[Scientists have been chasing the dream of harnessing the
reactions that power the Sun since the dawn of the atomic era.
Interest, and investment, in the carbon-free energy source is heating
up.]
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THE CHALLENGE OF FUSION POWER  
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M. Mitchell Waldrop
November 1, 2023
Knowable Magazine
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_ Scientists have been chasing the dream of harnessing the reactions
that power the Sun since the dawn of the atomic era. Interest, and
investment, in the carbon-free energy source is heating up. _

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For the better part of a century now, astronomers and physicists have
known that a process called thermonuclear fusion has kept the Sun and
the stars shining for millions or even billions of years. And ever
since that discovery, they’ve dreamed of bringing that energy source
down to Earth and using it to power the modern world.

It’s a dream that’s only become more compelling today, in the age
of escalating climate change. Harnessing thermonuclear fusion and
feeding it into the world’s electric grids could help make all our
carbon dioxide-spewing coal- and gas-fired plants a distant memory.
Fusion power plants could offer zero-carbon electricity
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that flows day and night, with no worries about wind or weather —
and without the drawbacks of today’s nuclear fission plants
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such as potentially catastrophic meltdowns and radioactive waste that
has to be isolated for thousands of centuries.

In fact, fusion is the exact opposite of fission: Instead of splitting
heavy elements such as uranium into lighter atoms, fusion generates
energy by merging various isotopes of light elements such as hydrogen
into heavier atoms.

To make this dream a reality, fusion scientists must ignite fusion
here on the ground — but without access to the crushing levels of
gravity that accomplish this feat at the core of the Sun. Doing it on
Earth means putting those light isotopes into a reactor and finding a
way to heat them to hundreds of millions of degrees centigrade —
turning them into an ionized “plasma” akin to the insides of a
lightning bolt, only hotter and harder to control. And it means
finding a way to control that lightning, usually with some kind of
magnetic field that will grab the plasma and hold on tight while it
writhes, twists and tries to escape like a living thing.

Both challenges are daunting, to say the least. It was only in late
2022, in fact, that a multibillion-dollar fusion experiment in
California finally got a tiny isotope sample to put out more
thermonuclear energy than went in to ignite it
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And that event, which lasted only about one-tenth of a nanosecond, had
to be triggered by the combined output of 192 of the world’s most
powerful lasers.

Today, though, the fusion world is awash in plans for much more
practical machines. Novel technologies such as high-temperature
superconductors are promising to make fusion reactors smaller,
simpler, cheaper and more efficient than once seemed possible. And
better still, all those decades of slow, dogged progress seem to have
passed a tipping point, with fusion researchers now experienced enough
to design plasma experiments that work pretty much as predicted.

“There is a coming of age of technological capability that now
matches up with the challenge of this quest,” says Michl
Binderbauer, CEO of the fusion firm TAE Technologies
[[link removed]] in Southern California.

Indeed, more than 40 commercial fusion firms have been launched since
TAE became the first in 1998 — most of them in the past five years,
and many with a power-reactor design that they hope to have operating
in the next decade or so. “‘I keep thinking that, oh sure, we’ve
reached our peak,” says Andrew Holland, who maintains a running
count
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as CEO of the Fusion Industry Association
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founded in 2018 in Washington, DC. “But no, we keep seeing more and
more companies come in with different ideas.”

None of this has gone unnoticed by private investment firms, which
have backed the fusion startups with some $6 billion and counting.
This combination of new technology and private money creates a happy
synergy, says Jonathan Menard, head of research at the Department of
Energy’s Princeton Plasma Physics Laboratory in New Jersey, and not
a participant in any of the fusion firms.

Compared with the public sector, companies generally have more
resources for trying new things, says Menard. “Some will work, some
won’t. Some might be somewhere in between,” he says. “But
we’re going to find out, and that’s good.”

Granted, there’s ample reason for caution — starting with the fact
that none of these firms has so far shown that it can generate net
fusion energy even briefly, much less ramp up to a commercial-scale
machine within a decade. “Many of the companies are promising things
on timescales that generally we view as unlikely,” Menard says.

But then, he adds, “we’d be happy to be proven wrong.”

With more than 40 companies trying to do just that, we’ll know soon
enough if one or more of them succeeds. In the meantime, to give a
sense of the possibilities, here is an overview of the challenges that
every fusion reactor has to overcome, and a look at some of the
best-funded and best-developed designs for meeting those challenges.

Prerequisites for fusion

The first challenge for any fusion device is to light the fire, so to
speak: It has to take whatever mix of isotopes it’s using as fuel,
and get the nuclei to touch, fuse and release all that beautiful
energy.

This means literally “touch”: Fusion is a contact sport, and the
reaction won’t even begin until the nuclei hit head on. What makes
this tricky is that every atomic nucleus contains positively charged
protons and — Physics 101 — positive charges electrically repel
each other. So the only way to overcome that repulsion is to get the
nuclei moving so fast that they crash and fuse before they’re
deflected.

This need for speed requires a plasma temperature of at least 100
million degrees C. And that’s just for a fuel mix of deuterium and
tritium, the two heavy isotopes of hydrogen. Other isotope mixes would
have to get much hotter — which is why “DT” is still the fuel of
choice in most reactor designs.

But whatever the fuel, the quest to reach fusion temperatures
generally comes down to a race between researchers’ efforts to pump
in energy with an external source such as microwaves, or high-energy
beams of neutral atoms, and plasma ions’ attempts to radiate that
energy away as fast as they receive it.

The ultimate goal is to get the plasma past the temperature of
“ignition,” which is when fusion reactions will start to generate
enough internal energy to make up for that radiating away of energy
— and power a city or two besides.

But this just leads to the second challenge: Once the fire is lit, any
practical reactor will have to keep it lit — as in, confine these
superheated nuclei so that they’re close enough to maintain a
reasonable rate of collisions for long enough to produce a useful flow
of power.

In most reactors, this means protecting the plasma inside an airtight
chamber, since stray air molecules would cool down the plasma and
quench the reaction. But it also means holding the plasma away from
the chamber walls, which are so much colder than the plasma that the
slightest touch will also kill the reaction. The problem is, if you
try to hold the plasma away from the walls with a non-physical
barrier, such as a strong magnetic field, the flow of ions will
quickly get distorted and rendered useless by currents and fields
within the plasma.

Unless, that is, you’ve shaped the field with a great deal of care
and cleverness — which is why the various confinement schemes
account for some of the most dramatic differences between reactor
designs.

Finally, practical reactors will have to include some way of
extracting the fusion energy and turning it into a steady flow of
electricity. Although there has never been any shortage of ideas for
this last challenge, the details depend critically on which fuel mix
the reactor uses.

With deuterium-tritium fuel, for example, the reaction produces most
of its energy in the form of high-speed particles called neutrons,
which can’t be confined with a magnetic field because they don’t
have a charge. This lack of an electric charge allows the neutrons to
fly not only through the magnetic fields but also through the reactor
walls. So the plasma chamber will have to be surrounded by a
“blanket”: a thick layer of some heavy material like lead or steel
that will absorb the neutrons and turn their energy into heat. The
heat can then be used to boil water and generate electricity via the
same kind of steam turbines used in conventional power plants.

Many DT reactor designs also call for including some lithium in the
blanket material, so that the neutrons will react with that element to
produce new tritium nuclei. This step is critical: Since each DT
fusion event consumes one tritium nucleus, and since this isotope is
radioactive and doesn’t exist in nature, the reactor would soon run
out of fuel if it didn’t exploit this opportunity to replenish it.

The complexities of DT fuel are cumbersome enough that some of the
more audacious fusion startups have opted for alternative fuel mixes.
Binderbauer’s TAE, for example, is aiming for what many consider the
ultimate fusion fuel: a mix of protons and boron-11. Not only are both
ingredients stable, nontoxic and abundant, their sole reaction product
is a trio of positively charged helium-4 nuclei whose energy is easily
captured with magnetic fields, with no need for a blanket.

But alternative fuels present different challenges, such as the fact
that TAE will have to get its proton-boron-11 mix to up fusion
temperatures of at least a billion degrees Celsius, roughly 10 times
higher than the DT threshold.

A plasma donut

The basics of these three challenges — igniting the plasma,
sustaining the reaction, and harvesting the energy — were clear from
the earliest days of fusion energy research. And by the 1950s,
innovators in the field had begun to come up with any number of
schemes for solving them — most of which fell by the wayside after
1968, when Soviet physicists went public with a design they called the
tokamak.

Like several of the earlier reactor concepts, tokamaks featured a
plasma chamber something like a hollow donut — a shape that allowed
the ions to circulate endlessly without hitting anything — and
controlled the plasma ions with magnetic fields generated by
current-carrying coils wrapped around the outside of the donut.

But tokamaks also featured a new set of coils that caused an electric
current to go looping around and around the donut right through the
plasma, like a circular lightning bolt. This current gave the magnetic
fields a subtle twist that went a surprisingly long way toward
stabilizing the plasma. And while the first of these machines still
couldn’t get anywhere close to the temperatures and confinement
times a power reactor would need, the results were so much better than
anything seen before that the fusion world pretty much switched to
tokamaks _en masse_.

Since then, more than 200 tokamaks of various designs have been built
worldwide, and physicists have learned so much about tokamak plasmas
that they can confidently predict the performance of future machines.
That confidence is why an international consortium of funding agencies
has been willing to commit more than $20 billion to build ITER (Latin
for “the way”): a tokamak scaled up to the size of a 10-story
building. Under construction in southern France since 2010, ITER is
expected to start experiments with deuterium-tritium fuel in 2035. And
when it does, physicists are quite sure that ITER will be able to hold
and study burning fusion plasmas for minutes at a time, providing a
unique trove of data that will hopefully be useful in the construction
of power reactors.

But ITER was also designed as a research machine with a lot more
instrumentation and versatility than a working power reactor would
ever need — which is why two of today’s best-funded fusion
startups are racing to develop tokamak reactors that would be a lot
smaller, simpler and cheaper.

First out of the gate was Tokamak Energy
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company has received some $250 million in venture capital over the
years to develop a reactor based on “spherical tokamaks” — a
particularly compact variation that looks more like a cored apple than
a donut.

But coming up fast is Commonwealth Fusion Systems
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even launched until 2018. Although Commonwealth’s tokamak design
uses a more conventional donut configuration, access to MIT’s
extensive fundraising network has already brought the company nearly
$2 billion.

Both firms are among the first to generate their magnetic fields with
cables made of high-temperature superconductors (HTS). Discovered in
the 1980s but only recently available in cable form, these materials
can carry an electrical current without resistance even at a
relatively torrid 77 Kelvins, or -196 degrees Celsius, warm enough to
be achieved with liquid nitrogen or helium gas. This makes HTS cables
much easier and cheaper to cool than the ones that ITER will use,
since those will be made of conventional superconductors that need to
be bathed in liquid helium at 4 Kelvins.

But more than that, HTS cables can generate much stronger magnetic
fields in a much smaller space than their low-temperature counterparts
— which means that both companies have been able to shrink their
power plant designs to a fraction of the size of ITER.

As dominant as tokamaks have been, however, most of today’s fusion
startups are _not _using that design. They’re reviving older
alternatives that could be smaller, simpler and cheaper than tokamaks,
if someone could make them work.

Plasma vortices

Prime examples of these revived designs are fusion reactors based on
smoke-ring-like plasma vortices known as the field-reversed
configuration (FRC). Resembling a fat, hollow cigar that spins on its
axis like a gyroscope, an FRC vortex holds itself together with its
own internal currents and magnetic fields — which means there’s no
need for an FRC reactor to keep its ions endlessly circulating around
a donut-shaped plasma chamber. In principle, at least, the vortex will
happily stay put inside a straight cylindrical chamber, requiring only
a light-touch external field to hold it steady. This means that an
FRC-based reactor could ditch most of those pricey, power-hungry
external field coils, making it smaller, simpler and cheaper than a
tokamak or almost anything else.

In practice, unfortunately, the first experiments with these whirling
plasma cigars back in the 1960s found that they always seemed to
tumble out of control within a few hundred microseconds, which is why
the approach was mostly pushed aside in the tokamak era.

Yet the basic simplicity of an FRC reactor never fully lost its
appeal. Nor did the fact that FRCs could potentially be driven to
extreme plasma temperatures without flying apart — which is why TAE
chose the FRC approach in 1998, when the company started on its quest
to exploit the 1-billion-degree proton-boron-11 reaction.

Binderbauer and his TAE cofounder, the late physicist Norman Rostoker,
had come up with a scheme
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stabilize and sustain the FRC vortex indefinitely: Just fire in beams
of fresh fuel along the vortex’s outer edges to keep the plasma hot
and the spin rate high.

It worked. By the mid-2010s, the TAE team had shown that those
particle beams coming in from the side would, indeed, keep the FRC
spinning and stable for as long as the beam injectors had power —
just under 10 milliseconds with the lab’s stored-energy supply, but
as long as they want (presumably) once they can siphon a bit of spare
energy from a proton-boron-11-burning reactor. And by 2022, they had
shown that their FRCs could retain that stability well above 70
million degrees C.

With the planned 2025 completion of its next machine, the
30-meter-long Copernicus, TAE is hoping to actually reach burn
conditions above 100 million degrees (albeit using plain hydrogen as a
stand-in). This milestone should give the TAE team essential data for
designing their DaVinci machine: a reactor prototype that will (they
hope) start feeding p-B11-generated electricity into the grid by the
early 2030s.

Plasma in a can

Meanwhile, General Fusion [[link removed]] of Vancouver,
Canada, is partnering with the UK Atomic Energy Authority to construct
a demonstration reactor for perhaps the strangest concept of them all,
a 21st-century revival of magnetized target fusion. This 1970s-era
concept amounts to firing a plasma vortex into a metal can, then
crushing the can. Do that fast enough and the trapped plasma will be
compressed and heated to fusion conditions. Do it often enough and a
more or less continuous string of fusion energy pulses back out, and
you’ll have a power reactor.

In General Fusion’s current concept, the metal can will be replaced
by a molten lead-lithium mix that’s held by centrifugal force
against the sides of a cylindrical container spinning at 400 RPM. At
the start of each reactor cycle, a downward-pointing plasma gun will
inject a vortex of ionized deuterium-tritium fuel — the
“magnetized target” — which will briefly turn the whirling,
metal-lined container into a miniature spherical tokamak. Next, a
forest of compressed-air pistons arrayed around the container’s
outside will push the lead-lithium mix into the vortex, crushing it
from a diameter of three meters down to 30 centimeters within about
five milliseconds, and raising the deuterium-tritium to fusion
temperatures.

The resulting blast will then strike the molten lead-lithium mix,
pushing it back out to the rotating cylinder walls and resetting the
system for the next cycle — which will start about a second later.
Meanwhile, on a much slower timescale, pumps will steadily circulate
the molten metal to the outside so that heat exchangers can harvest
the fusion energy it’s absorbed, and other systems can scavenge the
tritium generated from neutron-lithium interactions.

All these moving parts require some intricate choreography, but if
everything works the way the simulations suggest, the company hopes to
build a full-scale, deuterium-tritium-burning power plant by the
2030s.

It’s anybody’s guess when (or if) the particular reactor concepts
mentioned here will result in real commercial power plants — or
whether the first to market will be one of the many alternative
reactor designs being developed by the other 40-plus fusion firms.

But then, few if any of these firms see the quest for fusion power as
either a horse race or a zero-sum game. Many of them have described
their rivalries as fierce, but basically friendly — mainly because,
in a world that’s desperate for any form of carbon-free energy,
there’s plenty of room for multiple fusion reactor types to be a
commercial success.

“I will say my idea is better than their idea. But if you ask them,
they will probably tell you that their idea is better than my idea,”
says physicist Michel Laberge, General Fusion’s founder and chief
scientist. “Most of these guys are serious researchers, and
there’s no fundamental flaw in their schemes.” The actual chance
of success, he says, is improved by having more possibilities. “And
we do need fusion on this planet, badly.”

_Editor’s note: This story was changed on November 2, 2023, to
correct the amount of compression that General Fusion is aiming for in
its reactor; it is 30 centimeters, not 10. The text was also changed
to clarify that the blast of energy leads to the resetting of the
magnetized target reactor._

10.1146/knowable-110123-1

_M. MITCHELL WALDROP is a freelance journalist based in Washington,
DC. His previous stories for Knowable Magazine have covered topics
such as green concrete
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cosmology
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and the perils of cash bail
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_This article originally appeared in Knowable Magazine
[[link removed]], an independent journalistic
endeavor from Annual Reviews. Sign up for the newsletter
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