Researchers at the National Ignition Facility (NIF) say they have met many of the demanding challenges leading up achieving the highly stable, precisely directed implosion required for fusion ignition. The NIF, engaged in a collaborative project led by the Department of Energy’s Lawrence Livermore National Laboratory, reports that there is at least one significant obstacle to overcome.
It’s the same matter that bedevils every one else, crushing the fuel is like squeezing a balloon in one’s fist, a lot of the fuel or balloon squishes out of the lower and less restrained areas.
The dream of igniting (another way to say ‘breakeven’) a self-sustained fusion reaction with high yields of energy is a feat likened to creating a miniature star on Earth. The NIF project is a multi-institutional effort including partners from the University of Rochester’s Laboratory for Laser Energetics, General Atomics, Los Alamos National Laboratory, Sandia National Laboratory, and the Massachusetts Institute of Technology.
In what must be a dreadful sense of confusion the group was scooped by the mass media a few weeks ago with a less than stellar report that cast much doubt on their success. Oddly the groups research report, received back in March was accepted in June after the peer review and was published back at the end of July in the journal Physics of Plasma. The press release only came out this past Monday. Wow, that’s months late. Anyway, the progress is much better than the wire service writer and the assorted mass media stories made the progress out to be.
To reach ignition (defined as the point at which the fusion reaction produces more energy than is needed to initiate it), the NIF focuses 192 laser beams simultaneously in billionth-of-a-second pulses inside a cryogenically cooled hohlraum (from the German word for “hollow room”), a hollow cylinder the size of a pencil eraser. Within the hohlraum is a ball-bearing-size capsule containing two hydrogen isotopes, deuterium and tritium (D-T). The unified lasers deliver 1.8 megajoules of energy and 500 terawatts of power – 1,000 times more than the United States uses at any one moment – to the hohlraum creating an “X-ray oven” which implodes the D-T capsule to temperatures and pressures similar to those found at the center of the sun.
John Edwards, NIF associate director for inertial confinement fusion and high-energy-density science explains, “What we want to do is use the X-rays to blast away the outer layer of the capsule in a very controlled manner, so that the D-T pellet is compressed to just the right conditions to initiate the fusion reaction. In our new review article, we report that the NIF has met many of the requirements believed necessary to achieve ignition – sufficient X-ray intensity in the hohlraum, accurate energy delivery to the target and desired levels of compression – but that at least one major hurdle remains to be overcome, the premature breaking apart of the capsule.”
In the article, Edwards and his colleagues discuss how they are using diagnostic tools developed at NIF to determine likely causes for the problem. “In some ignition tests, we measured the scattering of neutrons released and found different strength signals at different spots around the D-T capsule,” Edwards said. “This indicates that the shell’s surface is not uniformly smooth and that in some places, it’s thinner and weaker than in others. In other tests, the spectrum of X-rays emitted indicated that the D-T fuel and capsule were mixing too much – the results of hydrodynamic instability – and that can quench the ignition process.”
There’s quite a lot to examine and improve. The lasers have to go off exactly at the right time, focused precisely correctly on a target so uniformly built that the fuel inside gets compressed and heated properly. That’s the simplest explanation of something that involves a huge amount of energy on a wee bit of material in a tiny moment.
Edwards said that the team is concentrating its efforts on NIF to define the exact nature of the instability and use the knowledge gained to design an improved, sturdier capsule. Achieving that milestone, he said, should clear the path for further advances toward laboratory ignition.
That may be the last step to a breakeven event. So far humans have only managed fusion breakeven inside explosions. Whoever gets it done first will prove lots of ideas might be possible ways to burn fusion fuel beyond breakeven. Then the engineering race will get underway.
It’s going to happen. Somewhere and maybe soon.