In a recent press release, Lawrence Livermore National Laboratory (LLNL) announced that it had achieved a yield of 3 x 1015 neutrons in the latest round of experiments at its National Ignition Facility, a giant, 192-beam laser facility designed, as its name implies, to achieve fusion ignition. That’s nowhere near “ignition,” but still encouraging as it’s three times better than results achieved in earlier experiments.
The easiest way to achieve fusion is with two heavy isotopes of hydrogen; deuterium, with a nucleus containing one proton and one neutron, and tritium, with a nucleus containing one proton and two neutrons. Deuterium is not radioactive, and occurs naturally as about one atom to every 6400 atoms of “normal” hydrogen, with a nucleus containing only a single proton. Tritium is radioactive, and occurs naturally only in tiny trace amounts. It has a half-life (the time it takes for half of a given amount to undergo radioactive decay) of 12.3 years, and must be produced artificially. When tritium and deuterium fuse, they release a neutron, a helium nucleus, or alpha particle, and lots of energy (17.6 million electron volts).
Fortunately (because otherwise it would be too easy to blow up the planet), or unfortunately (if you want to convert the energy into electricity), fusion is hard. The two atoms don’t like to get too close, because their positively charged nuclei repel each other. Somehow, a way must be found to make the heavy hydrogen fuel material very hot, causing the thermal motion of the atoms to become very large. Once they start moving fast enough, they can smash into each other with enough momentum to overcome the repulsion of the positive nuclei, allowing them to fuse. However, the amount of energy needed per atom is huge, and when atoms get that hot, the last thing they want to do is stay close to each other (think of what happens in the detonation of high explosive.) There are two mainstream approaches to solving this problem; magnetic fusion, in which the atoms are held in place by powerful magnetic fields while they are heated (the approach being pursued at ITER, the International Thermonuclear Experimental Reactor, currently under construction in France), and inertial confinement fusion (ICF), where the idea is to dump energy into the fuel material so fast that its own inertia holds it in place long enough for fusion to occur. The NIF is an ICF facility.
There are various definitions of ICF “ignition,” but, in order to avoid comparisons of apples and oranges between ICF and magnetic fusion experiments, LLNL has explicitly accepted the point at which the fusion energy out equals the laser energy in as the definition of ignition. In the experiment referred to above, the total fusion energy release was about 10,000 joules, give or take. Since the laser energy in was around 1.7 million joules, that’s only a little over one half of one percent of what’s needed for ignition. Paltry, you say? Not really. To understand why, you have to know a little about how ICF experiments work.
Recall that the idea is to heat the fuel material up so fast that its own inertia holds it in place long enough for fusion to occur. The “obvious” way to do that would be to simply dump in enough laser energy to heat all the fuel material to fusion temperatures at once. Unfortunately, this “volumetric heating” approach wouldn’t work. The energy required would be orders of magnitude more than what’s available on the NIF. What to do? Apply lots and lots of finesse. It turns out that if a very small volume or “hot spot” in the fuel material can be brought to fusion conditions, the alpha particles released in the fusion reactions might carry enough energy to heat up the nearby fuel to fusion conditions as well. Ideally, the result would be an alpha “burn wave,” moving out through the fuel, and consuming it all. But wait, it ain’t that easy! An efficient burn wave will occur only if the alphas are slammed to a stop and forced to dump their energy after traveling only a very short distance in the cold fuel material around the hot spot. Their range is too large unless the fuel is first compressed to a tiny fraction of its original volume, causing its density to increase by orders of magnitude.
In other words, to get the fuel to fuse, we need to make it very hot, but we also need to compress it to very high density, which can be done much more easily and efficiently if the material is cold! Somehow, we need to keep the fuel “cold” during the compression process, and then, just at the right moment, suddenly heat up a small volume to fusion conditions. It turns out that shocks are the answer to the problem. If a train of four shocks can be set off in the fuel material as it is being compressed, or “imploded,” by the lasers, precisely timed so that they will all converge at just the right moment, it should be possible, in theory at least, to generate a hot spot. If the nice, spherical symmetry of the fuel target could be maintained during the implosion process, everything should work just fine. The NIF would have more than enough energy to achieve ignition. But there’s the rub. Maintaining the necessary symmetry has turned out to be inordinately hard. Tiny imperfections in the target surface finish, small asymmetries in the laser beams, etc., lead to big deviations from perfect symmetry in the dense, imploded fuel. These asymmetries have been the main reason the NIF has not been able to achieve its ignition goal to date.
And that’s why the results of the latest round of experiments haven’t been as “paltry” as they seem. As noted in the LLNL press release,
Early calculations show that fusion reactions in the hot plasma started to self-heat the burning core and enhanced the yield by nearly 50 percent, pushing close to the margins of alpha burn, where the fusion reactions dominate the process.
“The yield was significantly greater than the energy deposited in the hot spot by the implosion,” said Ed Moses, principle associate director for NIF and Photon Science. “This represents an important advance in establishing a self-sustaining burning target, the next critical step on the path to fusion ignition on NIF.”
That’s not just hype. If the self-heating can be increased in future experiments, it may be possible to reach a threshold at which the alpha heating sets off a burn wave through the rest of the cold fuel, as described above. In other words, ignition is hardly a given, but the guys at LLNL still have a fighting chance. Their main challenge may be to stem the gradual evaporation of political support for NIF while the experiments are underway. Their own Senator, Diane Feinstein, is anything but an avid supporter. She recently turned down appeals to halt NIF budget cuts, and says the project needs to be “reassessed” in light of the failure to achieve ignition.
Such a “reassessment” would be a big mistake. The NIF was never funded as an energy project. Its support comes from the National Nuclear Security Administration (NNSA), a semi-autonomous arm of the Department of Energy charged with maintaining the safety and reliability of the nation’s nuclear arsenal. As a tool for achieving that end, the NIF is without peer in any other country. It has delivered on all of its performance design goals, including laser energy, illumination symmetry, shot rate, the precision and accuracy of its diagnostic instrumentation, etc. The facility is of exceptional value to the weapons program even if ignition is never achieved. It can still generate experimental conditions approaching those present in an exploding nuclear device, and, along with the rest of our suite of “above-ground experimental facilities,” or AGEX, it gives us a major leg up over the competition in maintaining our arsenal and avoiding technological surprise in the post-testing era.
Why is that important? Because the alternative is a return to nuclear testing. Do you think no one at NNSA wants to return to testing, and that the weapon designers at the National Weapons Laboratories wouldn’t jump at the chance? If so, you’re dreaming. It seems to me we should be doing our best to keep the nuclear genie in the bottle, not let it out. Mothballing the NIF would be an excellent start at pulling the cork!
I understand why the guys at LLNL are hyping the NIF’s potential as a source of energy. It’s a lot easier to generate political support for lots of electricity with very little radioactive waste and no greenhouse gases than for maintaining our aging arsenal of nuclear weapons. However, IMHO, ICF is hopeless as a source of electricity, at least for the next few hundred years. I know many excellent scientists will disagree, but many excellent scientists are also prone to extreme wishful thinking when it comes to rationalizing a technology they’ve devoted their careers to. Regardless, energy hype isn’t needed to justify the NIF. It and facilities like it will insure our technological superiority over potential nuclear rivals for years to come, and at the same time provide a potent argument against the resumption of nuclear testing.
