The National Ignition Facility, or NIF, is a huge, 192 beam laser system, located at Lawrence Livermore National Laboratory in California. It was designed, as the name implies, to achieve thermonuclear ignition in the laboratory. “Ignition” is generally accepted to mean getting a greater energy output from fusion than the laser input energy. Unlike magnetic confinement fusion, the approach currently being pursued at the International Thermonuclear Experimental Reactor, or ITER, now under construction in France, the goal of the NIF is to achieve ignition via inertial confinement fusion, or ICF, in which the fuel material is compressed and heated to the extreme conditions at which fusion occurs so quickly that it is held in place by its own inertia.
The NIF has been operational for over a year now, and a two year campaign is underway with the goal of achieving ignition by the end of this fiscal year. Recently, there has been a somewhat ominous silence from the facility, manifesting itself as a lack of publications in the major journals favored by fusion scientists. That doesn’t usually happen when there is anything interesting to report. Finally, however, some papers have turned up in the journal Physics of Plasmas, containing reports of significant progress.
To grasp the importance of the papers, it is necessary to understand what is supposed to occur within the NIF target chamber for fusion to occur. Of course, just as in magnetic fusion, the goal is to bring a mixture of deuterium and tritium, two heavy isotopes of hydrogen, to the extreme conditions at which fusion takes place. In the ICF approach, this hydrogen “fuel” is contained in a tiny, BB-sized target. However, the lasers are not aimed directly at the fuel “capsule.” Instead, the capsule is suspended in the middle of a tiny cylinder made of a heavy metal like gold or uranium. The lasers are fired through holes on each end of the cylinder, striking the interior walls, where their energy is converted to x-rays. It is these x-rays that must actually bring the target to fusion conditions.
It was recognized many years ago that one couldn’t achieve fusion ignition by simply heating up the target. That would require a laser driver orders of magnitude bigger than the NIF. Instead, it is first necessary to compress, or implode, the fuel material to extremely high density. Obviously, it is harder to “squeeze” hot material than cold material to the necessary high densities, so the fuel must be kept as “cold” as possible during the implosion process. However, cold fuel won’t ignite, begging the question of how to heat it up once the necessary high densities have been achieved.
It turns out that the answer is shocks. When the laser generated x-rays hit the target surface, they do so with such force that it begins to implode faster than the speed of sound. Everyone knows that when a plane breaks the sound barrier, it, too, generates a shock, which can be heard as a sonic boom. The same thing happens in ICF fusion targets. When such a shock converges at the center of the target, the result is a small “hot spot” in the center of the fuel. If the temperature in the hot spot were high enough, fusion would occur. Each fusion reaction would release a high energy helium nucleus, or alpha particle, and a neutron. The alpha particles would be slammed to a stop in the surrounding cold fuel material, heating it, in turn, to fusion conditions. This would result in a fusion “burn wave” that would propagate out through the rest of the fuel, completing the fusion process.
The problem is that one shock isn’t enough to create such a “hot spot.” Four of them are required, all precisely timed by the carefully tailored NIF laser pulse to converge at the center of the target at exactly the same time. This is where real finesse is needed in laser fusion. The implosion must be extremely symmetric, or the shocks will not converge properly. The timing must be exact, and the laser pulse must deliver just the right amount of energy.
One problem in the work to date has been an inability to achieve high enough implosion velocities for the above scenario to work as planned. One of the Physics of Plasmas papers reports that, by increasing the laser energy and replacing some of the gold originally used in the wall of the cylinder, or “hohlraum,” in which the fuel capsule is mounted with depleted uranium, velocities of 99% of those required for ignition have been achieved. In view of the recent announcement that a shot on the NIF had exceeded its design energy of 1.8 megajoules, it appears the required velocity is within reach. Another of the Physics of Plasmas papers dealt with the degree to which implosion asymmetries were causing harmful mixing of the surrounding cold fuel material into the imploded core of the target. It, too, provided grounds for optimism.
In the end, I suspect the success or failure of the NIF will depend on whether the complex sequence of four shocks can really be made to work as advertised. That will depend on the accuracy of the physics algorithms in the computer codes that have been used to model the experiments. Time and again, earlier and less sophisticated codes have been wrong because they didn’t accurately account for all the relevant physics. There is no guarantee that critical phenomena have not been left out of the current versions as well. We may soon find out, if the critical series of experiments planned to achieve ignition before the end of the fiscal year are carried out as planned.
One can but hope they will succeed, if only because some of our finest scientists have dedicated their careers to the quest to achieve the elusive goal of controlled fusion. Even if they do, fusion based on the NIF approach is unlikely to become a viable source of energy, at least in the foreseeable future. Laser fusion may prove scientifically feasible, but getting useful energy out of it will be an engineering nightmare, dangerous because of the need to rely on highly volatile and radioactive tritium, and much too expensive to compete with potential alternatives. I know many of the faithful in the scientific community will beg to differ with me, but, trust me, laser fusion energy aint’ gonna happen.
On the other hand, if ignition is achieved, the NIF will be invaluable to the country, not as a source of energy, but for the reason it was funded in the first place – to insure that our nation has an unmatched suite of experimental facilities to study the physics of nuclear weapons in a era free of nuclear testing. As long as we have unique access to facilities like the NIF, which can approach the extreme physical conditions within exploding nukes, we will have a significant leg up on the competition as long as the test ban remains in place. For that, if for no other reason, we should keep our fingers crossed that the NIF team can finally clear the last technical hurdles and reach the goal they have been working towards for so long.
surely the radioactivity of tritium is weak? it’s just a low energy beta particle, an electron.
The tritium beta is indeed less energetic than betas from many other radioactive isotopes. It’s sort of like the difference between a big knife and a small knife. The small knife may not look impressive, but it can still kill you.
According to the EPA fact sheets, tritium is one of the least dangerous radionuclides. And it is needed in tiny cryogenic quantities for fusion. I don’t buy your assertion that fusion energy is beyond us. And considering what we have spent on oil at the pump, oil subsidies, oil cleanups and oil wars, a few billion for fusion energy seems more than reasonable, a bargain, to me.
We have made breeder reactors before. Why not a tritium breeder.
You implied here or earlier that tritium is very dangerous because it is “highly radioactive”. But this really just means that it has a short half life – it decays both quickly and weakly. That does not sound dangerous to me!
Also, given the amount of sea water on the planet, surely we can find enough tritium for the .minute. quantities required for fusion.
As for volatility, how hard is it to keep the tritium cryogenically frozen for ease of handling? Surely we know how to do this very well by now!
I just don’t see the insurmountable problems that you appear to. You show a lack of will to tackle what needs to be done, yet you complain that the NIF has not finished – “are we there yet?”. Why don’t you give the NIF scientists and engineers a break and provide support or constructive criticism instead. They are working on the highest endeavor humans have attempted. Apollo pales in comparison.
I have no objection to fusion if it can be made to work, Sandra, and I support increased funding for fusion research, as opposed to white elephants like ITER. I just don’t think we should ignore the obvious drawbacks to the two current mainstream approaches; magnetic and ICF. There is no question that tritium is dangerous once it gets inside the body. If you doubt that you are simply willfully ignoring the truth. We can certainly breed tritium, and have already done so for the nuclear weapons program. The problem isn’t that we can’t do it, but that producing enough of it to keep a reactor going and extracting it and fabricating targets from it in real time would be an engineering nightmare even if it were scientifically feasible. I doubt that such an approach will ever be economically competitive with, for example, thorium breeders. Tritium does not occur naturally in sea water. You are probably thinking of the lighter isotope, deuterium, which does. I certainly support the work at NIF, but don’t think we should ignore the fact that the goal of ignition hasn’t proved as easy to reach as some had imagined. My greatest fear in that quarter is that, after the current ignition campaign ends at the end of the fiscal year, the government will lose interest and cut funding for further experiments. The idea that central hot spot ignition would work was always a stretch, but there are other approaches that might, such as fast ignitor and polar direct drive. My opinion is and always has been that NIF should be fully funded for operation with as many shifts as the guys at Livermore think they can handle. Building such a world class facility and then letting it sit idle or operating on half shifts would be short sighted indeed. ICF may or may not be viable as a future source of energy, but there is no doubt whatever that it will give us a major advantage over the competition in maintaining a safe and reliable nuclear stockpile in an era of no nuclear testing.
the alternative to a breeder reactor is a proton accelerator to produce tritium, independent from the fusion facility. I take it your work involves thorium reactors and you are a thorium advocate? Or nuclear weapons researcher? Every obstacle you presented has solutions, it seems. Which leads me to conclude that we can do fusion energy if we put our minds to it.
Do you know why the NIF chose indirect drive over direct drive? I wonder if the secondary heating from the hohlraum provides a more controlled environment for monitoring the implosion and tuning the lasers and timing. If so, I wonder if they could take off the training wheels and go to direct drive later with what they have learned using the hohlraum. Just speculating. I had not heard of “fast ignitor”. Does the DU in the hohlraum undergo fission? Or is it simply that the higher atomic mass of DU imparts greater momentum/kinetic energy to the DT fuel than lead or gold? You seem to know this subject, indulge me if you will.
I hope NIF does reach ignition this year, that will assure continued funding and silence many critics. Then, a serious discussion can begin on future direction for this capability, and I don’t just mean stockpile testing.
Back in the day, before Nova, the NIF’s predecessor at Livermore, was built, there was some hope that ignition could be reached on that facility. However, that facility had only 10 beams, and it would have been difficult to get the necessary illumination symmetry in direct drive, so it was decided to go with indirect drive. Subsequently, Livermore became the “lead lab” for indirect drive. The OMEGA laser at the University of Rochester, which is still in operation, had 60 beams, so Rochester became the lead lab for direct drive. There was also an ICF team at the Naval Research Laboratory led by a brilliant physicist named Steve Bodner. Steve was an advocate of gas lasers over glass, and also preferred direct drive. He was always a thorn in Livermore’s side, and that rivalry probably also had something to do with Livermore’s continued reliance on indirect drive.
Livermore could certainly try a version of direct drive known as polar direct drive if indirect drive doesn’t work. After the current ignition campaign ends at the end of the fiscal year, however, the weapons guys will have their turn, and I doubt their plans include direct drive. DU is being used in the hohlraum because models predicted and experiments confirmed that it would result in higher implosion velocities.
Thanks for the background.
I read Bodner’s memo “http://fire.pppl.gov/IFE_NAS_Bodner_PlanB.pdf” pages 7 though 9 and it was terrible. For him. What’s his problem, he can’t stand hippie scientists? He sounds like a Prima Donna or someone with a serious grudge. Perhaps he once was a great scientist but he has been retired since 1999 so I would not expect him to be current. At any rate, direct drive has not worked either. I guess that’s why he has his hand out for “another lab to compete” which btw will cost the tax payer twice as much.
Unfortunately these fights are mostly political, over funding and power, and the technical is merely a football and not centre stage as it should be.