demonvstration plant.

TAE Technologies:

reactors ‘ready for

commercialization’

CFS: aims to have

200 megawatt plant

supplying electricity

grid in early 2030s.

targets reactors for

sale in early 2030s.

Tokamak Energy:

fusion power plant

(pilot) in 2030s.

First Light Fusion:

anticipates its first

power plant in 2030s.

ITER: to run fusion with

deuterium–tritium fuel.

China Fusion

Engineering Test

Reactor might complete

construction in 2030s.

UK Atomic Energy

Authority hopes STEP

fusion power plant can

supply energy to

national grid.

to generate

more energy

than it uses.

demonvstration

CFS: aims to

megawatt plant

electricity grid

in early 2030s.

targets reactors

for sale in

early 2030s.

anticipates

its first power

plant in 2030s.

fusion power

plant (pilot)

ITER: to run

fusion with

deuterium–tritium

Reactor might

construction

Energy Authority

hopes STEP fusion

power plant can

Despite his scepticism, Donné adds: “I see the booming of private fusion companies as a good sign. There can be mutual benefits in keeping close ties between public and private fusion projects.” That’s certainly happening. Not only is the private fusion industry building on years of state investment in projects such as ITER, but it is benefiting from governments that see value in supporting it — which is why the UK government and the US Department of Energy are also investing in firms such as Tokamak Energy, CFS and GF. Mowry thinks that such public–private partnerships are the way forward — as they were for COVID-19 vaccines. And, as with the vaccines, fusion will be needed everywhere, especially as energy use rises in lower-income countries.

The vaccines showed “what you can do if you have the resources”, says Windridge. “If we had that kind of commitment in energy, I think it would be incredible to see what can be achieved.” As with the vaccines, too, society desperately needs more clean, carbon-free sources of energy. “This is an existential challenge,” says Mowry. “Fusion is the vaccine for climate change.”

Philip Ball is a science writer in London.

This article is also available as a pdf version .

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What is nuclear fusion and why is it such a big deal?

Scientists and engineers have been working on trying to harness this form of energy for decades.

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The U.S. Department of Energy on Tuesday announced a breakthrough in nuclear fusion, a method of producing clean energy, that has many people hopeful for the future.

The achievement was reached by scientists at the Lawrence Livermore National Laboratory  in California. 

Nuclear fusion may be a new concept to many, but scientists have been working on it  since the 1940s . However, they have faced a tough challenge: how to produce more energy than it takes to create it. It almost seemed like an insurmountable challenge.

Until today.

What is nuclear fusion?

Nuclear fusion is a process where two lighter elements combine to make a heavier element. 

It's the same process that powers our sun, where protons of hydrogen atoms collide violently and at incredibly high temperatures at the core, fusing together to produce a helium atom.

• Fusion energy 'breakthrough' revealed by U.S. scientists

Here on Earth, nuclear fusion is produced by fusing the elements deuterium and tritium. Deuterium is quite plentiful and can be found in water, but is most abundant in our oceans. Tritium, on the other hand, is less plentiful and is primarily found in our atmosphere, a result of cosmic radiation. Tritium is also made in nuclear explosions and is a byproduct from nuclear reactors.

The sun's massive gravitational force allows it to fuse hydrogen atoms, but to create fusion on Earth, scientists need to apply extremely high pressures and temperatures that are roughly 100 million degrees Celsius, or 10 times hotter than the sun's core.

On Dec. 5, 2022, a team at LLNL's <a href="https://twitter.com/lasers_llnl?ref_src=twsrc%5Etfw">@lasers_llnl</a> conducted the first controlled fusion experiment in history to achieve fusion ignition. Also known as scientific energy breakeven, the experiment produced more energy from fusion than the laser energy used to drive it. <a href="https://t.co/t9htICEcuh">pic.twitter.com/t9htICEcuh</a> &mdash; @Livermore_Lab

While there are different ways to try to produce nuclear fusion, scientists at the California lab used 192 lasers focused on the inner wall of a cylinder that contained a small capsule (about the size of a BB) of fusion fuel: deuterium and tritium. 

That generated X-rays from the wall that struck the capsule, squeezing the fuel. It stayed hot, dense and round enough for long enough that it ignited, producing more energy than the lasers used. 

While the energy produced was small — about three megajoules, or enough to power a light bulb — it marks a historic first in nuclear fusion energy because the lasers used just over two megajoules to fire into the target.

However, it's important to note that 300 megajoules of traditional energy — often referred to as "from the wall" energy — were needed to fire the lasers during the experiment, according to Mark Herrmann, program director for weapon physics and design at Lawrence Livermore National Laboratory.

How is it different from nuclear power we already use?

When most people think about nuclear energy, they likely think about the nuclear reactors we have today. But those reactors operate using nuclear fission. 

Fission is the exact opposite of fusion, which forces atoms together. Instead, nuclear reactors generate energy by separating heavy atoms.

As well, fusion produces clean energy. Unlike nuclear reactors, the process doesn't result in byproducts like the spent rods found in nuclear power plants.

With nuclear fusion, there is also no chance of a nuclear meltdown as there is with fission, and nuclear fusion cannot be used to make nuclear weapons.

The International Atomic Energy Agency explains that while hydrogen bombs do use fusion reactions, a second fission bomb is needed to detonate it. 

Why is nuclear fusion important?

Earth is facing a climate crisis caused by centuries of burning fossil fuels. As a result, there will be an increase in floods, droughts, rising sea levels and more. We are already seeing this happening, and the more the planet warms, the worse these disasters will become.

The planet has warmed by roughly 1.2 C, but if we are to limit that to 1.5 C by the end of the century, which is the ambitious goal set at the 2015 Paris climate accord, it could mean fewer climate-related disasters. So, scientists and engineers have been trying to develop cost-effective, clean energy. 

Bob McDonald explains nuclear fusion

That's where fusion comes in. 

It produces no harmful carbon dioxide or methane emissions and is highly efficient.

According to the International Atomic Energy Agency , "Fusion could generate four times more energy per kilogram of fuel than fission (used in nuclear power plants) and nearly four million times more energy than burning oil or coal." 

"It moves us closer to the possibility of zero carbon abundant fusion energy powering our society," said U.S. Secretary of Energy Jennifer M. Granholm at Tuesday's announcement.

When will we use fusion as an energy source?

While this is an historic first, it doesn't mean that we're ready to produce energy on a large scale yet. 

"There are very significant hurdles, not just in the science, but in technology," said Kim Budil, director of the Lawrence Livermore National Laboratory in Livermore, Calif. 

"This is one igniting capsule, one time and to realize commercial fusion energy, you have to … produce many, many fusion ignition events per minute and you have to have a robust system of drivers to enable that," said Budil. 

She explained that though it wouldn't take quite as long as scientists used to estimate, it will be least a few decades before the underlying technologies are developed enough to build a nuclear fusion power plant.

• Video European scientists demonstrate fusion energy record
• Analysis Nuclear fusion, a disruptive power source for crowded cities: Don Pittis

It's also important to remember that the U.S. isn't the only country working on nuclear fusion. 

In France, there is the collaborative  International Thermonuclear Experimental Reactor , a massive nuclear fusion reactor weighing 23,000 tonnes and standing nearly 30 metres tall, which is set to begin operations in roughly a decade. 

In Canada, there are also private companies like General Fusion , based out of British Columbia, among others. There are also private enterprises working on fusion  in China, the United Kingdom and Germany .

Clarifications

• This story has been updated to clarify how 300 megajoules of traditional energy were used by the lasers involved in the experiment. Dec 16, 2022 1:17 PM ET

ABOUT THE AUTHOR

Senior reporter, science

Based in Toronto, Nicole covers all things science for CBC News. As an amateur astronomer, Nicole can be found looking up at the night sky appreciating the marvels of our universe. She is the editor of the Journal of the Royal Astronomical Society of Canada and the author of several books. In 2021, she won the Kavli Science Journalism Award from the American Association for the Advancement of Science for a Quirks and Quarks audio special on the history and future of Black people in science. You can send her story ideas at [email protected].

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Here’s how scientists reached nuclear fusion ‘ignition’ for the first time.

The experiment, performed in 2022, also revealed a never-before-seen phenomenon

In December 2022, scientists at the National Ignition Facility (pictured) achieved nuclear fusion “ignition,” in which the energy produced by the fusing of atomic nuclei exceeds that needed to kick the fusion off.

Jason Laurea/LLNL

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By Emily Conover

February 16, 2024 at 9:30 am

One of nuclear fusion’s biggest advances wouldn’t have happened without some impeccable scientific artistry.

In December 2022, researchers at Lawrence Livermore National Laboratory in California created fusion reactions that produced an excess of energy — a first. In the experiment, 192 lasers blasted a small chamber, setting off fusion reactions — in which smaller atomic nuclei merge to form larger ones — that released more energy than initially kicked them off ( SN: 12/12/22 ). It’s a milestone known as “ignition,” and it has been decades in the making.

Now, researchers have released details of that experiment in five peer-reviewed papers published online February 5 in Physical Review Letters and Physical Review E . The feat demanded an extraordinary level of finesse, tweaking conditions just so to get more energy out of the lasers and create the ideal conditions for fusion.

The work is “exquisitely beautiful,” says physicist Peter Norreys of the University of Oxford. Norreys, who was not involved with the research, compares the achievement to conducting a world-class orchestra: Different elements of the experiment had to be meticulously coordinated and precisely timed.

Scientists also discovered a long-predicted heating effect that could expose the physics of other violent environments, such as exploding stars called supernovas. “People say [physics is] a dry subject,” Norreys says. “But I always think that physics is at the very forefront of creativity,”

The road to nuclear fusion’s big break

Fusion, the same process that takes place in the sun, is an appealing energy source. Fusion power plants wouldn’t emit greenhouse gases. And unlike current nuclear fission power plants, which split atomic nuclei to produce energy, nuclear fusion plants wouldn’t produce dangerous, long-lived radioactive waste. Ignition is the first step toward harnessing such power.

Generating fusion requires extreme pressures and temperatures. In the experiment, the lasers at LLNL’s National Ignition Facility pelted the inside of a hollow cylinder, called a hohlraum, which is about the size of a pencil eraser. The blast heated the hohlraum to a sizzling 3 million degrees Celsius — so hot that it emitted X-rays. Inside this X-ray oven, a diamond capsule contained the fuel: two heavy varieties of hydrogen called deuterium and tritium. The radiation vaporized the capsule’s diamond shell, triggering the fuel to implode at speeds of around 400 kilometers per second, forming the hot, dense conditions that spark fusion.

Previous experiments had gotten tantalizingly close to ignition ( SN: 8/18/21 ). To push further, the researchers increased the energy of the laser pulse from 1.92 million joules to 2.05 million joules. This they accomplished by slightly lengthening the laser pulse, which blasts the target for just a few nanoseconds, extending it by a mere fraction of a nanosecond. (Increasing the laser power directly, rather than lengthening the pulse, risked damage to the facility.)

The team also thickened the capsule’s diamond shell by about 7 percent — a difference of just a few micrometers — which slowed down the capsule’s implosion, allowing the scientists to fully capitalize on the longer laser pulse.  “That was a quite remarkable achievement,” Norreys says.

But these tweaks altered the symmetry of the implosion, which meant other adjustments were needed. It’s like trying to squeeze a basketball down to the size of a pea, says physicist Annie Kritcher of LLNL, “and we’re trying to do that spherically symmetric to within 1 percent.”

That’s particularly challenging because of the mishmash of electrically charged particles, or plasma, that fills the hohlraum during the laser blast. This plasma can absorb the laser beams before they reach the walls of the hohlraum, messing with the implosion’s symmetry.

To even things out, Kritcher and colleagues slightly altered the wavelengths of the laser beams in a way that allowed them to transfer energy from one beam to another. The fix required tweaking the beams’ wavelengths by mere angstroms — tenths of a billionth of a meter.

“Engineering-wise, that’s amazing they could do that,” says physicist Carolyn Kuranz of the University of Michigan in Ann Arbor, who was not involved with the work. What’s more, “these tiny, tiny tweaks make such a phenomenal difference.”

After all the adjustments, the ensuing fusion reactions yielded 3.15 million joules of energy — about 1.5 times the input energy, Kritcher and colleagues reported in Physical Review E . The total energy needed to power NIF’s lasers is much larger, around 350 million joules. While NIF’s lasers are not designed to be energy-efficient, this means that fusion is still far from a practical power source.

Another experiment in July 2023 used a higher-quality diamond capsule and obtained an even larger energy gain of 1.9, meaning it released nearly twice as much energy as went into the reactions ( SN: 10/2/23 ). In the future, NIF researchers hope to be able to increase the laser’s energy from around 2 million joules up to 3 million , which could kick off fusion reactions with a gain as large as 10.

What’s next for fusion

The researchers also discovered a long-predicted phenomenon that could be useful for future experiments: After the lasers heated the hohlraum, it was heated further by effects of the fusion reactions, physicist Mordy Rosen and colleagues report in Physical Review Letters .

Following the implosion, the ignited fuel expanded outward, plowing into the remnants of the diamond shell. That heated the material, which then radiated its heat to the hohlraum. It’s reminiscent of a supernova, in which the shock wave from an exploding star plows through debris the star expelled prior to its explosion ( SN: 2/8/17 ).

“This is exactly the collision that’s happening in this hohlraum,” says Rosen, of LLNL, a coauthor of the study. In addition to explaining supernovas, the effect could help scientists study the physics of nuclear weapons and other extreme situations.

NIF is not the only fusion game in town. Other researchers aim to kick off fusion by confining plasma into a torus, or donut shape, using a device called a tokamak. In a new record, the Joint European Torus in Abingdon, England, generated 69 million joules , a record for total fusion energy production, researchers reported February 8.

After decades of slow progress on fusion, scientists are beginning to get their atomic orchestras in sync.

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Can Nuclear Fusion Put the Brakes on Climate Change?

By Rivka Galchen

Let’s say that you’ve devoted your entire adult life to developing a carbon-free way to power a household for a year on the fuel of a single glass of water, and that you’ve had moments, even years, when you were pretty sure you would succeed. Let’s say also that you’re not crazy. This is a reasonable description of many of the physicists working in the field of nuclear fusion. In order to reach this goal, they had to find a way to heat matter to temperatures hotter than the center of the sun, so hot that atoms essentially melt into a cloud of charged particles known as plasma; they did that. They had to conceive of and build containers that could hold those plasmas; they did that, too, by making “bottles” out of strong magnetic fields. When those magnetic bottles leaked—because, as one scientist explained, trying to contain plasma in a magnetic bottle is like trying to wrap a jelly in twine—they had to devise further ingenious solutions, and, again and again, they did. Over decades, in the pursuit of nuclear fusion, scientists and engineers built giant metal doughnuts and Gehryesque twisted coils, they “pinched” plasmas with lasers, and they constructed fusion devices in garages. For thirty-six years, they have been planning and building an experimental fusion device in Provence. And yet commercially viable nuclear-fusion energy has always remained just a bit farther on. As the White Queen, in “ Through the Looking Glass ,” said to Alice, it is never jam today, it is always jam tomorrow.

The accelerating climate crisis makes fusion’s elusiveness more than cutely maddening. Solar energy gets more efficient and affordable each year, but it’s not continuously available, and it still relies on gas power plants for distribution. The same is true for wind power. Conventional nuclear power has extremely well-known disadvantages. Carbon capture, which is like a toothbrush for the sky, is compelling, but after you capture a teraton or two of carbon there’s nowhere to put it. All these tools figure extensively in decarbonization plans laid out by groups like the Intergovernmental Panel on Climate Change , but, according to those plans, even when combined with one another the tools are insufficient. Fusion remains the great clean-energy dream—or, depending on whom you ask, pipe dream.

Fusion, theoretically, has no scarcity issues; our planet has enough of fusion’s primary fuels, heavy hydrogen and lithium, which are found in seawater, to last thirty million years. Fusion requires no major advances in batteries, it would be available on demand, it wouldn’t cause the next Fukushima, and it wouldn’t be too pricey—if only we could figure out all the “details.” (A joke I heard is that fusion operates according to the law of the “conservation of difficulty”: when one problem is solved, a new one of equal difficulty emerges to take its place.) The details are tremendously complex, and the people who work to figure them out have for years been dealing with their own scarcities—scarcities of funding and scarcities of faith. Fusion, as of now, has no place in the Green New Deal .

In 1976, the U.S. Energy Research and Development Administration published a study predicting how quickly nuclear fusion could become a reality, depending on how much money was invested in the field. For around nine billion a year in today’s dollars—described as the “Maximum Effective Effort”—it projected reaching fusion energy by 1990. The scale descended to about a billion dollars a year, which the study projected would lead to “Fusion Never.” “And that’s about what’s been spent,” the British physicist Steven Cowley told me. “Pretty close to the maximum amount you could spend in order to never get there.”

“To be honest, I was feeling pretty despondent,” Dennis Whyte, the fifty-seven-year-old director of the Plasma Science and Fusion Center, at M.I.T., said. “And I was seeing that despondency in the faces of my students, too.” It was 2013, and M.I.T.’s experimental fusion device had lost its Department of Energy funding, for no clearly stated reason. The field of nuclear fusion, as a whole, was still moving forward, but agonizingly slowly. iter , an enormous fusion device being built in southern France, in an international collaboration, was progressing—the schedule is for ITER to demonstrate net fusion energy in 2035, and the majority of plasma physicists have high confidence that it will work—but Whyte knew that it wasn’t going to deliver affordable energy to the public in his lifetime, and maybe not in his students’ lifetimes, either. “ ITER is scientifically interesting. But it’s not economically interesting,” Whyte said. “I almost retired.”

Whyte is a gentle giant from Saskatchewan, Canada. “If you’ve ever been to the middle of nowhere, that’s where I grew up,” he told me. His family were farmers and electricians. By the time he was in the fifth grade, he knew he wanted to be a scientist, and in the eleventh grade he wrote a term paper on that wild idea which often appeared in science fiction—near-boundless energy generated by the fusing of two atoms, as happens in stars. “I remember getting that paper back, and my teacher saying, ‘Great job, but it’s too complicated.’ ” Whyte went on to major in engineering and physics at the University of Saskatchewan; for his Ph.D., he attended a new plasma-physics program at the University of Quebec, where he worked in a government-funded fusion lab. “I thought, Great: I’ll learn French and get to work on a tokamak,” he said, referring to the large doughnut-shaped machine whose design is commonly used for fusion devices. Later, Whyte took a job at a lab in San Diego. He intended to return home eventually, but in 1997 Canada cancelled its fusion program. “I was stranded in the U.S.,” he said.

At M.I.T., Whyte teaches an engineering-design class for graduate students which he organizes each year around a different practical problem in fusion. “I’ve always wanted to expose my students not only to the science questions but also to the technology questions,” he said. In 2008, he asked his students to design a device that would pump helium but not hydrogen—in most approaches to fusion, hydrogen is the fuel, and helium is, in effect, the ash. “Helium is one of the hardest things to pump in the periodic table, because it’s so inert,” Whyte said. The class came up with several very clever ideas. None of them was successful. “We’re still working on that one,” he said.

The next year, something happened that Whyte credits with restoring his interest in fusion. “I had passed my colleague Leslie in the hall, and he was holding a bundle of what looked like the spoolings of a cassette tape,” he said. It was a relatively new material: ribbons of high-temperature superconductor. Superconductors are materials that offer little to no resistance to the flow of electricity; for this reason, they make ideally efficient electromagnets, and magnets are the key component in tokamaks. A high-temperature superconductor—well, it opened up new possibilities, in the way that the vulcanization of rubber opened up possibilities in the mid-nineteenth century. The superconductor material that Whyte’s colleague was holding could in theory make a much more effective magnet than had ever existed, resulting in a significantly smaller and cheaper fusion device. “Every time you double a magnetic field, the volume of the plasma required to produce the same amount of power goes down by a factor of sixteen,” Whyte explained. Fusion happens when a contained plasma is heated to more than a hundred million degrees. Whyte asked his class to use this new material to design a compact fusion power plant of at least five hundred megawatts, enough to power a small city: “I was not sure what we would find with H.T.S., but I knew it would be innovative.”

The physicists Bob Mumgaard, Dan Brunner, and Zach Hartwig were in that class. The power plant that they came up with was in most respects familiar. At its center would be a doughnut-shaped tokamak, not unlike the type that Whyte had worked with as a graduate student. They named their design Vulcan. In the next iteration of the class, those ideas evolved into a design called ARC , for “affordable, robust, and compact.” (This also happens to be the name of the personal fusion device of the billionaire industrialist Tony Stark, in the “Iron Man” movies.) ARC would use an ordinary salt to translate its heat onto an electrical grid. It would be modular, for easy maintenance. It would not be able to recycle its own fuel. It was a “good enough” machine. But the use of H.T.S. magnets made it about the size of a conventional power plant—a tenth the size of ITER .

Physicists from both classes later formed a group that modified the arc design. The new model was two-thirds the size and intended to be ready as soon as possible — SPARC . SPARC would be the prototype that demonstrated the concept; ARC would be a long-lasting power plant capable of delivering affordable energy to the grid.

There were real reasons for skepticism. H.T.S. is fragile—it remained to be seen if it could even be made into a hardy magnet, and, if it could, how well that magnet would endure bombardment by charged particles. Plus, H.T.S. was not yet commercially available at sufficient scale and performance. “But those were engineering barriers, not scientific barriers,” Whyte said. “That class really changed my mind about where we were in fusion.”

Fusion scientists often speak of waiting for a “Kitty Hawk moment,” though they argue about what would constitute one. Only in retrospect do we view the Wright brothers’ Flyer as the essential breakthrough in manned flight. Hot-air balloons had already achieved flight, of a kind; gliders were around, too, though they couldn’t take off or land without a catapult or a leap. One of the Wright brothers’ first manned flights lasted less than a minute—was that flight? An A.P. reporter said, of that event, “Fifty-seven seconds, hey? If it had been fifty-seven minutes, then it might have been a news item.”

Our sun is a fusion engine. So are all the stars.

But we discovered that fusion powered the stars only about a hundred years ago, when the British physicist Arthur Eddington put together two pieces of knowledge into what was seen at the time as a wild surmise. The facts he combined were that the sun is made up mostly of hydrogen, with some helium, and that E=mc 2 .

Eddington noticed that four hydrogen atoms weigh a tiny bit more than one helium atom. If four hydrogen nuclei somehow fuse together, in a series of steps, and form helium, then a little bit of mass must be “lost” in the process. And if one takes seriously that most famous of equations, then that little bit of mass becomes a lot of energy—as much energy as that amount of mass multiplied by the speed of light, squared. To give a sense of this ratio: If you converted a baseball into pure energy, you could power New York City for about two weeks. Maybe that process—hydrogen crashing into hydrogen and forming helium, giving off an extraordinary amount of energy in the process—was how the sun and all the stars burned so bright and so long. Eddington, in a paper laying out this theory, closed with an unusual take on the story of Daedalus and his son Icarus. Eddington argued in defense of Icarus, saying it was better to fly too high, and in doing so see where a scientific idea begins to fail, than it was to be cautious and not try to fly high at all.

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When most people think of nuclear energy, they are thinking not of fusion but of fission. Fission is when an atom—most commonly uranium or plutonium—breaks in two. Fission generates waste that remains radioactive for tens of thousands of years; in contrast, the little bit of waste that fusion generates remains radioactive for only a few decades. Fission is pretty powerful, as evidenced by atomic bombs; fusion is much, much more powerful. (In 1952, a fusion bomb, known as the H-bomb, was tested, though it has never been used in warfare; it worked by using a fission bomb to set off a giant uncontrolled fusion reaction. One of the fathers of the H-bomb, Edward Teller, an aggrieved Shakespearean villain in most tellings, had other incautious ideas, such as using fusion bombs to dig canals or make diamonds.) The process of fusion sounds dangerous to a layperson—a sun in a magnetic bottle?—but it is easier to extinguish than a match.

The allure of fusion has attracted brilliant, imaginative minds; it has also attracted a crowd of shysters, cranks, and false messiahs. In 1951, Juan Perón, Argentina’s President, announced that the country had harnessed fusion energy. It would soon be available in litre and half-litre bottles, like milk. Perón had made the mistake of distrusting his own country’s scientific community, instead putting his faith in Ronald Richter, an Austrian immigrant whose apparatus, when inspected by scientists, didn’t even have a functioning Geiger counter, the device he was using to claim evidence of fusion radiation.

A few decades later, two respected chemists at the University of Utah, Stanley Pons and Martin Fleischmann, convinced the public that they had produced nuclear fusion at room temperature, in what looked like a jar with a little mixer stick in it. They announced their results in a press conference before they published their data or methods. Pons and Fleischmann were featured on the cover of Time . Meanwhile, the work of Steven Jones, a respected physicist at Brigham Young University, was also receiving press attention; he, too, was working on producing fusion at a low temperature, and, though he seemed to be on a promising path, he was ultimately unsuccessful. When Pons and Fleischmann finally published a paper, they were suspected of having fudged their data. No one was able to reliably reproduce their results. Jones later turned to proving that Jesus had visited Mesoamerica, and after that to explaining that the destruction of the World Trade Center was an inside job. Zach Hartwig, now a professor of nuclear science and engineering at M.I.T. and part of the ARC / SPARC team, has said, “The biggest problem in fusion is perception. It’s the perception that fusion is a joke.”

Estimates of the cost of the Manhattan Project, which produced atomic weapons in four years, vary, but it is commonly said that the scientists were given a “blank check.” This year, the U.S. government will spend some six hundred and seventy million dollars on nuclear fusion. That’s a lot of money, but six hundred and fifty billion—the amount the I.M.F. estimates that U.S. taxpayers spent on fossil-fuel subsidies last year—is quite a bit more.

During the oil crisis of the nineteen-seventies, fusion research briefly received the sort of funding that goes to national-defense projects. M.I.T.’s Plasma Fusion Center was established in 1976. The Joint European Torus, at the Culham Center for Fusion Energy, in the United Kingdom, which has heated hydrogen to temperatures hotter than the inside of the sun, began operating in 1983, and by 1997 had set important records, some still not surpassed. “It was such an exciting time,” Michael Mauel, a professor of applied physics at Columbia University, who did his undergraduate and graduate work in fusion at M.I.T., said. “And we were sure we were going to be the ones to solve it all.”

Steven Cowley, the former head of the U.K. Atomic Energy Authority, who now heads the Princeton Plasma Physics Laboratory, recalled his days as a graduate student at Princeton, in the nineteen-eighties. “Fusion was all we thought about, from the time we woke up in the morning to the last beer in the basement of the graduate college,” he said. “I remember when we got to ten million watts of fusion power on T.F.T.R.”—Princeton’s fusion device. “I still have a photo of that moment outside my office.” It was a tremendous milestone, but it also, basically, created enough energy to light up a single bulb for a day. More needed to be done.

But, by the nineties, oil was cheap again. Fusion research funding declined. “We had learned to extract oil and gas from all kinds of places,” Cowley said. “Now we have to learn how to leave it in the ground in order to survive, to save civilization. It’s that simple.”

Bob Mumgaard, a thirty-seven-year-old plasma physicist from Omaha, gets animated when talking about the laying of the transatlantic telegraph cable, in 1858, or the founding of Genentech, in 1976. He studied engineering at the University of Nebraska, though his first love was physics, a field he saw as compelling but impractical. “A lot of the engineers who came out of my school took jobs designing tractors,” he said. In 2008, Mumgaard was working in a lab studying computer hard drives when the MacBook Air came out, with its solid-state hard drive: “I said to myself, ‘O.K., normal hard drives are dead now. I need to go and do something else.’ ”

He applied to graduate programs in physics. He was accepted at Stanford, where he could investigate questions of cosmology and dark matter; he was also accepted to M.I.T.’s P.S.F.C., where he could work on nuclear fusion. The Midwestern pragmatist in him chose fusion over foundational questions about the universe, though he was not particularly motivated by the climate emergency. “Sometimes I think about the way we talked about climate back then, and I can’t believe we wasted so much time debating, like, whether or not Penn State had the best climate model,” he told me. By the time he was a student in Dennis Whyte’s design course, his perspective had changed—he saw fusion as something that needed to have happened yesterday.

He was also a student in a program with an iffy future. After M.I.T. was told that it would lose funding for its experimental fusion device, the P.S.F.C. negotiated an extension to 2016, but it was clear there would be no further reprieve. “We had this opportunity forced on us,” Mumgaard said. “We lost our funding just at the moment that we had this big shiny new lever, this new superconducting material that could move fusion forward.” By 2014, Mumgaard and his colleagues could write down their plans for ARC/SPARC in the form of a concrete risk retirement plan—a venture-capital term for tightly focussed research, with discrete benchmarks. “At M.I.T., venture capital is something you learn about at the university bar,” Mumgaard said. As they saw it, the biggest risk to retire would be making an H.T.S. magnet for SPARC .

In 2015, the Institute of Electrical and Electronics Engineers Symposium on Fusion Engineering was held in Austin, Texas. Many key members of the plasma-physics community were there, and there were two especially noteworthy talks. The first was by the Austrian physicist Guenter Janeschitz, who not only sounds but also looks like Arnold Schwarzenegger. He gave a presentation on DEMO , a proposed fusion device that would be almost twice the size of ITER and produce five gigawatts of power. Janeschitz envisions that, if funded, a prototype could be built in twenty years. Demo is widely seen to be a clear-eyed, workable plan, and a step on the path to bringing practical fusion energy to your great-grandchildren.

Dennis Whyte gave a presentation on ARC . He estimated that it could demonstrate net fusion energy in 2025 and bring fusion to the electric grid by 2030, with individual plants producing a gigawatt of power each—about what a conventional power plant provides today. DEMO would cost an initial thirty billion dollars; ARC would be a million-dollar machine. “It was very dramatic,” Mumgaard said. “The difference was so stark. The room was split.” Roughly speaking, the younger people were buzzing with hope; the older people had perhaps been hopeful one too many times.

The doubters weren’t simply killjoys—they were imaginative thinkers who had devoted decades of their lives to fusion research. It wouldn’t be easy to make H.T.S. into a magnet of sufficient size. And the powerful magnetic field created by H.T.S. was sure to have consequences, which hadn’t been fully studied. There was every reason in the history of experimental science to expect surprises. And funding for fusion projects was already tight; another idea might draw money away from projects that many scientists considered more promising. It was entirely reasonable to ask whether the members of the M.I.T. team were the Wright brothers or Samuel Pierpont Langley—the head of the Smithsonian who in 1903 crashed his very expensive Aerodrome into the Potomac, and then a couple of years later did it again.

After Whyte’s keynote, the M.I.T. crowd went out for lunch at Stubb’s Bar-B-Q. “It’s the kind of place with red-checked tablecloths and food that comes with a lot of napkins,” Whyte said. Everyone around the table knew that the primary funding for their work would end within a year. As Mumgaard recalls, “Basically, we all had pink slips, and yet we were still there. And the question was, Why? We had to learn to listen to ourselves. Did we really believe the field was where we were saying we thought it was?” Was H.T.S. really the shiny new lever that would move fusion dramatically forward? Whyte and his colleagues started to write on a napkin details of how they could make SPARC and then ARC a reality. They wrote down estimates of how much money it would cost to develop it. “It was like this collective dawning, that this thing was really possible,” he told me. Over ribs, they decided that they would fund their work with lottery tickets or with venture capital or with philanthropy—one way or another, they would make their good-enough fusion power plant real.

On September 30, 2016, M.I.T.’s old experimental fusion device, which had been running for twenty-five years, was obliged to shut down by midnight. “This device graduated more than a hundred and fifty Ph.D.s,” Whyte said wistfully. “It set records, even though it’s a hundred times smaller than ITER .” Although M.I.T. was never told why the device was shut down—the Department of Energy continued to fund two other tokamak projects in the U.S.—there was speculation that the reason was that it was the smallest. “Which is ironic, because smaller is where we’re trying to go,” Whyte said. The researchers ran experiments on the machine until the last permitted minute. At 10:30 P . M ., they set a world record for temperature and pressure. At midnight, they shared champagne.

“I went home a little after midnight, but I couldn’t sleep,” Whyte said. In his home office, with his wife’s paintings of trees and flowers on the wall, he started going over the data from the final experiments: “I was just sort of plugging in what our results would mean in a machine with a higher magnetic field,” as would be produced with H.T.S. magnets. “It meant spARC could provide a hundred million watts.” This was even more than the team had speculated in Austin. Whyte was seeing fusion’s holy grail.

The M.I.T. team continued to dedicate its time to ARC / SPARC , quilting together fellowships and grants. At one point, to make payroll, technicians went into the basement and loaded trucks with scrap copper to sell. SPARC Underground was set up—a group of interested scientists who met regularly, to discuss plans and work through difficulties. They needed to buy as much H.T.S. as they could, in order to learn more about the material’s characteristics—hammer it, heat it, freeze it, send current through it. “I remember so well the first shipment of H.T.S.,” Mumgaard said. “We waited for months to get this reel of material. It was only five hundred metres. Now, if we’re not talking ten kilometres, we’re not talking anything. These days, you can order this stuff on Alibaba.com. But then—it was such a moment.”

The team had to solve engineering problems—it also had to solve business problems, including convincing suppliers that there was a market for the material, so that more would be made. “We met with them and asked them if they had considered fusion as a market,” Mumgaard told me. “They were, like, ‘No way, that’s not a real thing.’ ” After two years of extensive lab work and dreamy conversations over five-dollar pitchers of Miller High Life at the Muddy Charles Pub, SPARC Underground became Commonwealth Fusion Systems, a seven-person private fusion-energy company with an ongoing relationship with M.I.T. (C.F.S. funds research at M.I.T., which shares its intellectual resources and some lab space with C.F.S.; patents are filed jointly.) Some of C.F.S.’s funders are European energy companies, and some are philanthropists. By 2021, the company employed more than a hundred and fifty people, many of them veterans of SpaceX and Tesla.

“Energy is a market ,” Mumgaard said. “If you knew there was a ten-trillion-dollar market out there—that is a pull. You couldn’t even have said there was a market that big for computers, or for social media. But you can say that about energy.”

The Plasma Science and Fusion Center, at the northwest corner of the M.I.T. campus, is only a few minutes’ walk from the Cambridge campuses of Pfizer and Moderna. In March, Whyte and Mumgaard met me at the front steps. Mumgaard is now the C.E.O. of C.F.S.; Whyte, a co-founder, remains at M.I.T. They wore T-shirts and had pandemic-untrimmed wavy hair, giving them the look of ambitious surfers. I was there to meet them, but also to meet their magnet, which was still under construction. Maybe it would work, or maybe it would send the team back to the planning stages for years. It was a warm and sunny day. If Kool-Aid had been on offer, I would have drunk not one glass but two.

Aristotle described magnetism as the workings of the soul inside a stone. Magnets have been used to navigate ships, to levitate high-speed trains, to image the inside of a human body, and to move iron filings to make a silly beard on a plastic-bubble-encased drawing of a face. In 1951, the physicist Lyman Spitzer suggested that a magnetic field could serve as a bottle in which to contain a plasma that re-created the pressure and the temperature inside a star. Magnets have been a centerpiece of fusion research ever since.

Mumgaard and Whyte gave me a tour of their lab spaces. The first stop was at what looked like a lectern, in a cubicled room. The room’s distant wall was the control board for M.I.T.’s first experimental fusion device, from the nineteen-eighties. The lectern featured pictures of common plasmas: the sun, lightning, the northern lights, magnetic fusion, and a neon sign reading “ OPEN .” Mounted on the lectern was a hollow glass tube with copper wire coiled around it in two places. The wire was set up so that a current could be run through it, and the glass tube was suspended over a metal plate. You may remember a demonstration, from your high-school science class, of an electric current being run through coiled wires, generating an electromagnetic field—this was basically a fancier version of that. “You can turn it on,” Mumgaard said.

I pushed a black button. A purring noise began. “That’s the sound of the vacuum draining the air from the glass tube,” Mumgaard said. He turned a valve, releasing a tiny bit of hydrogen gas into the tube. A hot-pink glowing light appeared, nested within the glass tube like a matryoshka doll. The magnetic field that contained the pink plasma was visible in the form of empty space between the glass and the glow. “That pink is the superheated plasma,” Mumgaard said. “It’s at least a thousand degrees. But touch the glass.” The glass was cool. “Now touch the copper wires.” They were warm, but not hot. The warmth of the copper wires was not on account of their proximity to the superheated plasma but, rather, because copper is not a perfect conductor; some of the energy running through it is lost in the form of heat. Superconductors lose almost no heat—which is energy.

It seemed impossible that the pink plasma inside the tube, which was as hot as lightning, wasn’t in some way dangerous. Couldn’t some of it leak out of the magnetic bottle, with catastrophic consequences? As an answer, Mumgaard twisted a valve to let a tiny bit of air into the glass tube; the plasma vanished. “People think of fusion like they think of fission, as this overwhelming reaction, but, really, it’s such a delicate process,” Whyte said. “It’s like a candle in the wind. Anything can blow it out. Even a single human breath.”

Much of what Mumgaard and Whyte showed me at P.S.F.C. was the standard part of fusion science. A magnetic bottle is an old idea, and plasma is the most common state of matter; it’s the state that 99.9 per cent of the universe is in. Scientists have been studying plasmas, and magnetic bottles, for decades. Much of what seems difficult about fusion to a plasma physicist—How will tritium be produced and recycled? How can edge-localized modes be anticipated and countered? Will quantum computing enable the study of electromagnetic waves in a plasma?—is so much Greek to a layperson. In contrast, much of what seems difficult about fusion to a layperson—super-hot plasmas, magnetic bottles, toroidal coils—is bread and butter for a fusion scientist.

“As energy, fusion is in some sense very prosaic,” Whyte said. “It’s an intense source of heat.”

“And we’ve been turning heat into electricity since James Watt,” Mumgaard added, referring to the eighteenth-century Scotsman whose development of the steam engine enabled the Industrial Revolution. Mumgaard often stresses that C.F.S. is building a “standard, even boring” machine, using “boring, non-innovative” technology, “but for very non-boring reasons.”

The one exception is the H.T.S. magnet—the most exciting element of the research, and the one that raises the most doubt within the scientific community. “I just wonder about the material stresses of such a powerful magnetic field,” one scientist said to me. “H.T.S. magnets will definitely be used in future tokamaks, no doubt, but I suspect they’ll be used with a weaker magnetic field.”

“Most of the criticism we hear is not about the science but about the timeline,” Mumgaard said. The magnets inside ITER took thirty years to develop. “It took us three years.” He could barely repress a grin; it was the one moment of boyish bullishness and ego that I saw in him.

SPARC will have eighteen H.T.S. magnets; each will be composed of sixteen “pancakes”—eight-foot-tall stackable D-shaped slices. I met a pancake in the West Cell, an enormous open laboratory space at M.I.T. which resembles an airplane hangar. What with all the pancakes and doughnuts being tested there, the West Cell has come to be called the West Cell Diner. The pancakes were given names in alphabetical order. The first production pancake was named Egg. When I was there, I saw Strawberry. “We originally planned to have a pancake breakfast for the team when we finished,” Whyte said. “ COVID is making that look less likely.”

Strawberry was, incidentally, beautiful. It comprised coils of steel, copper, H.T.S., and helium coolant, because even a high-temperature superconductor has to be kept very cold. (In its internal structure, the magnet was more croissant than pancake.) “I remember when the first pancake was done, and we were moving it so delicately,” Whyte said. “Our hearts were in our mouths—it was, like, Holy cow. Then, the other week, it was the fifteenth pancake. We rolled it over, connected it, like we’d done it a thousand times.”

C.F.S. is not the only enterprise trying to be the Wright brothers. In 2001, Michel Laberge left his job as a physicist and engineer at a printing company and began work on a fusion project that evolved into General Fusion, a Canada-based company developing a technology called magnetized target fusion. General Fusion has the backing of Jeff Bezos , though some plasma physicists note that they haven’t seen enough published work to know how the fusion device is progressing. The U.K. Energy Agency has commissioned General Fusion to build a demonstration plant in Culham, Oxfordshire, where major fusion records were set in the nineteen-nineties. General Fusion has announced its intention to open the plant in 2025, the year that C.F.S. plans to turn on its switch at a SPARC demonstration plant being built in Devens, Massachusetts. There are at least twenty fusion startups now, all benefitting from technological advances in 3-D printing and artificial intelligence. The companies have different risks. TAE, in Orange County, California, uses a fuel, boron, that requires higher temperatures but generates no radioactive by-products. Physicists describe boron fusion as “elegant” and even “perfect,” if also, in certain ways, more difficult. Michl Binderbauer, the head of TAE, told me, “I don’t call these other companies my competitors, I call them my compatriots. We have the same goals, and it will be wonderful for any of us to get there.”

C.F.S.’s seventh hire was Joy Dunn, an aerospace engineer recruited from SpaceX and made head of manufacturing. Dunn, who is thirty-five, has a youthful face and short, rockabilly hair; she loves scuba diving, which made leaving California difficult. She had attended M.I.T. as an undergraduate, and at one of the early C.F.S. meetings she found herself seated next to her fluid-dynamics professor. “I was thinking, I hope he doesn’t remember what grade I got in his class,” she said.

One of Dunn’s main tasks has been producing the magnets, including the pancakes I saw in the West Cell Diner. When I met her, a test of the magnets was imminent, but Dunn told me that she wasn’t really worried about failure. “When they were hiring me, they stressed that it wasn’t a physics problem but an engineering problem,” she said. “That appealed to me. You can’t change the laws of physics, but an engineering problem—that can be solved.”

Dunn showed me around the C.F.S. headquarters, a modest one-story building a fifteen-minute walk from the M.I.T. campus. There were wooden presses and lazy Susans and people spooling H.T.S. wire onto metal plates in what I can only describe as an artisanal atmosphere. There was no hum of machinery. The pancakes that were being tested in the West Cell Diner had evolved from being hand-fabricated here to being made by repeatable mechanized processes.

Dunn said that her time at SpaceX had accustomed her to productive failure. “We’d all watch the early rocket-landing attempts,” she said. “One would miss the boat entirely. The next one would land on the boat, but then slide off into the water. Another would land, then tip over.” She went on, “But I remember having a good feeling before the first time we landed successfully. I made sure to go to the front row for the viewing.” The spirit in the crowd that day was something that still motivates her. Dunn sees her work at SpaceX as not very different from her work at C.F.S.: “It’s large metallic structures under stress.”

The day of the crucial magnet demonstration came about six months after I met Dunn. At around 5:30 a . m . on September 5th, Dunn gathered with much of her team at an outdoor tent—on account of COVID —near the magnet she and her team had worked for three years to develop. The magnet had spent the past week being cooled down to twenty degrees Kelvin; the air inside it had been pumped down to a vacuum. The plan was to run a current through it, resulting in a magnetic field of twenty tesla. (A kitchen magnet is about 0.001 tesla; an M.R.I. machine operates at about 1.5 tesla; the magnets that levitate high-speed trains are about five tesla.) Under the tent, a screen displayed a reading of the amps into the magnet, and of the magnetic field out.

As both the current and the magnetic-field numbers rose, Dunn said, “Our anxieties were about the pumps, the valves, the vacuum system, all that—but really it was about the unknown unknowns.” The magnetic field reached twenty tesla. There were hugs, cheers, high fives, and a crowd of very happy people. Whyte made remarks, as did Mumgaard. Dunn and her colleague Brandon Sorbom hosted “The Joy and Brandon Show,” in which they interviewed members of the team about their contributions. “I think for me, personally, a lot of the nervous excitement—it was existential,” Dunn said. “I feel we proved the science. I feel we can make a difference. When people ask me, ‘Why fusion? Why not other renewables?,’ my thinking is: This is a solution at the scale of the problem.”

Soon after the demonstration, Paul Dabbar, the former Under-Secretary for Science and a visiting fellow at Columbia University’s Center on Global Energy policy, declared in an op-ed for The Hill that “the fusion age is upon us.” He urged more government support for the field. Dabbar, like many fusion scientists, takes seriously C.F.S.’s claims that by 2025 it will be demonstrating a fusion device that gives out considerably more energy than it takes to run.

But many, many technological challenges remain before fusion will turn on the lights in your kitchen. Will these fusion devices sustain plasmas for sufficient periods of time? Will they solve their daunting fuel-cycle issues, and manage their exhaust, and will the stresses of the extreme conditions destroy the devices themselves? Will there come a time when there is jam today, and the day after, and the day after that?

“This is difficult to judge,” Cowley, of Princeton, told me. “What C.F.S. has done—it’s a big contribution, absolutely.” He went on, “I’m always cautious. That’s my personality. I do worry that this is fitting luxury seats into a hot-air balloon—and that won’t take you across the Atlantic. I do worry that if this doesn’t work, after all this attention, then the whole field will have a pall over it again for a long time.”

Cowley wavered between seeing his perspective as sober and seeing it as too cautious. He was the one who drew my attention to the argument, in Eddington’s fusion paper, that there is something to be said for Icarus. “My feeling is that there’s still an idea that we haven’t had yet, and that once we have it we’ll think what fools we were not to have had it earlier,” Cowley said. “But the Wright brothers weren’t like me. They weren’t scientists in a lab—they were mechanically minded people who had some new ideas but also who had some luck on their side in terms of other technologies that came of age at the right time. C.F.S. has that youthful spirit. C.F.S. thinks, We know more than we think we know.” The realm of science and invention is not free from psychology. Cowley circled back over his doubts, then suddenly said, “I can’t believe there aren’t a series of steps that will get us there. I can’t believe that we won’t be able to do it eventually.”

In 1901, the chief engineer of the United States Navy wrote, of heavier-than-air flight, “A calm survey of natural phenomenon leads the engineer to pronounce all confident prophecies for future success as wholly unwarranted, if not absurd.” At the time, the Wright brothers were studying aerodynamics in a makeshift wind tunnel; after one particularly disheartening summer at Kitty Hawk, Wilbur confided to Orville his feeling that “not in a thousand years will man fly.” Two years later, they flew their plane for twelve seconds; not too many years after that, they were flying for hours, performing figure eights for large crowds. In response to a report that President Theodore Roosevelt intended to fly with Orville soon, Orville said that, though he wouldn’t turn down a request from the President, he did not think it wise for the President to take such chances. ♦

An earlier version of this article misstated the number of people employed by Commonwealth Fusion Systems, and the period in which an earlier M.I.T. experimental fusion device was invented. It also misidentified James Watt’s nationality.

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Nuclear Fusion is the acknowledged world-leading journal specializing in fusion. The journal covers all aspects of research, theoretical and practical, relevant to controlled thermonuclear fusion.

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Santanu Banerjee et al 2024 Nucl. Fusion 64 046026

We present observations, numerical simulations, and analysis from experiments in the Lithium Tokamak Experiment-Beta (LTX- β ) in which the electron temperature profile ( T e ( r )) shifts from flat to peaked and a tearing mode is also destabilized when the average density ( n e ave ) exceeds ∼10 19 m −3 . Flat T e ( r ) is obtained routinely in LTX- β , with a lithium coated, low-recycling first wall, once the external fueling is stopped and density decays [Boyle et al 2023 Nucl. Fusion   63 056020]. In the present experiment, flat T e profiles can be sustained while maintaining constant n e ave below a line averaged density threshold ( n e ave th ) of ∼10 19 m −3 . Above n e ave th , T e ( r ) shifts from flat to peaked and a tearing mode is destabilized. Due to low recycling, the achieved n e ave can be controlled precisely by external fueling and hence, a certain threshold of the edge neutral inventory from the external fueling is experimentally manifested through n e ave th . The goal of the present work is to investigate the role of edge neutrals in determining T e ( r ) and MHD stability in the unique low-recycling regime of LTX- β . Our hypothesis is that the peaking of T e ( r ) beyond n e ave th is due ultimately to the edge cooling by the cold neutrals beyond a critical fueling flux. At lower fueling flux, flat T e ( r ) results in broader pressure profile and lower resistivity, which in turn stabilizes the tearing mode. This hypothesis is supported by edge neutral density estimation by DEGAS 2 code. Mode analysis by singular value decomposition confirms the tearing mode structure to be m / n = 2/1 ( m and n being the poloidal and toroidal mode numbers). Linear tearing stability analysis with M3D-C1 predicts that plasmas with n e ave > 10 19 are highly susceptible to a n = 1 tearing mode. ORBIT simulations, however, confirmed that the tearing modes do not contribute to the loss of fast ions from neutral beam injection. This study shows for the first time that the neutral inventory at the edge could be one of the deciding factors for the achievability of the unique operation regime of flat T e ( r ) and the excitation of tearing activity that could be disruptive for the plasmas.

G. Federici et al 2024 Nucl. Fusion 64 036025

High temperature superconductors (HTSs) offer the promise of operating at higher magnetic field and temperature. Recently, the use of high field magnets (by adopting HTS) has been promoted by several groups around the world, including new start-up entries, both to substantially reduce the size of a fusion power reactor system and as a breakthrough innovation that could dramatically accelerate fusion power deployment. This paper describes the results of an assessment to understand the impact of using high field magnets in the design of DEMO in Europe, considering a comprehensive list of physics and engineering limitations together with the interdependencies with other important parameters. Based on the results, it is concluded that increasing the magnetic field does not lead to a reduction in device size with relevant nuclear performance requirements, because (i) large structures are needed to withstand the enormous electromagnetic forces, (ii) thick blanket and n-shield structures are needed to protect the coils from radiation damage effects, and (iii) new divertor solutions with performances well beyond today's concepts are needed. Stronger structural materials allow for more compact tokamaks, but do not change the conclusion that scalability is not favourable when increasing the magnetic field, beyond a certain point, the machine size cannot be further reduced. More advanced structural support concepts for high-field coils have been explored and concluded that these solutions are either unfeasible or provide only marginal size reduction, by far not sufficient to account for the potential of operating at very high field provided by HTS. Additionally, the cost of high field coils is significant at today's price levels and shows to scale roughly with the square of the field. Nevertheless, it is believed that even when not operated at high field and starting within conventional insulated coils, HTS can still offer certain benefits. These include the simplification of the magnet cooling scheme thanks to increased temperature margin (indirect conduction cooling). This in turn can greatly simplify coil construction and minimize high-voltage risks at the terminals.

Sehila M. Gonzalez de Vicente et al 2022 Nucl. Fusion 62 085001

In the absence of official standards and guidelines for nuclear fusion plants, fusion designers adopted, as far as possible, well-established standards for fission-based nuclear power plants (NPPs). This often implies interpretation and/or extrapolation, due to differences in structures, systems and components, materials, safety mitigation systems, risks, etc. This approach could result in the consideration of overconservative measures that might lead to an increase in cost and complexity with limited or negligible improvements. One important topic is the generation of radioactive waste in fusion power plants. Fusion waste is significantly different to fission NPP waste, i.e. the quantity of fusion waste is much larger. However, it mostly comprises low-level waste (LLW) and intermediate level waste (ILW). Notably, the waste does not contain many long-lived isotopes, mainly tritium and other activation isotopes but no-transuranic elements. An important benefit of fusion employing reduced-activation materials is the lower decay heat removal and rapid radioactivity decay overall. The dominant fusion wastes are primarily composed of structural materials, such as different types of steel, including reduced activation ferritic martensitic steels, such as EUROFER97 and F82H, AISI 316L, bainitic, and JK2LB. The relevant long-lived radioisotopes come from alloying elements, such as niobium, molybdenum, nickel, carbon, nitrogen, copper and aluminum and also from uncontrolled impurities (of the same elements, but also, e.g. of potassium and cobalt). After irradiation, these isotopes might preclude disposal in LLW repositories. Fusion power should be able to avoid creating high-level waste, while the volume of fusion ILW and LLW will be significant, both in terms of pure volume and volume per unit of electricity produced. Thus, efforts to recycle and clear are essential to support fusion deployment, reclaim resources (through less ore mining) and minimize the radwaste burden for future generations.

F. Sun et al 2024 Nucl. Fusion 64 046011

The safety of future fusion reactors is critically dependent on the tritium (T) retention in plasma-facing materials. Hydrogen isotope (HI) exchange offers a method to redistribute HIs within solid materials, presenting a feasible approach for removing T from bulk materials and trapped by strong trapping sites. Nonetheless, unraveling the intricate mechanism behind HI exchange remains an urgent yet formidable challenge. This study undertakes a comprehensive investigation into the mechanism of HI exchange in tungsten materials across multiple scales. First, we developed a multi-component hydrogen isotope transport and exchange model (HIDTX) based on classical rate theory. The model validation was further carried out, demonstrating good consistency with the well-controlled laboratory experiments. From the results of different comparative models in HIDTX, it is found that the reduction in deuterium retention due to HI exchange was primarily driven by three synergistic effects: competitive re-trapping, collision, and swapping effects. Through molecular dynamics (MD) and first-principles calculations, the microscopic mechanism of HI exchange was revealed to be that the presence of hydrogen atoms in the interstitial sites surrounding a vacancy in tungsten decreased the binding energy between the vacancy and hydrogen. Meanwhile, we discovered that the combination of thermal desorption and HI exchange can significantly lower the temperature required for the hydrogen removal and enhance the removal rate. Particularly, the hydrogen removal time can be shortened by approximately 95% with simultaneous HI exchange compared to that with only thermal desorption. This work provides a practical guideline for comprehending and subsequently designing for efficient T removal in future nuclear fusion materials.

Ethan E. Peterson et al 2024 Nucl. Fusion 64 056011

We present the first fully open-source capabilities for shutdown dose rate (SDR) calculations of fusion energy facilities based on the Rigorous 2-Step (R2S) methodology. These capabilities have been implemented in the OpenMC Monte Carlo particle transport code, building on its existing capabilities while also leveraging new features that have been added to the code to support SDR calculations, such as decay photon source generation. Each of the individual physics components in the R2S workflow—neutron transport, activation, decay photon source generation, and photon transport—have been verified through code-to-code comparisons with MCNP6.2 and FISPACT-II 4.0. These comparisons generally demonstrate excellent agreement between codes for each of the physics components. The full cell-based R2S workflow was validated by performing a simulation of the first experimental campaign from the Frascati Neutron Generator (FNG) ITER dose rate benchmark problem from the Shielding INtegral Benchmark Archive and Database (SINBAD). For short cooling times, the dose calculated by OpenMC agrees with the experimental measurements within the stated experimental uncertainties. For longer cooling times, an overprediction of the shutdown dose was observed relative to experiment, which is consistent with previous studies in the literature. Altogether, these features constitute a combination of capabilities in a single, open-source codebase to provide the fusion community with a readily-accessible option for SDR calculations and a platform for rapidly analyzing the performance of fusion technology.

T. Qian et al 2022 Nucl. Fusion 62 084001

J. Mailloux et al 2022 Nucl. Fusion 62 042026

The JET 2019–2020 scientific and technological programme exploited the results of years of concerted scientific and engineering work, including the ITER-like wall (ILW: Be wall and W divertor) installed in 2010, improved diagnostic capabilities now fully available, a major neutral beam injection upgrade providing record power in 2019–2020, and tested the technical and procedural preparation for safe operation with tritium. Research along three complementary axes yielded a wealth of new results. Firstly, the JET plasma programme delivered scenarios suitable for high fusion power and alpha particle ( α ) physics in the coming D–T campaign (DTE2), with record sustained neutron rates, as well as plasmas for clarifying the impact of isotope mass on plasma core, edge and plasma-wall interactions, and for ITER pre-fusion power operation. The efficacy of the newly installed shattered pellet injector for mitigating disruption forces and runaway electrons was demonstrated. Secondly, research on the consequences of long-term exposure to JET-ILW plasma was completed, with emphasis on wall damage and fuel retention, and with analyses of wall materials and dust particles that will help validate assumptions and codes for design and operation of ITER and DEMO. Thirdly, the nuclear technology programme aiming to deliver maximum technological return from operations in D, T and D–T benefited from the highest D–D neutron yield in years, securing results for validating radiation transport and activation codes, and nuclear data for ITER.

P. Rodriguez-Fernandez et al 2022 Nucl. Fusion 62 042003

The SPARC tokamak project, currently in engineering design, aims to achieve breakeven and burning plasma conditions in a compact device, thanks to new developments in high-temperature superconductor technology. With a magnetic field of 12.2 T on axis and 8.7 MA of plasma current, SPARC is predicted to produce 140 MW of fusion power with a plasma gain of Q ≈ 11, providing ample margin with respect to its mission of Q > 2. All tokamak systems are being designed to produce this landmark plasma discharge, thus enabling the study of burning plasma physics and tokamak operations in reactor relevant conditions to pave the way for the design and construction of a compact, high-field fusion power plant. Construction of SPARC is planned to begin by mid-2021.

N.S. Buchelnikova et al 1966 Nucl. Fusion 6 255

The authors investigate the development of instability when a current is passed through a strongly ionized potassium plasma of non-uniform density. The collisionless plasma, having a density of 10 9 to 10 10 cm −3 , is subjected to magnetic fields of 400 to 2000 Oe. In accordance with earlier experiments, "universal" instability in the form of drift waves is observed when no current is present. The coefficient of diffusion across the magnetic field is some two orders of magnitude greater than the classical coefficient. When a current is passed through the plasma two oscillation branches are excited: drift and ion-sound waves. With the first branch the oscillation frequency is inversely proportional to the magnetic field and the waves have an azimuthal component, the azimuthal phase velocities of all the harmonics being the same. With the second branch the oscillation frequency is independent of the magnetic field and varies inversely with the length of the system. For practical purposes, the phase velocity of the longitudinal component coincides with the ion-sound velocity. The critical drift velocity of the electrons and of the ions, at which ion-sound waves are excited, is approximately 2 × 10 6 cm/ sec, an order of magnitude less than the theoretical critical drift velocity for an homogeneous, almost isothermal, plasma. When the current instability is excited, the diffusion coefficient increases by a factor of 1.5 to 2.5, diffusion beginning to increase simultaneously with the increase in oscillation amplitude.

Latest articles

M. Bergmann et al 2024 Nucl. Fusion 64 056024

Combining the analysis of multiple diagnostics and well-chosen prior information in the framework of Bayesian probability theory, the Integrated Data Analysis code (IDA Fischer et al 2010 Fusion Sci. Technol. 58 675–84) can provide density and temperature radial profiles of fusion plasmas. These IDA-fitted measurements are then used for further analysis, such as discharge simulations and other experimental data analysis. Since IDA considers measurement data, which is frequently fragmentary, with statistical and systematic uncertainties, which are often difficult to quantify, from a heterogeneous set of diagnostics, the fitted profiles and their gradients may be in contradiction to well-established expectations from transport theory. Using the modeling suite ASTRA coupled with the quasi-linear transport solver TGLF, we have created a loop in which simulated profiles and their uncertainties are fed back into IDA as an additional prior, thus providing constraints about the physically reasonable parameter space. We apply this physics-motivated prior to several different plasma scenarios and find improved heat flux match, while still matching the experimental data. This work feeds into a broader effort to make IDA more robust against measurement uncertainties or lack of measurements by combining multiple transport solvers with different levels of complexity and computing costs in a multi-fidelity approach.

You Li et al 2024 Nucl. Fusion 64 056023

Scrape-off layer (SOL) profiles and turbulence in ion cyclotron range of frequency (ICRF)-heated plasmas are investigated by the reciprocating probe diagnostic system (FRPs) and gas puff imaging (GPI) diagnostic in EAST. A radio-frequency (RF) sheath potential reaching up to 100 V is identified proximate to the ICRF antennas. Notably, the amplitude of this RF sheath potential escalates in response to rising ICRF power and inversely with plasma density. When a RF sheath is present in the far SOL, a pronounced density 'shoulder' forms in front of the ICRF antennas, while the 'shoulder' fade away as the antenna and associated RF sheath shift outwards. A strong E r shear is revealed by measurements from both FRPs and GPI. Analysis of the poloidal wave number-frequency spectrum reveals suppression of high-frequency turbulence in the far SOL due to the RF sheath. This effect is manifested in the reduced autocorrelation time τ c and reduced average blob size δ blob of the SOL plasma. Intriguingly, the poloidal propagation direction of the low-frequency turbulence reverses from the electron to the ion diamagnetic drift direction at the RF sheath location. A surge of tungsten impurity is potentially attributed to the heightened interaction between the SOL plasmas and the wall material. Shifting the ICRF antennas outward, to alleviate heat spots, results in the relocation of the RF sheath to the shaded region of the main limiter. This shift amplifies the radial velocity of blobs in the far SOL and concurrently diminishes the SOL density when compared to conditions without ICRF injection. The properties of ion saturation current fluctuations are consistent with the stochastic model predictions.

O. Samant et al 2024 Nucl. Fusion 64 056022

Meng-Chong Ren et al 2024 Nucl. Fusion 64 056021

Surface damage and microscopic defect evolution of tungsten (W) armor under transient heat loads are key factors for fuel retention in fusion reactors. In this work, experiments were conducted to investigate the effects of cyclic thermal shocks on deuterium (D) retention and surface blistering in W. Thermal shock experiments were conducted on recrystallized W using an electron beam with a power density of 0.15 GW m −2 across 100–1500 cycles, followed by D plasma exposure with high-fluence (∼1 × 10 26 D m −2 ). The results demonstrate that samples subjected to 500 and 1500 cycles exhibit a significant presence of sub-grains within 90 μ m. Notably, the inhibition of blistering induced by thermal shock leads to a substantial reduction in D retention (5.45 × 10 19 D m −2 ) at lower cycle numbers (100 cycles) compared to the reference sample (2.35 × 10 20 D m −2 ) which was only exposed to D plasma. When cycle numbers increase to 500 and 1500, D retention reaches 1.98 × 10 20 D m −2 and 4.56 × 10 20 D m −2 , respectively. Based on the tritium migration analysis program, we propose that total D retention is a consequence of the competition between defects reduced by thermal shock-induced suppression of blistering and defects generated by plastic deformation induced by thermal stress. D retention initially decreases with the increase in cycle numbers, followed by a subsequent rise, with the inflection point slightly higher than 500 cycles. Additionally, due to the extensive scope of thermal stress, an escalated exposure period will result in substantial D captured by heat-induced defects, consequently intensifying the D retention. Whether there exists an upper limit to D retention induced by the increasing thermal shock cycles necessitates further experimental analysis. Nonetheless, it is evident that thermal shock significantly contributes to D retention within a profoundly deep bulk region under high cycles.

O. Grover et al 2024 Nucl. Fusion 64 056020

Review articles

G.D. Conway et al 2022 Nucl. Fusion 62 013001

Geodesic acoustic modes (GAMs) are ubiquitous oscillatory flow phenomena observed in toroidal magnetic confinement fusion plasmas, such as tokamaks and stellarators. They are recognized as the non-stationary branch of the turbulence driven zonal flows which play a critical regulatory role in cross-field turbulent transport. GAMs are supported by the plasma compressibility due to magnetic geodesic curvature—an intrinsic feature of any toroidal confinement device. GAMs impact the plasma confinement via velocity shearing of turbulent eddies, modulation of transport, and by providing additional routes for energy dissipation. GAMs can also be driven by energetic particles (so-called EGAMs) or even pumped by a variety of other mechanisms, both internal and external to the plasma, opening-up possibilities for plasma diagnosis and turbulence control. In recent years there have been major advances in all areas of GAM research: measurements, theory, and numerical simulations. This review assesses the status of these developments and the progress made towards a unified understanding of the GAM behaviour and its role in plasma confinement. The review begins with tutorial-like reviews of the basic concepts and theory, followed by a series of topic orientated sections covering different aspects of the GAM. The approach adopted here is to present and contrast experimental observations alongside the predictions from theory and numerical simulations. The review concludes with a comprehensive summary of the field, highlighting outstanding issues and prospects for future developments.

L. Marrelli et al 2021 Nucl. Fusion 61 023001

This paper reviews the research on the reversed field pinch (RFP) in the last three decades. Substantial experimental and theoretical progress and transformational changes have been achieved since the last review (Bodin 1990 Nucl. Fusion 30 1717–37). The experiments have been performed in devices with different sizes and capabilities. The largest are RFX-mod in Padova (Italy) and MST in Madison (USA). The experimental community includes also EXTRAP-T2R in Sweden, RELAX in Japan and KTX in China. Impressive improvements in the performance are the result of exploration of two lines: the high current operation (up to 2 MA) with the spontaneous occurrence of helical equilibria with good magnetic flux surfaces and the active control of the current profile. A crucial ingredient for the advancements obtained in the experiments has been the development of state-of-art active feedback control systems allowing the control of MHD instabilities in presence of a thin shell. The balance between achievements and still open issues leads us to the conclusion that the RFP can be a valuable and diverse contributor in the quest for fusion electricity.

Mohamed Abdou et al 2021 Nucl. Fusion 61 013001

The tritium aspects of the DT fuel cycle embody some of the most challenging feasibility and attractiveness issues in the development of fusion systems. The review and analyses in this paper provide important information to understand and quantify these challenges and to define the phase space of plasma physics and fusion technology parameters and features that must guide a serious R&D in the world fusion program. We focus in particular on components, issues and R&D necessary to satisfy three 'principal requirements': (1) achieving tritium self-sufficiency within the fusion system, (2) providing a tritium inventory for the initial start-up of a fusion facility, and (3) managing the safety and biological hazards of tritium. A primary conclusion is that the physics and technology state-of-the-art will not enable DEMO and future power plants to satisfy these principal requirements. We quantify goals and define specific areas and ideas for physics and technology R&D to meet these requirements. A powerful fuel cycle dynamics model was developed to calculate time-dependent tritium inventories and flow rates in all parts and components of the fuel cycle for different ranges of parameters and physics and technology conditions. Dynamics modeling analyses show that the key parameters affecting tritium inventories, tritium start-up inventory, and tritium self-sufficiency are the tritium burn fraction in the plasma ( f b ), fueling efficiency ( η f ), processing time of plasma exhaust in the inner fuel cycle ( t p ), reactor availability factor (AF), reserve time ( t r ) which determines the reserve tritium inventory needed in the storage system in order to keep the plant operational for time t r in case of any malfunction of any part of the tritium processing system, and the doubling time ( t d ). Results show that η f f b > 2% and processing time of 1–4 h are required to achieve tritium self-sufficiency with reasonable confidence. For η f f b = 2% and processing time of 4 h, the tritium start-up inventory required for a 3 GW fusion reactor is ∼11 kg, while it is <5 kg if η f f b = 5% and the processing time is 1 h. To achieve these stringent requirements, a serious R&D program in physics and technology is necessary. The EU-DEMO direct internal recycling concept that carries fuel directly from the plasma exhaust gas to the fueling systems without going through the isotope separation system reduces the overall processing time and tritium inventories and has positive effects on the required tritium breeding ratio (TBR R ). A significant finding is the strong dependence of tritium self-sufficiency on the reactor availability factor. Simulations show that tritium self-sufficiency is: impossible if AF < 10% for any η f f b , possible if AF > 30% and 1% ⩽ η f f b ⩽ 2%, and achievable with reasonable confidence if AF > 50% and η f f b > 2%. These results are of particular concern in light of the low availability factor predicted for the near-term plasma-based experimental facilities (e.g. FNSF, VNS, CTF), and can have repercussions on tritium economy in DEMO reactors as well, unless significant advancements in RAMI are made. There is a linear dependency between the tritium start-up inventory and the fusion power. The required tritium start-up inventory for a fusion facility of 100 MW fusion power is as small as 1 kg. Since fusion power plants will have large powers for better economics, it is important to maintain a 'reserve' tritium inventory in the tritium storage system to continue to fuel the plasma and avoid plant shutdown in case of malfunctions of some parts of the tritium processing lines. But our results show that a reserve time as short as 24 h leads to unacceptable reserve and start-up inventory requirements. Therefore, high reliability and fast maintainability of all components in the fuel cycle are necessary in order to avoid the need for storing reserve tritium inventory sufficient for continued fusion facility operation for more than a few hours. The physics aspects of plasma fueling, tritium burn fraction, and particle and power exhaust are highly interrelated and complex, and predictions for DEMO and power reactors are highly uncertain because of lack of experiments with burning plasma. Fueling by pellet injection on the high field side of tokamak has evolved to be the preferred method to fuel a burning plasma. Extrapolation from the DIII-D penetration scaling shows fueling efficiency expected in DEMO to be <25%, but such extrapolations are highly uncertain. The fueling efficiency of gas in a reactor relevant regime is expected to be extremely poor and not very useful for getting tritium into the core plasma efficiently. Gas fueling will nonetheless be useful for feedback control of the divertor operating parameters. Extensive modeling has been carried out to predict burn fraction, fueling requirements, and fueling efficiency for ITER, DEMO, and beyond. The fueling rate required to operate Q = 10 ITER plasmas in order to provide the required core fueling, helium exhaust and radiative divertor plasma conditions for acceptable divertor power loads was calculated. If this fueling is performed with a 50–50 DT mix, the tritium burn fraction in ITER would be ∼0.36%, which is too low to satisfy the self-sufficiency conditions derived from the dynamics modeling for fusion reactors. Extrapolation to DEMO using this approach would also yield similarly low burn fraction. Extensive analysis presented shows that specific features of edge neutral dynamics in ITER and fusion reactors, which are different from present experiments, open possibilities for optimization of tritium fueling and thus to improve the burn fraction. Using only tritium in pellet fueling of the plasma core, and only deuterium for edge density, divertor power load and ELM control results in significant increase of the burn fraction to 1.8–3.6%. These estimates are performed with physics models whose results cannot be fully validated for ITER and DEMO plasma conditions since these cannot be achieved in present tokamak experiments. Thus, several uncertainties remain regarding particle transport and scenario requirements in ITER and DEMO. The safety standard requirements for protection of the public and release guidelines for tritium have been reviewed. General safety approaches including minimizing tritium inventories, reducing tritium permeation through materials, and decontaminating material for waste disposal have been suggested.

Boris N. Breizman et al 2019 Nucl. Fusion 59 083001

Of all electrons, runaway electrons have long been recognized in the fusion community as a distinctive population. They now attract special attention as a part of ITER mission considerations. This review covers basic physics ingredients of the runaway phenomenon and the ongoing efforts (experimental and theoretical) aimed at runaway electron (RE) taming in the next generation tokamaks. We emphasize the prevailing physics themes of the last 20 years: the hot-tail mechanism of runaway production, RE interaction with impurity ions, the role of synchrotron radiation in runaway kinetics, RE transport in presence of magnetic fluctuations, micro-instabilities driven by REs in magnetized plasmas, and vertical stability of the plasma with REs. The review also discusses implications of the runaway phenomenon for ITER and the current strategy of RE mitigation.

M.K.A. Thumm et al 2019 Nucl. Fusion 59 073001

In many tokamak and stellarator experiments around the globe that are investigating energy production via controlled thermonuclear fusion, electron cyclotron heating and current drive (ECH&CD) are used for plasma start-up, heating, non-inductive current drive and magnetohydrodynamic stability control. ECH will be the first auxiliary heating method used on ITER. Megawatt-class, continuous wave gyrotrons are employed as high-power millimeter (mm)-wave sources. The present review reports on the worldwide state-of-the-art of high-power gyrotrons. Their successful development during recent years changed ECH from a minor to a major heating method. After a general introduction of the various functions of ECH&CD in fusion physics, especially for ITER, section 2 will explain the fast-wave gyrotron interaction principle. Section 3 discusses innovations on the components of modern long-pulse fusion gyrotrons (magnetron injection electron gun, beam tunnel, cavity, quasi-optical output coupler, synthetic diamond output window, single-stage depressed collector) and auxiliary components (superconducting magnets, gyrotron diagnostics, high-power calorimetric dummy loads). Section 4 deals with present megawatt-class gyrotrons for ITER, W7-X, LHD, EAST, KSTAR and JT-60SA, and also includes tubes for moderate pulse length machines such as ASDEX-U, DIII-D, HL-2A, TCV, QUEST and GAMMA-10. In section 5 the development of future advanced fusion gyrotrons is discussed. These are tubes with higher frequencies for DEMO, multi-frequency (multi-purpose) gyrotrons, stepwise frequency tunable tubes for plasma stabilization, injection-locked and coaxial-cavity multi-megawatt gyrotrons, as well as sub-THz gyrotrons for collective Thomson scattering. Efficiency enhancement via multi-stage depressed collectors, fast oscillation recovery methods and reliability, availability, maintainability and inspectability will be discussed at the end of this section.

Accepted manuscripts

Kong et al 

The pre-thermal quench (pre-TQ) dynamics of a pure deuterium (D 2 ) shattered pellet injection (SPI) into a 3MA/7MJ JET H-mode plasma is studied via 3D non-linear MHD modelling with the JOREK code. The interpretative modelling captures the overall evolution of the measured density and radiated power. The simulations also identify the importance of the drifts of ablation plasmoids towards the tokamak low field side (LFS) and the impurities in the background plasma in fragment penetration, assimilation, radiative cooling and MHD activity in D 2 SPI experiments. It is found that plasmoid drifts lead to an about 70% reduction of the central line-integrated density (compared to a simulation without drifts) in the JET D 2 SPI discharge considered. Impurities that pre-exist before the SPI as well as those from possible impurity influxes related to the SPI are shown to dominate the radiation in the considered discharge. With inputs from JOREK simulations, modelling with the Lagrangian particle-based pellet code PELOTON reproduces the deviation of the SPI fragments in the direction of the major radius as observed by the fast camera. This confirms the role of rocket effects and plasmoid drifts in the considered discharge and reinforces the validity of the JOREK modelling. The limited core density rise due to plasmoid drifts and the strong radiative cooling and MHD activity with impurities (depending on their species and concentration) could limit the effectiveness of LFS D 2 SPI in runaway electron avoidance and are worth considering in the design of the ITER disruption mitigation system.

Ko et al 

The KSTAR has been focused on exploring the key physics and engineering issues for future fusion reactors by demonstrating the long pulse operation of high beta steady-state discharge. Advanced scenarios are being developed with the goal for steady-state operation, and significant progress has been made in high ℓi, hybrid and high beta scenarios with βN of 3. In the new operation scenario called FIRE, fast ions play an essential role in confinement enhancement. GK simulations show a significant reduction of the thermal energy flux when the thermal ion fraction decreases and the main ion density gradient is reversed by the fast ions in FIRE mode. Optimization of 3D magnetic field techniques, including adaptive control and real-time machine learning (ML) control algorithm, enabled long-pulse operation and high-performance ELM-suppressed discharge. Symmetric multiple shattered pellet injections and real-time DECAF are being performed to mitigate and avoid the disruptions associated with high-performance, long-pulse ITER-like scenarios. Finally, the near-term research plan will be addressed with the actively cooled tungsten divertor, a major upgrade of the NBI and helicon current drive heating, and transition to a full metallic wall.

Chang et al 

The magnetic separatrix surface is designed to provide the final and critical confinement to the hot stationary-operation core plasma in modern tokamak reactors in the absence of an external magnetic perturbation or transient MHD perturbation, while diverting the exhaust heat to divertor plates. All the stationary operational boundary plasma studies and reactor designs have been performed under this assumption. However, there has been a long-standing suspicion that a stationary-operation tokamak plasma even without an external magnetic perturbations (MPs) or edge localized modes (ELMs) activities may not have a stable closed separatrix surface, especially near the magnetic X-point. Here, the first gyrokinetic numerical observation is reported that the divertor separatrix surface, due to homoclinic tangles caused by intrinsic electromagnetic turbulence, is not a stable closed surface in a stationary operation phase even without MPs or ELMs. Unlike the MP- or ELM-driven homoclinic tangles that could cause deleterious effects to core confinement or divertor plates, it is found that the micro-turbulence driven homoclinic tangles could connect the divertor plasma to the pedestal plasma in a constructive way by broadening the divertor heat-exhaust footprint and weakening the pedestal slope to the ELM-safe direction. Micro-turbulent homoclinic tangles can open a new research direction in understanding and controlling these two most troublesome and non-locally connected edge-plasma issues in a tokamak fusion reactor.

Islam et al 

The SOLPS-ITER code is utilized to analyze the boundary plasma associated with a fast-flow lithium (Li) divertor configuration in the Fusion Nuclear Science Facility (FNSF) tokamak and identify operational regimes with acceptable divertor and core conditions. Plasma transport from the SOLPS-ITER code has been coupled with a liquid metal (LM) MHD/heat transfer code to model a Li open-surface divertor design and assess its impact on the scrape-off-layer (SOL) and core plasma performance. Simulations with only Neon (Ne) impurity seeding have been conducted to evaluate its impact on meeting FNSF design demands for the divertor and upstream plasma parameters. Simulation results indicate that Ne seeding significantly mitigates divertor heat flux but potentially reduces both upstream electron and main ion density due to fuel dilution. The combined application of Ne seeding and deuterium (D 2 ) puffing is required to satisfy the FNSF design requirements on upstream density (1e20) and divertor energy flux ( < 10 MW/m 2 ). D 2 puffing plays a role in counteracting upstream density drops and augmenting energy and momentum losses through atomic and molecular processes.&#xD;&#xD;The inlet Li flow velocity is systematically varied across a wide range to identify acceptable flows and corresponding LM surface temperatures. This comprehensive analysis identifies the acceptable Li flow parameters, LM surface temperature, and emitted Li fluxes necessary to meet the major design constraints. The emitted Li fluxes exhibit minimal impact on the main plasma at surface temperatures up to approximately 525^C, corresponding emitted Li fluxes of up to 2e23 atoms/s. Uncertainties in the Li emission processes from the surface are also investigated, primarily influencing Li loss in the lower surface temperature range (525C). Conversely, evaporation predominantly drives the Li loss processes at higher surface temperature ranges (525C), contaminating both the divertor and upstream plasma.

Liu et al 

The reversed field pinch (RFP) is a toroidal magnetic configuration in which plasmas can spontaneously transform into different self-organized states. Among various states, the ''quasi-single-helical'' (QSH) state has a dominant component for the magnetic field and significantly improves confinement. Many theoretical and experimental efforts have investigated the transitions among different states. This paper employs the multi-region relaxed magnetohydrodynamic (MRxMHD) model to study the properties of QSH and other states. The stepped-pressure equilibrium code (SPEC) is used to compute MHD equilibria for the Keda Torus eXperiment (KTX). The toroidal volume of KTX is partitioned into two subvolumes by an internal transport barrier. The geometry of this barrier is adjusted to achieve force balance across the interface, ensuring that the plasma in each subvolume is force-free and that magnetic helicity is conserved. By varying the parameters, we generate distinct self-organized states in KTX. Our findings highlight the crucial role of magnetic helicity in shaping these states. In states with low magnetic helicity in both subvolumes, the plasma exhibits axisymmetric behavior. With increasing core helicity, the plasma gradually transforms from an axisymmetric state to a double-axis helical (DAx) state and finally to a single-helical-axis (SHAx) state. Elevated core magnetic helicity leads to a more pronounced dominant mode of the boundary magnetic field and a reduced core magnetic shear. This is consistent with previous experimental and numerical results in other RFP devices. We find a linear relationship between the plasma current and helicity in different self-organized states. Our findings suggest that KTX may enter the QSH state when the toroidal current reaches 0.72 MA. This study demonstrates that the stellarator equilibrium code SPEC unveils crucial RFP equilibrium properties, rendering it applicable to a broad range of RFP devices and other toroidal configurations.&#xD;

More Accepted manuscripts

Open access

Mengdi Kong et al 2024 Nucl. Fusion

Won-Ha Ko et al 2024 Nucl. Fusion

Choong Seock Chang et al 2024 Nucl. Fusion

MD Shahinul Islam et al 2024 Nucl. Fusion

Ke Liu et al 2024 Nucl. Fusion

Jason Parisi et al 2024 Nucl. Fusion

A theoretical model is presented that for the first time matches experimental measurements of the pedestal width-height Diallo scaling in the low-aspect-ratio high-$\beta$ tokamak NSTX. Combining linear gyrokinetics with self-consistent pedestal equilibrium variation, kinetic-ballooning, rather than ideal-ballooning plasma instability, is shown to limit achievable confinement in spherical tokamak pedestals. Simulations are used to find the novel Gyrokinetic Critical Pedestal constraint, which determines the steepest pressure profile a pedestal can sustain subject to gyrokinetic instability. Gyrokinetic width-height scaling expressions for NSTX pedestals with varying density and temperature profiles are obtained. These scalings for spherical tokamaks depart significantly from that of conventional aspect ratio tokamaks.&#xD;

Chunjie Niu et al 2024 Nucl. Fusion

A novel theoretical model based on modified diffusion rate equations is proposed to simulate the retention of hydrogen isotopes and the dynamics of bubble growth in tungsten (W) when exposed to simultaneous hydrogen (H) and helium (He) plasma irradiations. Simulation is conducted to assess the influence of temperature as well as simultaneous H and He irradiation at an increasing fluence. Not only to develop a holistic understanding but also to substantiate simulation findings about synergy between H and He plasma irradiation, a W sample is exposed sequentially to H and He plasma at 873 K using the large-power material irradiation experimental system (LP-MIES). The topographical changes in the W sample are investigated using atomic force microscopy (AFM) after each plasma irradiation exposure sequence. Simulation results reveal that the ability of a bubble containing both H and He to trap adjacent H/He atoms is primarily governed by their individual partial pressure within the bubble. Furthermore, at elevated temperatures, the synergy between H and He significantly enhances the retention of H isotopes in W. AFM micrographs of the W sample exposed to both H and He plasma irradiation show a severely damaged and locally delaminated layer, absent in the sample exposed only to either H or He, conclusively establishing evidence of synergy between H and He irradiation effects. The average bubble radius computed using the model aligns excellently with experimentally determined values obtained through SEM/AFM analysis. The robustness of the proposed model is also assessed by comparing bubble radius and H isotopes retention at various temperatures with experimental data reported in the literature.

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• 1960-present Nuclear Fusion doi: 10.1088/issn.0029-5515 Online ISSN: 1741-4326 Print ISSN: 0029-5515

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Is Nuclear Fusion the Answer to Our Energy Needs?

More from our inbox:, mayor eric adams’s efforts to help mentally ill new yorkers, a supreme court case about the power of state legislatures, no to appeasement over the ukraine war, donald trump, action figure.

To the Editor:

Re “ A Blast of 192 Lasers Achieves a Breakthrough in Nuclear Fusion ” (front page, Dec. 14):

California gives us a glimpse of the incredible impact a working fusion reactor could offer to the world. Clean, limitless energy without greenhouse gas emissions and virtually no toxic residues.

While the engineering challenges are enormous, they are not insurmountable. Sort of akin to where we were in 1961, when President John F. Kennedy set the goal of putting a man on the moon by the end of the decade. Many of those who knew anything about rocketry said it was impossible.

President Biden should promise that the U.S. will have a working fusion power plant by 2035. The costs will be enormous, but the benefits from a working fusion energy resource far outweigh the benefits from landing on the moon and it may help to save the planet from a climate disaster.

Cheap fusion energy would be the ultimate energy security and economic engine for a prosperous future for the world.

Gary Krellenstein New York

The ability of Lawrence Livermore National Laboratory to direct 192 laser beams to ignite a fusion reaction and fleetingly produce a small amount of heat is a scientific breakthrough, but hardly one that should inform energy policy.

The hype in the 20th century was that nuclear energy would be cheap and boundless. Nuclear turned out to be increasingly costly. The only perpetual product was radioactive waste. So now the jive is that “advanced” fission reactors and fusion will magically solve all problems in future decades.

Meanwhile, don’t look too carefully at the billions of dollars being diverted away from renewables, efficiency and energy storage solutions that could be rolled out rapidly now for far less money, with present-time climate benefits — and without the risk of nuclear weapons proliferation.

Michel Lee Scarsdale, N.Y. The writer is an attorney who has worked pro bono on nuclear-related policy matters.

I will always remember when and where I first heard about the nuclear fusion breakthrough: on the morning of Dec. 14, 2022, at approximately 8:30 a.m. at the breakfast table. That’s when I learned that planet Earth has a genuine chance of remaining habitable over the long term and that humankind — along with innumerable other deserving species — has a genuine chance of surviving over the long term. What incredibly hopeful news!

Now we can hope that the nations of the world will somehow manage to keep our beleaguered planet viable and chugging along until that long-dreamed-about future day finally arrives.

Nancy Stark New York

We have had unlimited free and clean fusion energy for over four billion years. All we need to do is scoop up the energy and make electricity from it. We know how to do this now.

The clean, free and unlimited energy is called “sunlight.” Every country on earth has access to it.

Melvin Dorin Cambria, Calif.

Re “ The Root Cause of Violent Crime Is Not What We Think It Is ,” by Phillip Atiba Goff (Opinion guest essay, Dec. 13):

Dr. Goff’s critique of the efforts of Mayor Eric Adams’s administration to reach homeless New Yorkers who are suffering from severe mental illness amounts to a serious misreading of our stated policy.

We are not criminalizing mental illness or trying to arrest our way out of the problem, but trying to get people with untreated severe mental illness help before a tragedy occurs.

Our approach is based on compassion, with the goal of protecting both the public and those in crisis — and we have the strong support of many advocates, including the families of those who are in desperate need of involuntary mental health treatment.

Mr. Adams’s job as mayor of New York City is to protect public safety, including the safety of those New Yorkers who have been left behind by our broken mental health system.

Anne Williams-Isom New York The writer is New York City’s deputy mayor for health and human services.

Re “ This Case Shouldn’t Be With the Supreme Court ” (editorial, Dec. 11), about a case that would greatly expand the power of state legislatures:

Any Supreme Court justice claiming to be an “originalist” must dismiss this case out of hand, for the authors of the Constitution almost universally agreed that a basic purpose was to rein in the state legislatures.

In the 1780s, state legislatures, newly including small farmers and tradesmen, appeared to be running amok in promoting too much populism, particularly undermining property rights, allowing easy money to stoke inflation and enabling people to avoid paying their debts, among other things.

A sensible explanation of the phrase assigning the “Times, Places and Manner of holding Elections for Senators and Representatives” to each state legislature is purely for the administration of elections to accommodate local conditions. The framers certainly rejected the idea of “independent state legislatures,” believing strongly in judicial review at the state and national levels.

To undo the fundamental principle of checks and balances designed to preserve liberty and reduce the possibility of governmental oppression at the state and national level would be to violate the very purpose of the Constitution itself.

Richard H. Kohn Kenneth Bowling Mr. Kohn is professor emeritus of history at the University of North Carolina at Chapel Hill. Mr. Bowling, a historian, was co-editor of the 22-volume “Documentary History of the First Federal Congress.”

Re “ Are We in the West Weaker Than Ukrainians? ,” by Nicholas Kristof (column, Dec. 15):

Western democracies have tried appeasement before, for example in the 1930s in the face of Hitler’s demands. The Nazis then invaded country after country in World War II. Many Americans wanted to stay out of the war in Europe.

Appeasement did not work very well then. We are facing a similarly dangerous situation now. We should do everything we can to support Ukraine without spreading the conflict unnecessarily.

Gregory Filice Siem Reap, Cambodia

Re “ Trump Hawks Images of Self as Superhero ” (news article, Dec. 16):

Former President Donald Trump — the Man Who Would Be King (with apologies to Rudyard Kipling) — has announced from his Truth Social digital podium that he is now selling digital trading cards of himself in heroic poses.

We can purchase him as a caped superhero, an Old West sheriff, an astronaut, etc., for only $99. Each.

This is beyond curious. It’s beyond cute. It’s actually sad to watch a former president — an announced candidate for 2024 — reducing himself to the level of a carnival sideshow barker. Many of us are saddened to witness his decline.

Mike Barrett Ashburn, Va.

An earlier version of the third letter misstated the nature of the nuclear breakthrough. It involved fusion, not fission.

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Is nuclear fusion the answer to the climate crisis?

Promising new studies suggest the long elusive technology may be capable of producing electricity for the grid by the end of the decade

If all goes as planned, the US will eliminate all greenhouse gas emissions from its electricity sector by 2035 – an ambitious goal set by President-elect Joe Biden, relying in large part on a sharp increase in wind and solar energy generation. That plan may soon get a boost from nuclear fusion, a powerful technology that until recently had seemed far out of reach.

Researchers developing a nuclear fusion reactor that can generate more energy than it consumes have shown in a series of recent papers that their design should work, restoring optimism that this clean, limitless power source will help mitigate the climate crisis.

While the new reactor still remains in early development, scientists hope it will be able to start producing electricity by the end of the decade. Martin Greenwald, one of the project’s senior scientists, said a key motivation for the ambitious timeline is meeting energy requirements in a warming world. “Fusion seems like one of the possible solutions to get ourselves out of our impending climate disaster,” he said.

Nuclear fusion, the physical process that powers our sun, occurs when atoms are pushed together at extremely high temperatures and pressure, causing them to release tremendous amounts of energy by merging into heavier atoms.

Since it was first discovered last century, scientists have sought to harness fusion, an extremely dense form of power whose fuel – hydrogen isotopes – are abundant and replenishable. Moreover, fusion produces no greenhouse gases or carbon, and unlike fission nuclear reactors, carries no risk of meltdown.

Harnessing this form of nuclear power, though, has proven extremely difficult, requiring heating a soup of subatomic particles, called plasma, to hundreds of millions of degrees – far too hot for any material container to withstand. To work around this, scientists developed a donut-shaped chamber with a strong magnetic field running through it, called a tokamak, which suspends the plasma in place.

MIT scientists and a spinoff company, Commonwealth Fusion Systems , began designing the new reactor, which is more compact than its predecessors, in early 2018, and will start construction in the first half of next year. If their timeline goes as planned, the reactor, called Sparc , will be capable of producing electricity for the grid by 2030, according to researchers and company officials. This would be far faster than existing major fusion power initiatives.

Existing reactor designs are too large and expensive to realistically generate electricity for consumers. Through the use of cutting-edge, ultra-strong magnets , the team at MIT and Commonwealth Fusion hope to make a tokamak reactor that is compact, efficient and scalable. “What we’ve really done is combine an existing science with new material to open up vast new possibilities,” Greenwald said

Having demonstrated that the Sparc device can theoretically produce more energy than it requires to run in the research papers published in September, the next step involves building the reactor, followed by a pilot plant that will generate electricity onto the grid.

Scientists and entrepreneurs have long made promises about fusion being just around the corner, only to encounter insurmountable problems. This has created reluctance to invest in it, particularly as wind, solar and other renewables — although less powerful than fusion — have become more efficient and cost effective.

But the tide is changing. In Biden’s $2tn plan , he named advanced nuclear technologies as part of the decarbonization strategy, the first time the Democrats have endorsed nuclear energy since 1972. There is also significant investment coming from private sources, including some major oil and gas companies , who see fusion as a better long term pivot than wind and solar.

According to Bob Mumgaard, chief executive of Commonwealth Fusion, the aim is not to use fusion to replace solar and wind, but to supplement them. “There are things that will be hard to do with only renewables, industrial scale things, like powering large cities or manufacturing,” he said. “This is where fusion can come in.”

The plasma science community is generally enthusiastic about Sparc’s progress, though some question the ambitious timeline, given engineering and regulatory hurdles.

Daniel Jassby, who worked as a research scientist at the Princeton Plasma Physics Lab for 25 years, is skeptical about whether a fusion reactor like SPARC will ever provide a feasible alternative source of energy. Tritium, one of the hydrogen isotopes that will be used as fuel by Sparc, is not naturally occurring and will need to be produced, he said.

The team at MIT propose that this substance will be regenerated continuously by the fusion reaction itself. But Jassby believes that this will require a huge amount of electricity, which will make the reactor prohibitively expensive. “When you consider we get solar and wind energy for free, to rely on fusion reaction would be foolish,” he said.

Mumgaard concedes that the challenges that lie ahead are daunting. But he remains confident.

“There is a broader trend in acknowledging how important climate is and that we need all hands on deck,” he said. “We got into this problem with technology, but with fusion there are big opportunities to solve this with technology.”

• Environment
• Climate crisis
• Renewable energy

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• The Inventory

New Kind of Fusion Reactor Built at Government Lab

The stellarator has permanent magnets, a first for a fusion experiment..

A team of physicists and engineers at Princeton Plasma Physics Laboratory built a twisting fusion reactor known as a stellarator that uses permanent magnets, showcasing a potentially cost-effective way of building the powerful machines. Their experiment, called MUSE, relies on 3D-printed and off-the-shelf parts.

Nuclear fusion, the reaction that powers stars like our Sun, produces huge amounts of energy by merging atoms (not to be confused with nuclear fission, which produces less energy by splitting atoms). Nuclear fission is the reaction at the core of modern nuclear reactors that power electric grids; scientists have yet to crack the code on nuclear fusion as an energy source. Even once that long-sought goal is reached, scaling the technology and making it commercially viable is its own beast .

Stellarators are cruller-shaped devices that contain high-temperature plasmas, which can bed tuned to foster the conditions for fusion reactions. They are similar to tokamaks, doughnut-shaped devices that run fusion reactions . Tokamaks rely on solenoids , which are magnets that carry electric current. MUSE is different.

“Using permanent magnets is a completely new way to design stellarators,” said Tony Qian, a graduate student at Princeton Plasma Physics Laboratory and lead author of two papers published in the Journal of Plasma Physics and Nuclear Fusion that describe the design of the MUSE experiment. “This technique allows us to test new plasma confinement ideas quickly and build new devices easily.”

Permanent magnets don’t need electric current to generate their magnet fields and can be purchased off-the-shelf. The MUSE experiment stuck such magnets onto a 3-D printed shell.

“I realized that even if they were situated alongside other magnets, rare-earth permanent magnets could generate and maintain the magnetic fields necessary to confine the plasma so fusion reactions can occur,” Michael Zarnstorff, a research scientist at the laboratory and principal investigator of the MUSE project, in a press release. “That’s the property that makes this technique work.”

Last year, scientists at the Department of Energy’s Lawrence Livermore National Laboratory (LLNL) achieved breakeven in a fusion reaction ; that is, the reaction produced more energy than it took to power it . However, that accolade neglects to account for the “wall power” necessary to induce the reaction. In other words, there’s still a long, long road ahead.

The LLNL breakthrough was done by shining powerful lasers at a pellet of atoms, a different process than the plasma-based fusion reactions that occur in tokamaks and stellarators. Little tweaks to the devices, like the implementation of permanent magnets in MUSE or an upgraded tungsten diverter in the KSTAR tokamak , make it easier for scientists to replicate the experimental setups and perform experiments at high temperatures for longer.

Taken together, these innovations will allow scientists to do more with the plasmas at their fingertips, and maybe—just maybe—reach the vaunted goal of usable and scalable fusion energy.

Correction: A previous version of this article incorrectly referred to Princeton Plasma Physics Laboratory as part of Princeton University. Though the university manages the lab for the Department of Energy, the lab is not part of the university.

Essay on Nuclear Fusion and Energy | Energy Management

Here is an essay on ‘Nuclear Fusion and Energy’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Nuclear Fusion and Energy’ especially written for school and college students.

Essay on Nuclear Fusion and Energy

Essay Contents:

• Essay on the Advantages of Fusion Energy

Essay # 1. Introduction to Nuclear Fusion and Energy:

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Of all the currently known available sources, only nuclear and solar energy have the potential of supplying large amounts of power within the available time frame. These two sources can complement each other to meet the demand economically and safely. However, considerable technical and economic prob­lems must be solved before large-scale utilization of these sources can become possible.

Energy is produced in the-sun and stars by continuous fusion reactions. Fusion of light nuclei to form a heavy nucleus releases large amount of energy. If the controlled fusion of light elements is carried out on our planet, enough energy can be generated to meet all the energy requirements of future genera­tions.

The primary fuel for fusion is deuterium, sometimes called heavy hydrogen (H 2 ). There is one atom of deuterium for every 6500 atoms of ordinary hydrogen in sea water. The total energy content of deuterium as fossil fuel is about 100,000 kWh of energy per gram of deuterium. This is about 10 million times more than released per gram in the combustion of fossil fuels.

The entire amount of world energy consumption of 6 × 10 13 kWh per year could be supplied by the fusion of only 600 metric tons of deuterium. The amount of deuterium in the world oceans would be enough to sustain the present total world energy require­ment for 100 billion years. Nuclear fusion would indeed present an ultimate solution of mankind’s energy needs for all the years to come.

A nuclear fusion reaction can occur when two atomic nuclei approach very close to each other at velocities at least large enough to overcome their mutual electrostatic repulsion called “Coulomb barrier” . Fusion reaction occurs at very high temperature. Therefore, there is a problem of confinement of fusion. There are other essential difficulties lying to be overcome before this future energy source can be put to man’s use.

Fusion research is indeed being taken seriously by the major industrial nations. There are serious efforts by individual countries as well as combined effort by consortium of countries. It is almost certain that large practical fusion power plants will be built in the twenty-first century. Once this technology is developed, an almost unlimited supply of energy will be available for the world’s needs ushering in a better living standard for the human kind all over the world.

Essay # 2. Principle of Fusion Process:

Energy is produced in the sun and stars by the following continuous fusion reactions:

The following symbols have been used:

The plasma is contained inside an evacuated tube of about 4m. The surrounding vacuum wall through which 14 MeV neutrons from the plasma pass, is maintained at about 750°C. Outside this wall are two concentric regions, i.e., the lithium breeder modulator and the magnetic shield. Tritium is manufac­tured in the lithium blanket. Large cryogenic superconducting magnets of 7 to 8 m diameter maintain the magnetic field.

The breeding of tritium takes place as follows:

6 Li + n → H e + T + 4.78 MeV.

The heavy lithium isotope 6 Li acts as breeding material and moderator.

The design parameters of a projected fusion reactor are given below:

Essay # 6. Advantages of Fusion Energy:

i. The supply of deuterium is inexhaustible.

ii. No radioactive waste is produced.

iii. It is very safe to operate.

iv. High conversion efficiency (60%) can be achieved.

v. Low heat rejection per kW of electricity generated to atmosphere.

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Ccientists Announce Highest Nuclear Fusion Output Ever. Is Unlimited Power Coming Sooner Than We Think?

T he world – at least, the scientific one –  has been on the hunt for the cleanest, most efficient way to fuel our needs for decades now.

Every year, they get a little closer and a little cleaner with nuclear fuel.

Now, scientists in the UK are saying they’ve surpassed the previous record for nuclear fusion output.

The test, performed at the Joint European Torus (JET), resulted in high-fusion power for 5 seconds, while releasing 69.26 megajoules of energy from a mere 0.21 milligrams of fuel.

The fuel, in this case, is a mixture of two types of heavy hydrogen, deuterium and tritium.

This is about the same amount of energy we get from 4.4 pounds of coal.

Their record is especially impressive when you realize that JET is sort of the wayfinder for full-scale prototypes like ITER and DEMO.

The former is set to generate 10 times as much energy as put in, and DEMO could reach 25 times the amount it started with.

Professor Ambrogio Fasoli, CEO at EUROfusion, says the results at JET definitely make it clear that the technology has promise.

“Our successful demonstration of operational scenarios for future fusion machines like ITER and DEMO, validated by the new energy record, instil greater confidence in the development of fusion energy. Beyond setting a new record, we achieved things we’ve never done before and deepened our understanding of fusion physics.”

He’s also happy the fuel mixture worked so well.

“We can reliably create fusion plasmas using the same fuel mixture to be used by commercial fusion energy power plants, showcasing the advanced expertise developed over time.”

All three facilities use a design known as a tokamak. With it, the fusing plasma is contained in a donut-shaped chamber by powerful magnets. We have to get creative when it comes to achieving the pressures and temperatures required for fusion, which means heating plasma to at least 100 million degrees.

Once there, the plasma releases a lot of energy. One drawback is that it sometimes comes in bursts that damage the confinement walls. Another is that helium is a byproduct and has to be safely discarded.

The recent test at JET proves that both of these challenges are solved easily enough, at least according to Emmanuel Joffrin, EUROfusion Tokamak Exploitation Task Force Leader.

“Not only did we demonstrate how to soften the intense heat flowing from the plasma to the exhaust, we also showed in JET how we can get the plasma edge into a stable state thus preventing bursts of energy reaching the wall. Both techniques are intended to protect the integrity of the walls of future machines. This is the first time that we’ve ever been able to test those scenarios in a deuterium-tritium environment.”

The goal with nuclear fusion is to reach a Q factor higher than one, which means getting as much energy out as you put in. So far, the only experiment to achieve that was one at the Inertial Fusion System, which earned a Q of 1.5.

Creative Commons License: ORNL History/CC BY 2.0 DEED

The best JET has managed is 0.69, but their output was 20x higher than what the Inertial Fusion has achieved.

No matter how exciting this news is for everyone at JET and around the industry, we’re still a couple of decades from commercial fusion power stations.

That said, it’s definitely encouraging.

If you think that’s impressive, check out this story about a “goldmine” of lithium that was found in the U.S. that could completely change the EV battery game.

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Model optimization of infrared homogenization for inertial confinement fusion cryotargets

• Xu, Zhaorui
• Peng, Shaojing
• Shen, Yibin

Inertial confinement fusion (ICF) is widely used to generate nuclear fusion energy worldwide. The key to successful ICF experiments is to achieve uniformity in cryotargets. To ensure the performance of the cryotarget, the fuel gas must create a uniform and smooth solid layer on the inner surface, with a roughness of less than 1μm and sphericity greater than 99%. Creating a uniform deuterium-deuterium (DD) ice layer is more challenging than making deuterium-tritium (DT) ice. This is due to the lack of self-smoothing caused by the absence of β decay heat. The DD ice layer also has a specific absorption peak in the mid-infrared band. This paper examines using 3.16μm light to heat and evenly distribute DD ice in a cryotarget. Additionally, the article introduces a technique to simulate the evenness of the heating process using the normalized volumetric weighted standard deviation of the volume heating rate, which involves annular light illumination. The simulation analysis reveals that reducing the flux threshold and increasing the number of rays used have little impact on the accuracy of the calculation, but it significantly reduces operational efficiency. Besides, the simulation error is less than 5% when the off-axis of the annular light source on the Z-axis is under 0.1mm, and the error is less than 10% when the Z-axis off-axis is less than 0.2mm. The system parameters' influence rules on the cryotarget's infrared uniformity effect are summarized through multi-group simulation analysis, laying a foundation for the subsequent related ICF experiments.