|Heavy Ion Fusion: Squeezing the Beams|
|Contact: Lynn Yarris, [email protected]|
| Spectacular results achieved by researchers in the Heavy Ion Fusion group at Lawrence Berkeley National Laboratory have the potential to substantially reduce the cost of dense-plasma physics experiments and could shorten the development timetable for the energy technology called heavy ion fusion (HIF) .
Working at the Neutralized Drift Compression Experiment facility (NDCX-1), which has been in operation for only a few months, the researchers compressed 200-nanosecond pulse lengths of a 25‑milliampere (25 mA), 255-kiloelectron volt (255 keV) beam of positively charged ions by an unprecedented fifty-fold an accomplishment that surprised even the scientists who did the experiment.
"The four-nanosecond pulse lengths we produced put heavy ion beams for the first time within range of the pulse lengths necessary for meaningful high-energy-density physics and for fusion power application," says Grant Logan, a physicist with Berkeley Lab's Accelerator and Fusion Research Division (AFRD), who leads the HIF research effort here and heads the U.S. Heavy-Ion Fusion Virtual National Laboratory (HIF-VNL).
HIF-VNL is a collaboration between Berkeley Lab, Lawrence Livermore National Laboratory, and the Princeton Plasma Physics Laboratory, formed in 1999 to develop heavy-ion accelerators capable of creating fusion reactions for commercial electricity. HIF is an alternative to fossil fuels that hasn't received the public attention given to magnetic-fusion energy (MFE), the tokomak-based concept behind the International Thermonuclear Experimental Reactor, ITER.
"When we began the NDCX-1 experiments, we believed that we would be able to compress the 200-nanosecond pulse lengths of our heavy-ion beam down to about 10 nanoseconds within two to three years," says Simon Yu, the AFRD physicist who leads the HIF-VNL's final beam transport and focusing research effort. "As far as we can determine, our fifty-fold compression ratio is by far the highest ever reported for an intense, space-charge dominated beam."
Fusion is the energy source that lights up the sun. It takes place when lighter atomic nuclei are fused together to form heavier nuclei. A typical fusion reaction releases roughly one million times the energy released by the burning of oil. Among the selling points of fusion are its safety unlike nuclear fission, fusion cannot sustain an uncontrolled chain reaction and its environmental cleanliness unlike the burning of fossil fuels, a fusion reaction does not contribute to global warming.
But the biggest selling point of fusion as an energy source is that it would last forever. A fusion power plant would be fueled by deuterium and tritium, the two isotopes of hydrogen. Both can be obtained from sea water deuterium is extracted directly through hydrolysis, and tritium is bred from the element lithium, which is also abundant in the earth's crust. Enough fusion water to supply a year's worth of electrical power to a city like San Francisco could be delivered in a pickup truck.
In an HIF reactor, an imploded pea-sized capsule of nuclear fuel burns quickly enough to keep it confined by its own inertia. This confinement lasts long enough for the reaction to produce energy. The implosion that ignites the fuel is set-off, or "driven," by high-powered beams of heavy ions such as xenon, mercury, or cesium, which are produced in a particle accelerator and focused on the capsule.
One of the big technical challenges facing HIF is the development of a particle accelerator that can transport and focus heavy-ion beams, whose total current might rise to 500,000 amperes by the time they reach the target. At such high current, space-charge forces the mutual repulsion between so many positively charged ions become a serious impediment. The same problem is faced by those who want to build high-energy particle accelerators for the study of dense-plasma physics.
In the NDCX-1 experiments, the Berkeley Lab researchers were able to neutralize the space-charge effects of their heavy-ion beams by sending them through a preformed plasma during compression rather than through a vacuum, as is typical for particle-beam acceleration. Electrons in the plasma counteracted the mutual repulsion of the positively charged heavy ions, which might otherwise have blown the beam apart. Beam compression was achieved by accelerating ions in the tail end of each pulse at a significantly faster rate than particles at the head of the pulse were accelerated.
"Using an 'induction tilt core,' we create a special waveform that produces a velocity ramp on each beam pulse," says Logan. "As the ions in the tail race to catch up with the ions in the head, it causes the pulse length of the beam to compress."
The NDCX-1 experimental results were achieved within four months of initial operation, which began in December of 2004. To confirm these results, improved diagnostics were applied for two more months. Logan and Yu credit much of their ability to score such a stunning success so quickly to an extremely effective team of experimentalists and theorists from both the HIF-VNL collaboration and the Mission Research Corporation, a private firm located in Albuquerque, New Mexico. They were also helped by highly sophisticated and accurate computer simulations, some of which were conducted at Berkeley Lab's National Energy Research Scientific Computing Center (NERSC).
Down the road, the HIF-VNL collaboration will need to achieve radial compression of their heavy ion beam pulses as well as longitudinal compression. They will also need to boost the accelerated energy of the beam. To reach this next step, they envision the development of a second, more elaborate experiment called NDCX-2. This upgraded sequel to NCDX-1 would cost approximately $5 million of hardware and would result, within about five years, in a facility fully integrated with all the components needed to demonstrate target physics.
"This would be a key step towards making heavy-ion fusion energy an affordable means of producing commercial electricity," says Logan. "Throughout the history of the HIF-VNL program, we've had to find innovative ways to make progress under constrained funding. The NDCX-2 facility would keep with that tradition, and we're very excited about its prospects."
Among the collaborators with Logan and Yu on the NCDX-1 experiment were Prabir Roy, Enrique Henestroza, Will Waldron, Wayne Greenway, Shmuel Eylon and Frank Bieniosek, of the HIF group; Adam Sefkow, with the Princeton Plasma Physics Laboratory; Josh Coleman, with the Nuclear Engineering Department at UC Berkeley; Andre Anders, who leads AFRD's Plasma Applications Group; Dale Welch, of Mission Research Corporation; and Wim Leemans at AFRD's Center for Beam Physics.