In order to investigate something called inertial confinement fusion (or ICF, which we'll talk about below), scientists at the National Ignition Facility built the biggest damn laser in the world. Located in Livermore, California, the NIF's laser is capable of producing something like 500TW of power per pulse, and the facility is so futuristic that parts of Star Trek Into Darkness were filmed there.

So imagine the disappointment felt by everyone at the NIF when, even though their laser worked fine, the amount of fusion it appeared to produce was shockingly low. Researchers now may have identified the problem and have shown that many neutrons can be observed at the NIF. Neutrons are a result of fusion, so the number of neutrons is a measure of how many atoms have fused. The problem is that, to produce efficient fusion, we can't currently use the facility at its full power.

Small hydrogen bombs

The idea behind inertial confinement fusion is simple. To get two atoms to fuse together, you need to bring their nuclei into contact with each other. Both nuclei are positively charged, so they repel each other, which means that force is needed 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 huge amount of energy.

Being killjoys, scientists prefer not to detonate nuclear weapons every time they want to study 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 pellet of hydrogen ice inside a heavy metal casing. The casing is illuminated by powerful lasers, which burn off a large portion of the material. The reaction force from the vaporized material exploding outward causes the remaining shell to implode. The resulting shockwave compresses the center of the core of the hydrogen pellet so that it begins to fuse.

If confinement fusion ended there, the amount of energy released would be tiny. But the energy released due to the initial fusion burn in the center generates enough heat for the hydrogen on the outside of the pellet to reach the required temperature and pressure. So, in the end (at least in computer models), all of the hydrogen is consumed in a fiery death, and massive quantities 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 in a symmetric fashion, it would work. And, with the lasers that scientists had then, the experiment worked well, generating a small amount of fusion. But, to get complete burn, a bigger laser would be required. So they built one at the NIF in California.

Early success turned to disappointment. Even with the new laser cranked up to 11, the amount of fusion measured was orders of magnitude smaller than the amount of fusion expected. As the power went up, the way the pellet collapsed seemed to be increasingly sensitive to imperfections in its spherical shape. This resulted in material squirting out of the pellet rather than being crushed. This was a big problem, because making hydrogen ice of the correct density and shape is already difficult.

Liquid spherical cows

So researchers went back to an old idea: liquid hydrogen. Liquids, thanks to surface tension, form perfect spheres in vacuum (and zero gravity). In this context, differences in density don't matter; the high energies involved mean that the ice and the 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 to hang around stuck to ice spheres; it just kind of drips off or freezes. To get around this, the liquid hydrogen is actually contained within 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 has taken some 30 years to develop a foam that doesn't fall apart on contact. But, with that small task out of the way...

The pellet was reworked so that it consisted of an ice core surrounded by a liquid hydrogen shell that was held in place by a low-density foam. This composite was then placed in a metallic 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 the laser power down. The implications are basically that, in a perfect world, bigger is better, but once your cow is no longer spherical, it will clobber you with its hooves, udder, and other sticking-out bits given the opportunity. Less energy means less clobbering

Next came the actual experiments. As with all such experiments, only a few pellets were actually crushed, but each crushing provides enormous amounts of data. The researchers showed that, for laser powers less than half the maximum available, they were crushing the pellets by a factor of three (or more) less than for full power. But, for these same parameters, they observed close to the same number of neutrons as observed for earlier experiments at high energy. More importantly, the neutron yields were predicted fairly accurately by models, giving researchers a path toward optimization.

So it's a bit of a good news, bad news story. The good news is that scientists may have found a way around the problems encountered in the earlier inertial confinement fusion work. The bad news? Scientists have to turn the power down to the point that they cannot actually achieve complete burn. But partially liquid spherical hydrogen drops appear to be the way forward.

Physical Review Letter, DOI: 10.1103/PhysRevLett.117.245001