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June10, 2009

Vol. 109, No. 15

Features

Firing up fusion

In a resurgence of worldwide fusion research, the Princeton Plasma Physics Lab tries to carve out a starring role

By Eric Hand ’97
Published in the June10, 2009, issue


Stewart Prager, foreground, and from left, Richard Hawryluk, Robert Goldston *77, and Masayuki Ono *78 stand in front of the NSTX fusion machine, which is hidden by hardware.
Ricardo Barros
Stewart Prager, foreground, and from left, Richard Hawryluk, Robert Goldston *77, and Masayuki Ono *78 stand in front of the NSTX fusion machine, which is hidden by hardware.

The quiet room has the airless feel of a mausoleum. Hutch Neilson, the former project manager for the National Compact Stellarator Experiment at the Princeton Plasma Physics Laboratory (PPPL), is standing in a cavernous, concrete-lined bunker at the lab. Stacked around him are the plywood boxes and plastic-wrapped remnants of what was, for him, his baby — strange-looking, perhaps even mutant, but no less dear because of it. Neilson peels the plastic from one of the 6,000-pound steel forms and admires its craftsmanship: the bizarrely sculpted surface, shaped to within 3/8ths of an inch to hold powerful magnetic coils precisely in place. The room contains $90 million of mothballed work. But the loss Neilson feels is not mainly about money. The work represents the dedication of scientists who see a future powered not by wind or water or even the rays of the sun. They see a future powered by a sun-on-Earth: fusion, the nuclear fire of stars. The stellarator — known as the NCSX — was supposed to stoke that fire within its peculiarly shaped core. Neilson touches a steel flange, checkmarked by a quality-control inspector’s felt-tip pen, and considers the decade of his life spent on the project. “One has to move on,” he says.

In May 2008, the U.S. Department of Energy canceled the stellarator, PPPL’s crown jewel, even as it had passed the halfway point in its construction. The culprit was its ballooning budget, which had swelled from $102 million to $170 million. Times were tough for most fusion scientists: The world’s great hope for fusion research, an international reactor in France called ITER — the largest and most advanced in the world — was far over its €10 billion budget, years behind schedule, and facing a major design change (see related article, on page 27). In the United States, the Department of Energy’s fiscal-year 2008 budget for fusion had been slashed dramatically, ending U.S. contributions to ITER. Meanwhile, Princeton, which manages PPPL for the Energy Department, was in the process of competing for a five-year contract renewal. Longtime director Robert Goldston *77 stepped down so that a new director could be a part of the contract competition. “It couldn’t have been a worse time to compete for a new contract,” says Princeton’s dean for research, A.J. Stewart Smith *66, who oversees the lab. “It was a perfect storm.”

Though the Energy Department is being overhauled to reflect the renewable-energy priorities of President Barack Obama’s administration, it is unclear where fusion fits within the overall vision. There are some hopeful signs: Funding for ITER was restored in fiscal-year 2009, and a $3.5-billion fusion machine that uses a different approach — powerful lasers — was dedicated in May at the Lawrence Livermore National Laboratory in California (the technology has implications for nuclear weapons, as well as energy). Still, critics have argued that fusion always seems to be a decade or two away, but never any closer to reality. Stewart Prager, who began work as PPPL’s new director Jan. 21 — less than a week after the Energy Department renewed its contract with Princeton — will have to counter this thought if he is to put PPPL back on the map as a way station en route to viable fusion. The longtime director of a fusion center at the University of Wisconsin in Madison, Prager has seen fusion budgets rise and fall — usually in sync with the price of oil. Expensive oil tends to remind bureaucrats that the world eventually will need a limitless energy source. “There is a fundamental need for an abundant, clean, and safe energy supply available to all nations,” says Prager, using a well-rehearsed line, but one that he utters no less earnestly. Fusion may not make economic sense now, and it may not yet be ready in 30 years. But eventually, he says, it has to happen. The goal of fusion power, he says, “transcends economics.”

Variations on a theme: ITER in France, the biggest fusion reactor ever attempted, has a doughnut-shaped tokamak design. PPPL hopes to play a role in its development.
ITER Organization
Variations on a theme: ITER in France, the biggest fusion reactor ever attempted, has a doughnut-shaped tokamak design. PPPL hopes to play a role in its development.


PPPL worked for a decade designing and building the stellarator, known as the NCSX, but the project was canceled last year. Stellarator plasma is believed to be less susceptible to turbulent disruptions than tokamak plasma.
Renderings courtesy Princeton Plasma Physics Laboratory
PPPL worked for a decade designing and building the stellarator, known as the NCSX, but the project was canceled last year. Stellarator plasma is believed to be less susceptible to turbulent disruptions than tokamak plasma.


PPPL’s primary experiment is the NSTX, which allows fusion to occur in smaller containers using less electrical power.
Renderings courtesy Princeton Plasma Physics Laboratory
PPPL’s primary experiment is the NSTX, which allows fusion to occur in smaller containers using less electrical power.

Since the dawn of the nuclear age, fusion has been a sort of holy grail. First came the discovery of fission, in which heavy radioactive atoms like uranium, split by neutrons, give off heat. While nuclear reactors have tamed fission into a viable energy source, the by-products are radioactive, and persistently so. Also, reactors can breed and enrich their own fission fuel — a good thing for the reactor operator, but bad for those who want to clamp down on the proliferation of bomb-grade uranium and plutonium produced in the guise of a civilian program.

Fusion, on the other hand, is pretty much as Prager describes it: abundant (the basic fuel is heavy isotopes of hydrogen, easily obtained from seawater), clean (the only radioactive ingredient is tritium, a short-lived heavy hydrogen that’s easily cleaned up), and safe (there is no chance of runaway reactions, nor can the technology be siphoned off into the production of a bomb). Plus, like solar, wind, and hydropower, it doesn’t emit carbon.

But those other renewable energy sources will require the development of a large number of available sunny desert spaces, windy plains, and steep river gorges. The basic fuel for fusion, however, is essentially limitless. In just a tablespoon of seawater there is the fusion-energy equivalent of a gallon of gasoline.

The only problem with fusion is that it is an extraordinarily tough thing to do. It requires sunlike temperatures and pressures to convince hydrogen atoms to do something awkward: fuse into helium. Stars get around this problem by being big: They are so heavy that their own gravity does all the necessary squeezing. On Earth, it’s more difficult. Fusion occurs when the atoms are in a hot plasma — a state of matter where ionized gases behave somewhat like a liquid. One can heat the plasma so the hydrogen atoms strike each other fiercely enough to fuse, but that reaction releases extra energy, agitating the atoms all the more. They move faster, try to expand, and therefore are more reluctant to fuse. The atoms need to be confined — but few materials would last long touching a 100 million-degree-Celsius plasma.

However, since plasma is charged, it can be moved and controlled at a distance by magnetic fields. Because of this, many scientists, including those at PPPL, think the best way to contain plasma is to create hollow structures ringed by magnetic coils, such as the doughnut-shaped tokamak machine. The tokamak is one particular shape and approach, but there are variations. There are stellarators, invented by Lyman Spitzer Jr. *38 (PPPL’s founder and first director) — supposedly, while he was daydreaming on a ski lift. A stellarator is like a twisted tokamak, and the NCSX is one particular incarnation (millions of slightly different twisted stellarator shapes are possible). Stellarators add a degree of complexity to the magnetic fields in return for eliminating the need for the plasma to be moving within a tokamak. A stellarator plasma is thought to be less susceptible to turbulent disruptions and more conducive to steady-state operations. But a third approach ultimately could win out: This one is similar to a tokamak, but fatter, with the central hole of the doughnut so small that the overall structure is better thought of as a sphere. Used in facilities such as PPPL’s National Spherical Torus Experiment (NSTX), spheres can maintain higher-pressure plasmas with lower magnetic fields. This means fusion can occur in smaller containers using less electrical power.

So far, tokamaks seem to be the most likely way to go, and using them, fusion scientists have made great strides, says PPPL deputy director Richard Hawryluk. PPPL’s Tokamak Fusion Test Reactor (TFTR), which shut down in 1997, produced tens of megawatts of power for seconds on end — millions of times more energy than that created by early-era machines — and neared the “break-even” point, where output energy from fusion would surpass the input energy needed to heat up the plasma. By another measure — a combination of the heat, density, and duration of a plasma — fusion reactors have improved faster than Moore’s Law (which said that the number of transistors that can be placed on a computer chip would double about every two years). “The long view is that we’ve made unbelievable progress,” says Hawryluk. ITER should be the machine that achieves a self-sustaining chain reaction called “burn” or “ignition,” where no heat has to be added in order to achieve fusion. A demonstration power plant could come next, with the ability to burn plasma continuously, rather than in bursts. That, Hawryluk says, could happen by 2040.

Yet the perception has been that progress toward fusion has been slow — perhaps because the goal is so big. Prager says the situation wasn’t helped by quasi-research into “cold fusion” and “bubble fusion” — both of which made claims that fusion eventually might be achieved on a tabletop. Those “fiascos,” Prager says, branded all of fusion. Even now, many in the public confuse cold fusion with hot. “It’s almost like confusing astrology and astronomy,” he says. But he acknowledges that fusion scientists need to be better at explaining their progress, calling for patience, and noting that good things can come from the research along the way — such as the way the semiconductor industry has learned to use plasma to finely etch its microchips. Fusion scientists would be wise to imitate the cancer-research community, which, Prager says, manages to explain the advances and side benefits of its work well enough for the public to understand. “[People] don’t simply ask, ‘When are you going to cure cancer?’”

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Miklos Porkolab, PPPL physicist 1976, Says:

2009-06-19 09:33:36

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