By Andrew Grant
A laser-driven technique could reignite research into fusing protons and boron nuclei, which make up the most seductive and challenging fuel for generating energy from nuclear fusion.
While the researchers admit that this type of fusion won’t be used to produce energy anytime soon, their work opens new avenues for exploring what many physicists consider the ideal fusion fuel. “The holy grail of holy grails is proton-boron fusion,” says Steven Cowley, a fusion physicist at Imperial College London who was not involved in the new work.
Since the Manhattan Project in the 1940s, scientists pursuing nuclear fusion have primarily focused on combining two varieties of hydrogen nuclei, deuterium and tritium. That’s the fuel of choice for hydrogen bombs and energy production experiments that try to squeeze more energy out of fusion reactions than they take to get started. Physicists have never reached this break-even point in controlled fusion. Plus hydrogen has drawbacks, including the scarcity of tritium (any potential fusion power plant would have to manufacture it) and the production of neutrons. Neutrons can make ordinary materials radioactive, and their energy is difficult to capture.
Decades ago, those shortcomings inspired scientists to explore fusing protons with boron-11, a nucleus of five protons and six neutrons. The reaction produces no stray neutrons, and boron is much easier to obtain than tritium. But while the National Ignition Facility in Livermore, Calif., and other research centers cultivate fusion reactions by crushing and heating hydrogen in the hopes of creating a self-sustaining burn (SN: 4/20/13, p. 26), that approach won’t work for proton-boron reactions; they require much higher temperatures to ignite. No other techniques coaxed protons and boron to fuse in significant numbers, so research stalled.
Three years ago Christine Labaune, a plasma physicist at the French National Center for Scientific Research, teamed with Johann Rafelski, a theoretical nuclear physicist at the University of Arizona in Tucson, to try a different approach. They set up two lasers. One accelerated a torrent of protons and electrons toward a small lump of boron. Just before the particles arrived, the other laser zapped the lump, heating it and stripping electrons off the boron nuclei to form a plasma.
Labaune, Rafelski and colleagues report October 8 in Nature Communications that about 80 million protons fused with boron nuclei during the 1.5 nanoseconds that the latter laser fired. That’s at least 100 times the reaction rate of any previous proton-boron experiment, they say. A major benefit of the approach, Rafelski says, is that the laser-accelerated electrons arrive at the boron plasma just before the protons. The team hypothesizes that the electrons zip through the boron and push electrons within the plasma out of the way. By the time the protons arrive, they have a better chance of fusing with boron nuclei, rather than losing energy to stray electrons.
Unfortunately, all those fusion reactions add up to only millijoules of energy — about a millionth of the lasers’ energy. Samuel Cohen of the Princeton Plasma Physics Laboratory points out that because the new approach requires so much energy to accelerate protons, nearly 10 percent of those protons would have to fuse to produce an energy surplus; in the experiment, no more than 1 in 300 protons fused with boron.
Labaune and Rafelski stress that they are not proposing to use their scheme as the basis of a nuclear reactor design. But they say their approach gives physicists a new way to look at an old problem. Cohen agrees that the technique warrants further exploration.
Rafelski says the team has seen much higher reaction rates in follow-up experiments. In the meantime, he says that clashes of protons and plasma similar to those in the experiment occurred frequently during the first several minutes after the Big Bang. Astrophysicists could use lasers to simulate how the first elements heavier than hydrogen formed, he says.
