The National Ignition Facility’s preamplifiers are the first step in increasing the energy of laser beams as they travel to the target chamber. NIF recently hit a 500-terawatt shot — then it had to reject it.
To investigate something called inertial confinement fusion (or ICF, which we’ll talk about below), scientists at the National Ignition Facility built the largest damn laser in the world. Located in Livermore, California, the NIF’s laser is capable of producing about 500 TW of power per pulse, and the facility is so futuristic that parts of Star Trek Into Darkness were filmed there.
So imagine the disappointment everyone at the NIF felt when, even though their laser worked well, the amount of fusion it seemed to produce was shockingly low. Researchers may now have identified the problem and have shown that many neutrons can be observed at the NIF. Neutrons are the result of fusion, so the number of neutrons is a measure of the number of atoms that have fused. The problem is that to produce efficient fusion, we can’t run the facility at full capacity right now.
Small hydrogen bombs
The idea behind inertial confinement fusion is simple. To fuse two atoms, you need to bring their nuclei into contact. Both nuclei are positively charged, so they repel each other, meaning it takes force to convince two hydrogen nuclei to touch. In a hydrogen bomb, force is generated when a small fission bomb explodes, compressing a core of hydrogen. This fuses to create heavier elements, releasing a tremendous amount of energy.
Scientists are spoilsports, preferring not to detonate nuclear weapons every time they want to study nuclear fusion or use it to generate electricity. Which brings us to inertial confinement fusion.
In inertial confinement fusion, the hydrogen core consists of a spherical globule of hydrogen ice in a heavy metal case. The housing is illuminated by powerful lasers, which burn away much of the material. The reaction force of the vaporized material exploding outwards causes the remaining shell to implode. The resulting shock wave compresses the center of the hydrogen pellet’s core so that it begins to fuse.
If the confinement fusion ended there, the amount of energy released would be small. But the energy released as a result of the initial fusion combustion in the center generates enough heat for the hydrogen on the outside of the pellet to reach the required temperature and pressure. So eventually (at least in computer models) all of the hydrogen is consumed in a fiery death and huge amounts of energy are released.
In the early days of inertial confinement fusion, the way forward seemed clear. As long as the target was sufficiently spherical and the laser beams all hit the target symmetrically, it would work. And with the lasers that scientists had at the time, the experiment worked well and generated a small amount of fusion. But to burn completely would require a larger laser. So they built one at the NIF in California.
Early success turned to disappointment. Even with the new laser boosted to 11, the measured amount of fusion was an order of magnitude smaller than the expected amount of fusion. As power increased, the way the pellet collapsed seemed to become more and more sensitive to imperfections in its spherical shape. This resulted in material spraying out of the pellet instead of being crushed. This was a big problem, because making hydrogen ice with the right density and shape is already difficult.
Liquid spherical cows
So researchers went back to an old idea: liquid hydrogen. Thanks to surface tension, liquids form perfect spheres in a vacuum (and without gravity). In this context, differences in density don’t matter; the high energies involved mean that the ice and liquid look almost identical to the incoming shock wave. So the liquid can fill in the imperfections of the ice core and present a smooth, seemingly uniform face.
The problem is that liquid doesn’t like being stuck on ice balls; it just drips off or freezes. To get around this, the liquid hydrogen is actually contained in a low-density foam where, again, surface tension holds it in place. This sounds simple, but liquid hydrogen is nasty to work with, so it’s taken about 30 years to develop a foam that doesn’t disintegrate on contact. But with that little job done…
The pellet was reworked to consist of an ice core surrounded by a liquid hydrogen shell held in place by low density foam. This composite was then placed in a metal armor so that the laser had something to crush.
The next step was to take advantage of new modeling techniques. Models of the implosion process showed that once you take imperfections into account, you can achieve higher pressures and densities if you turn off the laser power. The implications are essentially that, in a perfect world, bigger is better, but once your cow is no longer spherical, she will beat you up with her hooves, udder, and other protruding parts if given the chance. Less energy means less chatter
Next came the actual experiments. As with all such experiments, only a few pellets were crushed, but each crush yields huge amounts of data. The researchers showed that at a laser power of less than half the maximum available power, they crushed the pellets a factor of three (or more) less than at full power. But for the same parameters, they observed almost the same number of neutrons as observed for previous high-energy experiments. More importantly, the neutron yields were predicted fairly accurately by models, giving researchers a path to optimization.
So it’s kind of a good news, bad news story. The good news is that scientists may have found a way around the problems that posed the earlier inertial confinement fusion work. The bad news? Scientists have to turn the power so low that they can’t actually burn completely. But partially liquid spherical hydrogen droplets appear to be the way forward.
Physical assessment letterDOI: 10.1103/PhysRevLett.117.245001