NEWS FEATURE

Small-scalefusiontacklesenergy,spaceapplicationsEfforts are underway to exploit a strategy that could generate fusion with relative ease.

M. Mitchell Waldrop, Science Writer

On July 14, 2015, nine years and five billion kilometersafter liftoff, NASA’s New Horizons spacecraft passedthe dwarf planet Pluto and its outsized moon Charonat almost 14 kilometers per second—roughly 20 timesfaster than a rifle bullet.

The images and data that New Horizons pains-takingly radioed back to Earth in the weeks thatfollowed revealed a pair of worlds that were far morevaried and geologically active than anyone hadthought possible. The revelations were breathtak-ing—and yet tinged with melancholy, because NewHorizons was almost certain to be both the first andthe last spacecraft to visit this fascinating world inour lifetimes.

Unless, that is, Samuel Cohen succeeds with theoffbeat fusion reactor that he’s developing at thePrinceton Plasma Physics Laboratory in New Jersey.

Cohen’s current prototype is a clear plastic cylinderthat sits in the middle of his lab amidst a dense mass ofcables, magnets, and power supplies, emitting a violetpulse of light every two seconds like a two-meter-longstrobe light. “We’re only using hydrogen right now,”

Cohen explains, referring to the ionized plasma insidethe tube that’s emitting the flashes. So there are noactual fusion reactions taking place; that’s not in hisresearch plan until the mid-2020s, when he hopes tobe working with a more advanced prototype at leastthree times larger than this one.

If that hope pans out and his future machine doesindeed produce more greenhouse gas–free fusion en-ergy than it consumes, Cohen and his team will havebeaten the standard timetable for fusion by about adecade—using a reactor that’s just a tiny fraction ofthe size and cost of the huge, donut-shaped “tokamak”devices that have long devoured most of the researchfunding in this field. The flagship of this tokamak ap-proach, the International Thermonuclear ExperimentalReactor (ITER) now under construction in France, will betwice as large as any fusion reactor before it, will cost atleast $20 billion to build, and isn’t expected to startproducing fusion energy until the mid-2030s.

If and when Cohen does reach his fusion en-ergy milestone, he will likely have company. His deviceis just one of a family of small, alternative reactor projectsdesigned to exploit a phenomenon known as the field-reversed configuration (FRC): a dense mass of ionizedplasma that holds itself together something like a smokering and that could allow researchers to achieve fusionconditions with comparatively little effort. Among themembers of this family are some of the best-knownfusion upstarts: firms such as TAE Technologies (for-merly TriAlpha Energy) in Foothill Ranch, California,and Helion Energy in Redmond, Washington.

“There’s been a rejuvenation in that whole area” ofFRCs, says Stephen Dean, a nuclear engineer who haschampioned fusion energy for more than 50 years. “Allof the projects have good ideas, all of them are doinggood work.” But even if some or all of them do end upproducing fusion energy in the lab at some point in the2020s, he says, all of them are eventually going to haveto build a real, power-producing test reactor—some-thing that’s not likely to happen for a decade or more.

Pluto PowerThat’s why Cohen takes the long view. His goal is anultra-compact reactor that will use a fuel mix containing

Samuel Cohen and his team hope to beat the standard timetable for fusion byabout a decade using a reactor—initially for rocket propulsion—that’s a fractionof the size and cost of the huge tokamak devices. Cohen’s design takes advantageof the phenomenon of field reversed configuration (FRC), in which a dense mass ofionized plasma holds itself together. Image credit: Princeton Plasma PhysicsLaboratory.

Published under the PNAS license.

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helium-3, an isotope that yields a particularly clean formof fusion with minimal radiation risk. But the stuff isexceedingly rare, he says: “So we’re not trying to makepower for everybody.” Instead, the goal is niche usessuch as spacecraft propulsion, in which the reactorwould fire a very tenuous plasma from one end so that itfunctions as a rocket (1).

Such a direct fusion drive (DFD) would produceonly the most infinitesimal hint of acceleration, saysCohen—about like pushing an 18-wheel truck withyour fingertip. But in space, that push would havenothing to resist it. After a year or two, such a rocketcould get a 10-ton spacecraft halfway to Pluto, trav-eling well over 50 kilometers per second.

“Then you’d turn around and decelerate,” saysCohen. “And when you got to Pluto, you’d go intoorbit.” At that point, the reactor would turn off the ionrocket and convert itself into a one-megawatt elec-trical power source. “Some of that power you can useto send high-definition video back,” says Cohen.“And some of it you can beam down to a lander thatyou’ve placed on the surface, so it could drive aroundand drill holes in the ice.”

The same type of DFD rockets could also beused to explore the moons of Jupiter and Saturn,says Cohen, or the icy bodies of the Kuiper Beltbeyond Pluto, or anywhere else in the outer solarsystem.

Plasma ProblemOf course, there’s a reality check, says Dean: “If youwant to make a fusion exhaust system, you still have tobe able to make the fusion plasma.” It’s a trick thatneither Cohen nor anyone else has yet managed.Researchers have been trying to harness fusion powersince the 1920s and 1930s, when they first realizedthat stars like the sun get their energy from thermonuclearreactions at their core. And yet, as the many delays and

cost overruns on ITER have made clear, success is stillyears away at best.

Still, old hands like Cohen know the pitfalls of fu-sion research as well as anyone. Until the late 1990s,his professional life revolved around ITER, which issupposed to be the ultimate expression of the oldestand most promising approach to fusion energy:magnetic confinement. In theory, this is just a matterof ionizing an appropriate mix of light isotopes, trap-ping them in a magnetic field, and heating them tomillions of degrees while simultaneously squeezingthem to densities approximating the sun’s core. Theisotopes will then start fusing into larger nuclei whilereleasing vast amounts of energy.

In practice, though, hot, ionized plasma doesn’tlike being confined by a magnetic field; it twists andtries to escape like a living thing. Thus the appeal ofthe tokamak design, which was a major break-through when Soviet physicists introduced it in the1960s. Thanks to strong magnetic fields that guidethe ionized isotopes around and around its donut-shaped vacuum chamber, a tokamak could keepthe plasma under control better than almost any-thing else at the time. And thus the funding agen-cies’ willingness to keep sinking billions of dollarsinto ITER: a gargantuan tokamak whose 23,000-tonweight will be three times that of the Eiffel Tower,and whose 29 by 29-meter vacuum chamber will beas tall as a seven-story building. This is the scalethat a tokamak will need to achieve the elusive goalof “break-even,” in which the plasma producesmore fusion energy than the machine requires tooperate.

Except that to Cohen and an increasing number ofother fusion researchers, ITER has laid bare the toka-mak’s many drawbacks as a practical power source.These start with the facility’s size, cost, and complex-ity, which are so far beyond what power companiesare willing to accept that they have all but given up on

The International Thermonuclear Experimental Reactor (ITER), now under construction in France, will be twice as largeas any fusion reactor before it and will cost at least $20 billion to build. To keep the fusion plasma under control, thetokamak design uses strong magnetic fields to guide ionized isotopes around a donut-shaped vacuum chamber.(Left) Image credit: Wikimedia Commons/Oak Ridge National Laboratory. (Right) Image credit: Science Source/ITER.

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fusion, says Dean: “I can’t even talk to anyone in theutilities who knows what a tokamak is anymore.”

And then there’s the neutron problem. Thephysics of tokamaks limits them to burning a mix ofthe hydrogen isotopes deuterium and tritium. Thisfuel is by far the easiest to ignite, requiring compar-atively low plasma temperatures of about a hundredmillion degrees Kelvin. But when the two nuclei fuseto form a helium-4 nucleus (two protons plus twoneutrons), they eject the leftover neutron at highenergy. And because that particle is electricallyneutral and can’t be controlled with magnetic fields,it ends up smashing into the tokamak’s inner wallsand wreaking havoc with their structural integrity. Sothe walls will have to be replaced perhaps once peryear—a maintenance burden that no power com-pany wants to shoulder.

A Different ConfigurationBy the late 1990s such hurdles were spurring Cohenand others to take a fresh look at FRCs, which hadbeen discovered in the 1960s.

The key advantage was that an FRC doesn’t keepits plasma in line by brute force, the way a tokamakdoes. Instead, the FRC plasma is self-organizing. Thatis, the magnetic fields that hold it together are mostlygenerated by currents flowing through the plasmaitself, rather than in external coils. This self-organizingproperty can be found in other plasma structures,which have names such as “spheromak” and “denseplasma focus,” says Cohen. But all else being equal,FRC plasmas are much hotter and denser than theothers.

Once it’s set up, an FRC actually looks less like asmoke ring than an elongated American football, ormaybe a short cigar. The “field-reversed” namecomes from the way magnetic fields curve around thefootball’s outside and then loop backward through itslong axis. This structure tends to dissipate in less thana millisecond, unfortunately—one big reason why only

a handful of researchers stuck with the FRCs aftertokamaks came along. But the appeal remained: Finda way to stabilize the FRC, and the reactor wouldn’thave to be much more than a cylindrical vacuumchamber with a comparatively mild magnetic fieldrunning down the midline to hold the plasma footballin place.

Self-organization also should make it compara-tively easy for the dense, hot plasma inside the FRC toreach the threshold required for fusion. And not justdeuterium-tritium fusion, either: FRCs could poten-tially reach the much higher temperatures required toburn aneutronic fuels such as deuterium-helium–3 orproton-boron–11. These reactions emit most of theirfusion energy in the form of charged particles such asprotons or helium-4 nuclei, which—unlike neutrons—can be captured and controlled with magnetic fields.This would make it much easier to extract energy fromthe fusion products before they can damage the re-actor walls, and would allow the reactor to get by withminimal shielding.

So in principle, says Cohen, FRC-based reactorscould solve the tokamak’s size, complexity, and neutronproblems at a stroke. But to make that work in practice,he says, researchers have had tomake a series of criticaldesign choices: how to form, stabilize, and control theFRC, how to heat it, what kind of fusion fuel to use, andso on. “You multiply all those options,” he says, “youget roughly 80 different potential FRCs.”

TAE has been working on one such option since1998, when it was founded with the goal of fusingprotons with boron-11 nuclei. This pB11 reaction isin some ways the ultimate in neutron-free fusion: Itsoutput is just a triplet of positively charged helium-4 nuclei, which are commonly known as alpha par-ticles (thus the company’s original name, TriAlpha.) Butthe reaction also has some significant downsides. Forexample, its multibillion-degree threshold for fusion isabout 20 or 30 times higher than the temperatures re-quired for the deuterium-tritium reaction that ITER willuse. Also, it has about half as much energy yield perfusion event.

So to make pB11 work, TriAlpha’s design has to becorrespondingly ambitious (2). The idea is to cap thereactor on each end with two electromagnetic can-nons pointed barrel to barrel. To start things off, eachcannon fires a ring of plasma into a central chamber,where the rings merge into a single, furiously spinningFRC. From there, a beam of neutral atoms coming infrom the side will simultaneously heat the FRC, supplyit with fresh pB11 fuel, and stabilize it by keeping thespin rate up.

It took TAE until 2012 to demonstrate this wholeprocess in a prototype machine (albeit with a nonfusinghydrogen plasma), says the company’s CEO, MichlBinderbauer. “We showed these beautiful experimentswhere, if you start with the standard FRC and you don’tdo anything, it dies,” he says. “But if you start injectingparticles, you slow down the decay and expand howlong it lives.” Since then, says Binderbauer, the com-pany has shown that this process can sustain the FRCindefinitely—or at least, for the five or 10milliseconds it

TAE Technologies is designing a small fusion reactor capped on each end withelectromagnetic cannons pointed barrel to barrel. To start the reaction, eachcannon fires a ring of plasma into a central chamber, where the rings merge into asingle, furiously spinning FRC. A beam of neutral atoms coming in from the sidewill simultaneously heat the FRC, supply it with fresh fuel, and stabilize it bymaintaining the spin rate. Image credit: TAE Technologies.

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takes the 25-megawatt beam to exhaust the energythat researchers are able to store for each shot.

In a working reactor, of course, that beam powerwould come from the fusion reaction itself, so that thebeam and the FRC could keep going as long as theresearchers want. That’s a milestone TAE hopes tomeet with a pB11-burning prototype well before theend of the 2020s, says Binderbauer. This machine willbe roughly the size of four double-decker bussesparked end to end, he adds—not small, but still just afraction of the size of ITER.

In Bellevue, Washington, meanwhile, another FRC-based reactor is under development at Helion Energy,which was founded by University of Washington re-searchers in 2013. Company officials are not discus-sing their plans publicly at the moment, but they havebeen relatively open about their approach via theirwebsite and publications.

Helion’s reactor, like TAE’s, will be a linear tubethat uses twin plasma guns to form a stationary FRC inthe middle. But instead of trying to sustain the FRC,the Helion device will crush it with an ultrastrongmagnetic field until the plasma becomes denseenough and hot enough to fuse. The resulting burstof thermonuclear energy will then cause the ball ofplasma to explode outward again, pushing backagainst the magnetic field and allowing the system toharvest that energy. This cycle will then repeat onceper second, generating a steady average power out-put in much the same way that gasoline explosions doin an internal combustion engine.

The Helion reactor will also differ from TAE’s in itschoice of fuel. Instead of using pB11, it will burndeuterium and helium-3—an isotope often called a“helion.” This reaction requires a temperature ofseveral hundred million degrees, intermediate be-tween deuterium-tritium and pB11. But it, too, isaneutronic: the final products are two charged parti-cles, an alpha and a proton.

Or rather, this fuel is almost aneutronic: It’s impossibleto keep the deuterium nuclei in the fuel from reactingwith each other and producing at least some neutrons.But those neutrons are low energy and comparativelyeasy to shield against. And for Helion, the deuterium-deuterium side-reactions are a plus: the products arean almost equal mix of a neutron plus helium-3, and aproton plus tritium—a radioactive isotope that will decayinto helium-3 with a half-life of 12.3 years. So in principle,Helion’s reactor can make its own helium-3 fuel, which isotherwise available only in trace amounts extracted fromnatural gas fields, or produced as a byproduct in Cana-dian CANDU fission reactors.

Space ReactorCohen, for his part, has been pursuing his PrincetonField Reversed Configuration (PFRC) design since2002, with a strong emphasis on simplicity and com-pactness (3). The cylindrical plastic vacuum chamberof his current device, PFRC-2, is only 88 centimeterslong, with not a plasma cannon or neutral beam in-jector in sight. Instead, the FRC is generated via atechnique first explored by Austrian and Australian

physicists and then refined by Cohen himself. Hepoints to four rectangular copper coils that surroundthe middle of the tube: one each on its front, back,top, and bottom. Each rectangle, in turn, is dividedinto two smaller rectangles. The idea, says Cohen, is todrive oscillating currents through these coils in a waythat sets up a rotating magnetic field inside the tube: aloop of flux that whirls through the plasma like a flippedcoin and drags the plasma particles around and aroundthe waist of the cylinder. In the process, he says, “thefields create, stabilize, and heat the FRC”—all in asingle deft maneuver.

Indeed, that’s what Cohen routinely demonstrates inhis lab: Every two seconds, the hydrogen plasma insideis whipped into an FRC, causing a flash. Each flash lastsfor only about eight milliseconds, says Cohen, mainlybecause longer pulses risk melting the cables thatsupply the antennas with power. “So for the next ma-chine,” he says, “we’ve got to make better cables”—which should keep the FRCs going indefinitely.

In the meantime, Cohen and his group are workingon the main goal of PFRC-2, which is to improve theantennas’ ability to heat the plasma. This is crucial,notes Binderbauer: “I know Sam well, we root foreach other.” But compared with the temperatures

that TAE is already working with, he says, “his plas-mas are cold.” Also, he says, it remains to be seen howwell the rotating magnetic field approach will work in afull-scale reactor. “I’m not trying to say that it can’t bedone,” says Binderbauer, “but I think those are someof the things that they’re going to have to address.”

Still, Cohen remains confident. Sometime in thenext few years, if things progress as planned, heand his group will replace this machine with PFRC-3: a device twice as large that will hopefully allowthem to achieve FRCs lasting as long as 10 seconds,with plasma temperatures on the order of 60 millionK. And a few years after that, the plan calls formoving up to PFRC-4, an even larger machinedesigned to use live fusion fuel at temperatures of600 million K.

That fuel will be deuterium-helium-3, which Cohencalls “the Goldilocks approach” between deuterium-tritium and pB11. Unlike Helion, however, he plans tokeep things simple and forgo any attempt to breednew helium-3. Instead, Cohen will just live with thescarcity of helium-3, and focus on niche applicationslike spacecraft propulsion—an idea that emergedabout a decade ago in discussionswithMichael Paluszek,president of a space technology company, PrincetonSatellite Systems, New Jersey, which is located just a fewmiles from Cohen’s lab.

Of course, it could be another 10 to 15 years be-fore PFRC-based reactors are working reliably enough

“If you want to do this really ambitious stuff in the 2030s,you need to be developing new technology now.”

—Michael Paluszek

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for a multi-year deep-space mission, assuming thatthey work at all. But then, much the same could besaid about any of the alternative fusion designs—or forthat matter, ITER. Taking the long view is pretty much

a requirement for fusion energy research. “If youwant to do this really ambitious stuff in the 2030s,”Paluszek says, alluding to DFD missions, “you needto be developing new technology now.”

1 S. Thomas, Fusion-enabled Pluto orbiter and lander (NASA, 2016). https://www.nasa.gov/feature/fusion-enabled-pluto-orbiter-and-lander/. Accessed September 24, 2019.

2 H. Gota et al., Formation of hot, stable, long-lived field-reversed configuration plasmas on the C-2W device. Nucl. Fusion 59, 112009(2019).

3 C. Brunkhorst, B. Berlinger, N. Ferraro, S. A. Cohen, The Princeton FRC Rotating-Magnetic-Field-Experiment RF System in 2007 IEEE22nd Symposium on Fusion Engineering, (2007), pp. 1–4.

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