The news from California is encouraging. In an article recently published in Science and summarized on the website of Lawrence Livermore National Laboratory (LLNL), scientists working at the National Ignition Facility (NIF) report efficient coupling of energy from all 192 beams of the giant facility into a hohlraum target similar to the one that will be used later this year in the first attempts to achieve fusion ignition and “breakeven,” usually defined as more energy production from fusion than was carried in the laser beams used to hit the target. The design energy of the NIF is 1.8 megajoules, and, according to the latest reports from Livermore, the threshold of one megajoule has already been achieved.
In inertial confinement fusion, or ICF, the target, a thin, spherical shell containing a mixture of deuterium and tritium, two heavy isotopes of hydrogen, is first compressed and imploded to very high densities. A series of converging shocks then create a “hot spot” in the center of the compressed material, setting off fusion reactions which release enough energy to set off a “burn wave.” This wave propagates out through the remaining fuel material, heating it to fusion energies as well. The process is known as inertial confinement fusion because it takes place so fast (on the order of a nanosecond) that the material’s own inertia holds it in place long enough for the fusion reactions to occur. There are two basic approaches; direct drive, in which the laser beams hit the fusion target directly, and indirect drive, the process that will be used in the upcoming Livermore ignition experiments, in which the beams are shot into a hollow can or “hohlraum,” producing x-rays when they hit the inner walls. These x-rays then implode and ignite the target.
A potential problem that must be overcome in ICF is known as laser plasma interactions (LPI). These are parasitic interactions which can soak up laser energy and quench the fusion process. According to the Livermore paper, special grids at the hohlraum entrance holes were used in the latest experiments, allowing the use of LPI to “tweak” the incoming beams, steering them to just the right spots. This recent (and elegant) innovation allows the exploitation of a process that has always been considered a major headache in the past to actually improve the chances of achieving igntion.
The BBC and Spiegel both have articles about the latest experiments today, conflating the energy and military applications of the NIF as usual. According to the Spiegel article, for example, it will be necessary for the lasers in a fusion reactor to hit the target ten times a second, whereas hours are necessary between shots at the NIF. The reason, of course, is that the NIF was never designed as an energy project, but is being funded by the National Nuclear Security Administration (NNSA) to conduct nuclear weapons experiments. If ignition is achieved, the prospects for fusion energy will certainly be improved, but the prospects aren’t nearly as bright as the press releases from LLNL would imply. It will still be necessary to overcome a great number of scientific and engineering hurdles before the process can ever become useful and economical as a source of energy.
I am not optimistic about the success of the upcoming experiments. I suspect it will be too difficult to achieve the fine beam energy balance and symmetry that will be necessary to ignite the central “hot spot.” It will take more than one converging shock to do the job. Several will be necessary, moving inward through the target material at just the right speed to converge at a small spot at the center. If they really pull it off, I will be surprised, but will be more than happy to eat crow. A lot of very talented scientists have dedicated their careers to the quest for fusion, and I’m keeping my fingers crossed for them.
Even if these ignition experiments fail, it won’t mean the end for fusion by a long shot. We know we can achieve the high fuel densities needed for inertial fusion, and there are other ways of creating the “hot spot” needed to achieve ignition, such as “fast ignitor.” Other approaches to fusion keep showing up in the scientific literature, and I can’t help but think that, eventually, one of them will succeed.