“On January 12th Oxfordshire County
Council, in England, gave the go-ahead for a new building near the village of
Culham. The applicant, General Fusion, is a Canadian firm, and the edifice will
house its Fusion Demonstration Program, a seven-tenths-scale prototype of a
commercial nuclear-fusion reactor. The firm picked Culham because it is the
site of JET, the Joint European Torus, an experimental fusion reactor opened in
1983 by a consortium of governments. That means there is plenty of local talent
to be recruited.
General Fusion is not alone. On
February 10th Tokamak Energy, a British firm, announced plans for a
quarter-scale prototype, the ST80, also at Culham. And in 2024 they will be
joined there by Machine 4, a pre-commercial demonstrator from another British
outfit, First Light Fusion.
Meanwhile, across the ocean in
Massachusetts, Commonwealth Fusion Systems is already building, in Devens, a
town west of Boston, a half-scale prototype called SPARC. On the other side of
America, in Everett, Washington, Helion Energy is likewise constructing a
prototype called Polaris. And in Foothill Ranch, a suburb of Los Angeles, TAE
Technologies is similarly working on a machine it calls Copernicus.
These six firms, and 36 others
identified by the Fusion Industries Association (FIA), a trade body for this
incipient sector, are hoping to ride the green-energy wave to a carbon-free
future. They think they can succeed, where others failed, in taking fusion from
the lab to the grid—and do so with machines far smaller and cheaper than the
latest intergovernmental behemoth, ITER, now being built in the south of France
at a cost estimated by America's energy department to be $65bn. In some cases
that optimism is based on the use of technologies and materials not available
in the past; in others, on simpler designs.
Many of those on the FIA's rapidly
growing list are tiddlers. But General Fusion, Tokamak, Commonwealth, Helion
and TAE have all had investments in excess of $250m. TAE, indeed, has received
$1.2bn and Commonwealth $2bn. First Light is getting by on about $100m. But it
uses a simpler approach than the others ("fewer screws", as Bart
Markus, its chairman, puts it), so has less immediate need for cash.
All these firms have similar
timetables. They are, or shortly will be, building what they hope are
penultimate prototypes. Using these they plan, during the mid-to-late 2020s, to
iron out remaining kinks in their processes. The machines after that, all
agree, will be proper, if experimental, power stations—mostly rated between
200MW and 400MW—able to supply electricity to the grid. For most firms the
aspiration is to have these ready in the early 2030s.
Un peu d'histoire
The idea of harnessing the process
that powers the sun goes back almost as far as the discovery, in the 1920s and
1930s, of what that process is—namely the fusion of protons, the nuclei of
hydrogen atoms, to form helium nuclei (4He), also known as alpha particles.
This reaction yields something less than the sum of its parts, for an alpha
particle is lighter than four free protons. But the missing mass has not
disappeared; it has merely been transformed. As per Einstein's equation, E=mc2,
it has been converted into energy, in the form of heat.
This sounded technologically
promising. But it was soon apparent that doing it the way the sun does is a
non-starter.
Persuading nuclei to fuse requires
heat, pressure or both. The pressure reduces the space between the nuclei,
encouraging them to meet. The heat keeps them travelling fast enough that when
they do meet, they can overcome their mutual electrostatic repulsion, known as
the Coulomb barrier, and thus allow a phenomenon called the strong nuclear
force, which works only at short range, to take over. The strong force holds
protons and neutrons together to form nuclei, so once the Coulomb barrier is
breached, a new and larger nucleus quickly forms.
The temperature at which solar
fusion occurs, though high (15.5m°C), is well within engineers' reach.
Experimental reactors can manage 100m°C and there are hopes to go higher still.
But the pressure (250bn atmospheres) eludes them. Moreover, solar fusion's raw
material is recalcitrant. The first step on the journey to helium—fusing two
individual protons together to form a heavy isotope of hydrogen called
deuterium (a proton and a neutron)—is reckoned to take, on average, 9bn years.
What engineers propose is thus a
simulacrum of the solar reaction. The usual approach—that taken by General
Fusion, Tokamak Energy, Commonwealth Fusion and First Light, as well as
government projects like JET and ITER—is to start with deuterium and fuse it
with a yet-heavier (and radioactive) form of hydrogen called tritium (a proton
and two neutrons) to form 4He and a neutron. (Fusing deuterium nuclei directly,
though sometimes done on test runs, is only a thousandth as efficient.)
Ignition sequence start
The power released emerges as kinetic
energy of the reaction products, with 80% ending up in the neutron. The
proposal is to capture this as heat by intercepting the neutrons in an
absorptive blanket and then use it to raise steam to generate electricity.
Reactors will also, the idea goes, be able to make the tritium they need (for
tritium does not occur naturally) by including in the blanket some 6Li, an
isotope of lithium which reacts with neutrons to generate tritium and an alpha
particle. Deuterium is not a problem. One in every 3,200 water molecules
contains it.
Not everyone, though, is taking the
deuterium-tritium route. Helion and TAE are instead proposing versions of what
is known as aneutronic fusion.
Helion's suggestion is to start with
3He (two protons and a neutron), a light isotope of helium which is an
intermediate stage in the solar reaction. But instead of fusing two of these,
as happens in the sun (yielding 4He and two protons), it fuses them one at a
time with deuterium nuclei, to produce 4He and a proton. The 3He would be
replenished by tweaking conditions to promote a side reaction that makes it
from two deuteriums.
TAE proposes something yet more
intriguing. Its fuels are boron (five protons and six neutrons) and ordinary
hydrogen, both plentiful. When these fuse, the result breaks into three alpha
particles. Indeed, TAE originally stood for Tri-Alpha Energy. The problem is
that to work satisfactorily a boron-proton fusion reactor will have to generate
not a mere 100m°C but 1bn°C.
Even with deuterium-tritium fusion
there are many ways to encourage nuclear get-togethers. The aim is to create
conditions that match what is known as the Lawson criterion, after John Lawson,
who promulgated it in the 1950s. He realised that achieving power generation
means juggling temperature, density and the time for which the reaction can be
prolonged. This trinity gives rise to a value called the triple product which,
if high enough, results in "ignition", in which the reaction
generates enough energy to sustain itself.
The most common reactor design, a
tokamak, majors on temperature. It was invented in Russia in 1958, and pushed
aside two previous approaches, Z-pinching and stellarators, because it appeared
to offer better control over the deuterium-tritium plasma used as fuel. (A
plasma is a gas-like fluid in which atomic nuclei and electrons are separated.)
Its reaction chamber is a hollow torus which contains the plasma. This torus
has a set of toroidal electromagnetic coils wrapped around it, paired poloidal
coils above and below it, and a solenoid running through the middle (see panel
1).
A plasma's particles being
electrically charged, a tokamak's magnets can, in combination, control their
behaviour—containing and heating them to the point at which the nuclei will
fuse. The plasma must, though, be kept away from the reaction vessel's wall. If
it makes contact it will cool instantly and fusion will cease. Stellarators,
though also toroidal, required a more complex (and hard to control) arrangement
of magnets. Z-pinching used an electric current through the plasma to generate
a self-constraining magnetic field.
A conventional tokamak's torus
resembles a doughnut, but Tokamak Energy's design (the interior of the current
version is pictured, plasma-filled, above) looks like a cored apple. This was
calculated, in the 1980s, to be more efficient than a doughnut. The calculation
was done by Alan Sykes, who then worked on JET and who is one of the company's
founders.
The efficiency and compactness of Dr
Sykes's spherical layout have been greatly enhanced by using high-temperature
superconductor tapes for the coils' windings. ("High temperature"
means they operate below the boiling point of nitrogen, -196°C, rather than
that of liquid helium, -269°C). These offer no resistance to the passage of
electricity, and thus consume little power. Such tapes are now available
commercially from several suppliers.
Commonwealth Fusion also uses
high-temperature superconductors in its magnets. And, though its tokamak will
be a conventional doughnut rather than a cored apple, it, too, will be compact.
At least as important as the magnets
is the other improvement both firms have brought to tokamaks: plasma control.
Tokamak Energy's system, for example, is run from a control room that would not
disgrace the set of a James Bond film. The software involved is able to track
the plasma's behaviour so rapidly that it can tweak conditions every 100
microseconds, keeping it away from the reactor walls. Come the day a commercial
version is built, it will thus be able to operate continuously.
The pressure's on
General Fusion, by contrast, plans
to match the Lawson criteria using pressure, as well as temperature, in an
approach it calls magnetised target fusion. As Michel Laberge, its boss,
explains, the fuel is still a plasma, but the reaction vessel's lining is a
rotating cylinder of liquid metal—lithium in the prototype, and a mix of
lithium and lead in the putative commercial model.
Once the fuel has been injected into
the cavity inside this cylinder, pneumatic pistons will push the metal inward
(see panel 2), collapsing the cavity into a small sphere. That compresses and
heats the plasma to the point where it starts to fuse. If this system can
achieve ignition, the heat generated will be absorbed by the liquid
lithium—whence it can be extracted to raise steam. Also, some of the neutrons
will convert 6Li in the lining into tritium.
General Fusion, too, relies on
sophisticated software to control the pistons and so shape the plasma
appropriately. But Dr Laberge believes that doing without electromagnets has
simplified the design and removed potential points of failure.
TAE and Helion, meanwhile, both use
so-called field-reversed configurations (see panel 3) to confine their plasma.
Their reaction chambers resemble hollow barbells, but with a third
"weight" in the middle. The ends generate spinning plasma toroids
that are then fired at each other by magnetic fields. Their collision triggers
fusion. Again, this would not be possible without sophisticated control
systems.
Both Helion and TAE plan to generate
electricity directly, rather than raising steam to run a generator. Helion will
pluck it from the interaction between the magnetic field of the merged plasma
toroids and the external field. How TAE intends to do it is undisclosed, though
it says several approaches are being considered.
Several members of the FIA list's
"tail" of 36 are pushing the edges of the technological envelope in
other ways. Some are exploring yet further fuel cycles—reacting deuterium
nuclei to generate power, rather than just to test apparatus, for instance, or
fusing lithium with protons. Others are sticking to the deuterium-tritium
route, but examining different types of reactor.
Zap Energy, in Seattle, for example,
is using enhanced plasma control to revive Z-pinching. And several firms,
including Princeton Stellarators and Type One Energy Group, both in America,
and Renaissance Fusion, in France, are dusting off stellarators—again in the
belief that modern computing can deal with their quirks.
But the most immediate competition
for tokamaks, field-reversed configurations and General Fusion's hydraulic
design is an approach called inertial fusion. In this the fuel starts off in a
small capsule and the Coulomb barrier is overcome by applying an external shock.
At the moment, the leader of the
inertial-fusion pack is First Light. Its engineers apply the shock in the form
of a projectile fired by electromagnetic acceleration (see panel 4). The target
is a fuel capsule inside a cube-shaped amplifier. The amplifier boosts the
impact's shock wave (to 80km per second, it is hoped, in the case of Machine 4)
and refracts it so that it converges on the capsule simultaneously from all
directions. This will implode the fuel, achieving an ignition-level
triple-product.
First Light's approach is, however,
unusual. Most other proponents of inertial fusion plan to deliver the shock
with lasers. These include Focused Energy, of Austin, Texas; Marvel Fusion, of
Munich; and Xcimer Energy, of Redwood City, California. They are all following
a path pioneered by the National Ignition Facility (NIF), an American
government project to study the physics of atomic weapons.
Green grow my dollars-o
In December 2022 the NIF caused a
flutter by announcing it had reached ignition. But the energy released was less
than 1% of that expended, meaning it was nowhere near another sine qua non of
commercial fusion, Q>1. Q is the ratio of the energy coming out of a machine
to that going in. Different versions of Q have different definitions of "out"
and "in". But the one most pertinent to commerce is "plug to
plug"—the electricity drawn grid to run the whole caboodle versus the
energy delivered to back the grid. Focused, Marvel and Xcimer hope to match
that definition of Q>1.
It all, then, sounds very bubbly and
exciting. But bubbly—or, rather, a bubble—is precisely what some critics worry
it is.
First, many technological challenges
remain. Dr Markus's observation about the number of screws is shrewd. In
particular, his firm (and also General Fusion) have dealt with the need for
complex magnetic plasma-control systems by avoiding them.
Finance is also a consideration.
Fusion, like other areas of technology, has benefited from the recent period of
cheap money. The end of that may garrotte much of the tail. But the pack
leaders have stocked up with cash while the going was good. This should help
them to hang on until the moneymen and women can judge them on results, rather
than aspirations.
Nor should the arrival date of the
early 2030s be seen as set in stone. This is an industry with a record of
moving deadlines, and a British government project to build a spherical tokamak
called STEP has a more cautious target to be ready in 2040.
Moreover, even if a practical
machine does emerge, it will have to find its niche. The story told by the
companies is of supplying "baseline" power in support of intermittent
sources such as solar and wind—and doing so in a way that avoids the widespread
public fear of an otherwise-obvious alternative, nuclear fission. That might
work, but it will also have to be cheaper than other alternatives, such as
grid-scale energy-storage systems.
For fusion's boosters, though, there
is at least one good reason for hope. This is the sheer variety of approaches.
It would take only one of these to come good for the field to be transformed
from chimera to reality. And if that happened it could itself end up
transforming the energy landscape.” [1]
