National Ignition Facility

Nuclear Fusion Update

At the moment the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory is in a class by itself when it comes to inertial confinement fusion (ICF) facilities. That may change before too long. A paper by a group of Chinese authors describing a novel 3-axis cylindrical hohlraum design recently appeared in the prestigious journal Nature. In ICF jargon, a “hohlraum” is a container, typically cylindrical in form. Powerful laser beams are aimed through two or more entrance holes to illuminate the inner wall of the hohlraum, producing a burst of x-rays. These strike a target mounted inside the hohlraum containing fusion fuel, typically consisting of heavy isotopes of hydrogen, causing it to implode. At maximum compression, a series of shocks driven into the target are supposed to converge in the center, heating a small “hot spot” to fusion conditions. Unfortunately, such “indirect drive” experiments haven’t worked so far on the NIF. The 1.8 megajoules delivered by NIF’s 192 laser beams haven’t been enough to achieve fusion with current target designs, even though the beams are very clean and uniform, and the facility itself is working as designed. Perhaps the most interesting thing about the Chinese paper is not their novel three axis hohlraum design, but the fact that they are still interested in ICF at all in spite of the failure of the NIF to achieve ignition to date. To the best of my knowledge, they are still planning to build SG-IV, a 1.5 megajoule facility, with ignition experiments slated for the early 2020’s.

Why would the Chinese want to continue building a 1.5 megajoule facility in spite of the fact that U.S. scientists have failed to achieve ignition with the 1.8 megajoule NIF? For the answer, one need only look at who paid for the NIF, and why. The project was paid for by the people at the Department of Energy (DOE) responsible for maintaining the nuclear stockpile. Many of our weapons designers were ambivalent about the value of achieving ignition before the facility was built, and were more interested in the facility’s ability to access physical conditions relevant to those in exploding nuclear weapons for studying key aspects of nuclear weapon physics such as equation of state (EOS) and opacity of materials under extreme conditions. I suspect that’s why the Chinese are pressing ahead as well. Meanwhile, the Russians have also announced a super-laser project of their own that they claim will deliver energies of 2.8 megajoules.

Meanwhile, in the wake of the failed indirect drive experiments on the NIF, scientists in favor of the direct drive approach have been pleading their case. In direct drive experiments the laser beams are shot directly at the fusion target instead of at the inner walls of a hohlraum. The default approach for the NIF has always been indirect drive, but the alternative approach may be possible using an approach called “polar direct drive.” In recent experiments at the OMEGA laser facility at the University of Rochester’s Laboratory for Laser Energetics, the nation’s premier direct drive facility, scientists claim to have achieved results that, if scaled up to energies available on the NIF would produce five times more fusion energy output than has been achieved with indirect drive to date.

Meanwhile, construction continues on ITER, a fusion facility designed purely for energy applications. ITER will rely on magnetic plasma confinement, the other “mainstream” approach to harnessing fusion energy. The project is a white elephant that continues to devour ever increasing amounts of scarce scientific funding in spite of the fact that the chances that magnetic fusion will ever be a viable source of electric power are virtually nil. That fact should be obvious by now, and yet the project staggers forward, seemingly with a life of its own. Watching its progress is something like watching the Titanic’s progress towards the iceberg. Within the last decade the projected cost of ITER has metastasized from the original 6 billion euros to 15 billion euros in 2010, and finally to the latest estimate of 20 billion euros. There are no plans to even fuel the facility for full power fusion until 2035! It boggles the mind.

Magnetic fusion of the type envisioned for ITER will never come close to being an economically competitive source of power. It would already be a stretch if it were merely a question of controlling an unruly plasma and figuring out a viable way to extract the fusion energy. Unfortunately, there’s another problem. Remember all those yarns you’ve been told about how an unlimited supply of fuel is supposed to be on hand in the form of sea water? In fact, reactors like ITER won’t work without a heavy isotope of hydrogen known as tritium. A tritium nucleus contains a proton and two neutrons, and, for all practical purposes, the isotope doesn’t occur in nature, in sea water or anywhere else. It is highly radioactive, with a very short half-life of a bit over 12 years, and the only way to get it is to breed it. We are told that fast neutrons from the fusion reactions will breed sufficient tritium in lithium blankets surrounding the reaction chamber. That may work on paper, but breeding enough of the isotope and then somehow extracting it will be an engineering nightmare. There is virtually no chance that such reactors will ever be economically competitive with renewable power sources combined with baseline power supplied by proven fission breeder reactor technologies. Such reactors can consume most of the long-lived transuranic waste they produce.

In short, ITER should be stopped dead in its tracks and abandoned. It won’t be, because too many reputations and too much money are on the line. It’s too bad. Scientific projects that are far worthier of funding will go begging as a result. At best my descendants will be able to say, “See, my grandpa told you so!”

Fusion Update: The Turn of Direct Drive

Inertial confinement fusion, or ICF, is one of the two “mainstream” approaches to harnessing nuclear fusion in the laboratory. As its name would imply, it involves dumping energy into nuclear material, commonly consisting of heavy isotopes of hydrogen, so fast that its own inertia hold it in place long enough for significant thermonuclear fusion to occur. “Fast” means times on the order of billionths of a second. There are, in turn, two main approaches to supplying the necessary energy; direct drive and indirect drive. In direct drive the “target” of fuel material is hit directly by laser or some other type of energetic beams. In indirect drive, the target is mounted inside of a “can,” referred to as a “hohlraum.” The beams are aimed through holes in the hohlraum at the inner walls. There they are absorbed, producing x-rays, which supply the actual energy to the target.

To date, the only approach used at the biggest ICF experimental facility in the world, the National Ignition Facility, or NIF, at Lawrence Livermore National Laboratory (LLNL), has been indirect drive. So far, it has failed to achieve the goal implied by the facility’s name – ignition – defined as more fusion energy out than laser energy in. A lot of very complex physics goes on inside those cans, and the big computer codes used to predict the outcome of the experiments didn’t include enough of it to be right. They predicted ignition, but LLNL missed it by over a factor of 10. That doesn’t necessarily mean that the indirect drive approach will never work. However, the prospects of that happening are becoming increasingly dim.

Enter direct drive. It has always been the preferred approach at the Naval Research Laboratory and the Laboratory for Laser Energetics (LLE) at the University of Rochester, the latter home of the second biggest laser fusion facility in the world, OMEGA. They lost the debate to the guys at LLNL as the NIF was being built, but still managed to keep a crack open for themselves, in the form of polar direct drive. It would have been too difficult and expensive to configure the NIF beams so that they would be ideal for indirect drive, but could then be moved into a perfectly symmetric arrangement for direct drive. However, by carefully tailoring the length and power in each of the 192 laser beams, and delicately adjusting the thickness of the target at different locations, it is still theoretically possible to get a symmetric implosion. That is the idea behind polar direct drive.

With indirect drive on the ropes, there are signs that direct drive may finally have its turn. One such sign was the recent appearance in the prestigious journal, Physics of Plasmas, of a paper entitled Direct-drive inertial confinement fusion: A review. At the moment it is listed as the “most read” of all the articles to appear in this month’s issue, a feat that is probably beyond the ability of non-experts. The article is more than 100 pages long, and contains no less than 912 references to work by other scientists. However, look at the list of authors. They include familiar direct drive stalwarts like Bob McCrory, John Sethian, and Dave Meyerhofer. However, one can tell which way the wind is blowing by looking at some of the other names. They include some that haven’t been connected so closely with direct drive in the past. Notable among them is Bill Kruer, a star in the ICF business who specializes in theoretical plasma physics, but who works at LLNL, home turf for the indirect drive approach.

Will direct drive ignition experiments happen on the NIF? Not only science, but politics is involved, and not just on Capitol Hill. Money is a factor, as operating the NIF isn’t cheap. There has always been a give and take, or tug of war, if you will, between the weapons guys and the fusion energy guys. It must be kept in mind that the NIF was built primarily to serve the former, and they have not historically always been full of enthusiasm for ignition experiments. There is enough energy in the NIF beams to create conditions sufficiently close to those that occur in nuclear weapons without it. Finally, many in the indirect drive camp are far from being ready to throw in the towel.

In spite of that, some tantalizing signs of a change in direction are starting to turn up. Of course, the “usual suspects” at NRL and LLE continue to publish direct drive papers, but a paper was also just published in the journal High Energy Density Physics entitled, A direct-drive exploding-pusher implosion as the first step in development of a monoenergetic charged-particle backlighting platform at the National Ignition Facility. An exploding pusher target is basically a little glass shell filled with fusion fuel, usually in gaseous form. For various reasons, such targets are incapable of reaching ignition/breakeven. However, they were the type of target used in the first experiments to demonstrate significant fusion via laser implosion at the now defunct KMS Fusion, Inc., back in 1974. According to the paper, all of the NIF’s 192 beams were used to implode such a target, and they were, in fact, tuned for polar direct drive. However, they were “dumbed down” to deliver only a little over 43 kilojoules to the target, only a bit more than two percent of the design limit of 1.8 megajoules! Intriguingly enough, that happens to be just about the same energy that can be delivered by OMEGA. The target was filled with a mixture of deuterium (hydrogen with an extra neutron), and helium 3. Fusion of those two elements produces a highly energetic proton at 14.7 MeV. According to the paper copious amounts of these mono-energetic protons were detected. Ostensibly, the idea was to use the protons as a “backlighter.” In other words, they would be used merely as a diagnostic, shining through some other target to record its behavior at very high densities. That all sounds a bit odd to me. If all 192 beams are used for the backlighter, what’s left to hit the target that’s supposed to be backlighted? My guess is that the real goal here was to try out polar direct drive for later attempts at direct drive ignition.

All I can say is, stay tuned. The guys at General Atomics down in San Diego who make the targets for NIF may already be working on a serious direct drive ignition target for all I know. Regardless, I hope the guys at LLNL manage to pull a rabbit out of their hat and get ignition one way or another. Those “usual suspects” among the authors I mentioned have all been at it for decades now, and are starting to get decidedly long in the tooth. It would be nice if they could finally reach the goal they’ve been chasing for so long before they finally fade out of the picture. Meanwhile, I can but echo the words of Edgar Allan Poe:

Over the Mountains
Of the Moon,
Down the Valley of the Shadow,
Ride, boldly ride,
The shade replied —
If you seek for El Dorado.

Fusion Power Update: Hoping for a Shortcut

Think of a pile of bowling balls in a deep well. They don’t fly out because the force of gravity holds them in. If you roll an extra bowling ball to the edge of the well and let it drop in, energy is released when it hits the pile at the bottom. Atomic nuclei can be compared to the well. The neutrons and protons that make it up are the bowling balls, and the “gravity” is the far more powerful “strong force.” Roll some of these “bowling balls” into the well and energy will be released, just as in a real well. The process is called nuclear fusion, and it’s the source of energy that powers the sun. We’ve been trying to produce energy by repeating the process here on earth for a good many years now, but were only “lucky” enough to succeed in the case of thermonuclear weapons. We’ve been stymied in our efforts to harness fusion energy in less destructive forms. The problem is the Coulomb, or electrostatic force. It’s what causes unlike charges to attract and like charges to repel in the physics experiments you did in high school. It’s much weaker than the strong force that holds the neutrons and protons in an atomic nucleus together, but the strong force has a very short range. The trick is to get within that range. All atomic nuclei contain protons, and protons are positively charged. They repel each other, resisting our efforts to push them up to the edge of the “well,” where the strong force will finally overwhelm the Coulomb force, causing these tiny “bowling balls” to drop in. So far, only atomic bombs have supplied enough energy to provide a “push” big enough to result in a net release of fusion energy.

To date, we’ve tried two main approaches to supplying the “push” in a more controlled form; magnetic fusion and inertial confinement fusion, or ICF. In both approaches the idea is to heat the nuclei to extreme temperatures, causing them to bang into each other with enough energy to overcome the Coulomb repulsion. However, when you dump that much energy into a material, it tends to fly apart, as in a conventional explosion. Somehow a way must be found to hold it in place long enough for significant fusion to take place. In magnetic fusion that’s accomplished with magnetic lines of force that hold the hot nuclei within a confined space. Some will always manage to escape, but if enough are held in place long enough, the resulting fusion reactions will release enough energy to keep the process going. In inertial confinement fusion, as the name would imply, the magnetic fields are replaced by the material’s own inertia. The idea is to supply so much energy in such a short period of time that significant fusion will occur before the material has time to fly apart. That’s essentially what happens in thermonuclear weapons. In ICF the atomic bomb that drives the reaction is replaced by powerful arrays of laser or particle beams.

Both of these approaches are scientifically feasible. In other words, both will almost certainly work if the magnetic fields can be made strong enough, or the laser beams powerful enough. Unfortunately, after decades of effort, we still haven’t managed to reach those thresholds. Our biggest ICF facility, the National Ignition Facility or NIF, has so far failed to achieve “ignition,” defined as fusion energy out equal to laser energy in, by a wide margin. The biggest magnetic fusion facility, ITER, currently under construction in France, may reach the goal, but we’ll have to wait a long time to find out. The last time I looked there were no plans to even fuel it with deuterium and tritium, (D and T, heavy isotopes of hydrogen with one and two neutrons in the nucleus in addition to the usual proton) until 2028! The DT fusion reaction, shown below with some of the others, is the easiest to harness in the laboratory. For reasons I’ve outlined elsewhere, I doubt that either the “conventional” magnetic or inertial confinement approaches will ever produce energy at a cost competitive with the alternatives.

There are, however, other approaches out there. Over the years, startup companies have occasionally managed to attract millions in investment capital to explore these alternatives. Progress reports occasionally turn up on websites such as NextBigFuture. Examples may be found here, here and here, and many others may be found by typing in the search term “fusion” at the website. Typically, they claim they are three or four years away from building a breakeven device, or even a prototype reactor. So far none of them have panned out, but I keep hoping that eventually one of them will pull a rabbit out of their hat and come up with a workable design. The chances are probably slim, but at least marginally better than the odds that someone will perfect a perpetual motion machine.

I tend to be particularly dubious when I see proposals involving fusion fuels other than the usual deuterium and tritium. Other fusion reactions have their advantages. For example, some produce no neutrons, which can pose a radioactive hazard, and/or use fuels other than the highly radioactive tritium, which occurs in nature only in tiny trace amounts, and must therefore be “bred” in the reactor in order to keep the process going. Some of the most promising ones are shown along with the more “mainline” DT and DD reactions below.

D + T → ⁴He (3.5 MeV) + neutron (14.1 MeV)

D + D → T (1.01 MeV) + proton (3.02 MeV) 50%

D + D → ³He (0.82 MeV) + neutron (2.45 MeV) 50%

H + ¹¹B → 3(⁴He); Q = 8.68 MeV

H + ⁶Li → ³He + ⁴He; Q = 4.023 MeV

³He + ⁶Li → H + 2(⁴He); Q = 16.88 MeV

³He + ⁶Li → D + ⁷Be; Q = 0.113 MeV

The problem with the seemingly attractive alternatives to DT shown above as well as a number of others that have been proposed is that they all require significantly higher temperatures and/or confinement times for fusion “ignition” to occur. Take a look at the graph below.

The horizontal axis is in units of the “temperature” of the fuel in thousands of electron volts, and the vertical shows the “cross-section” for any of the reactions shown in units of “barns.” The cross-section is related to the probability that a particular reaction will occur. It is measured in units of 10⁻²⁴ cm², or “barns,” because, at least at the atomic scale, that’s as big as the broad side of a barn. Notice that the DT reaction is much higher at lower temperatures than all the others. Yet we failed to achieve fusion ignition on the NIF with that reaction in spite of the fact that the facility is capable of focusing a massive 1.8 megajoules of laser energy on a fusion target in a period of a few billionths of a second! Obviously, if we couldn’t get DT to work on the NIF, the other reactions will be difficult to harness indeed.

In short, I tend to be dubious when I read the highly optimistic progress reports, complete with “breakthroughs,” of the latest fusion startup. I tend to be a great deal more dubious when they announce they will dispense with DT altogether, as they are so sure of the superior qualities of their design that lithium, boron, or some other exotic fuel will work just as well. Still, I keep my fingers crossed.

Nuclear Update: Molten Salt, Rugby Balls, and the Advanced Hydrodynamic Facility

I hear at 7th or 8th hand that the folks at DOE have been seriously scratching their heads about the possibility of building a demonstration molten salt reactor. They come in various flavors, but the “default” version is a breeder, capable of extracting far more energy from a given quantity of fuel material than current reactors by converting thorium into fissile uranium 233. As they would have a liquid core, the possibility of a meltdown would be eliminated. The copious production of neutrons in such reactors would make it possible to destroy the transuranic actinides, such as americium and curium, and, potentially, also some of the most long-lived radioactive products produced in fission reactions. As a result, the residual radioactivity from running such a reactor for, say, 30 years, would potentially be less than that of the ore from which the fuel was originally extracted in under 500 years, a far cry from the millions commonly cited by anti-nuke alarmists. Such reactors would be particularly attractive for the United States, because we have the largest proven reserves of thorium on the planet. Disadvantages include the fact that uranium 233 is a potential bomb material, and therefore a proliferation concern, and the highly corrosive nature of the fluoride and/or chloride “salts” in the reactor core. More detailed discussions of the advantages and disadvantages may be found here and here.

The chances that the U.S. government will actually provide the funding necessary to build a molten salt or any other kind of advanced reactor are, unfortunately, slim and none. We could do such things in the 50’s and 60’s with alacrity, but those days are long gone, and the country seems to have fallen victim to a form of technological palsy. That’s too bad, because private industry won’t take up the slack. To the extent they’re interested in nuclear at all, the profit motive rules. At the moment, the most profitable way to generate nuclear energy is with reactors that simply burn naturally occurring uranium, wasting the lion’s share of the potential energy content, and generating copious amounts of long-lived radioactive waste for which no rational long term storage solution has yet been devised. In theory, DOE’s national laboratories should be stepping in to take up the slack, doing the things that industry can’t or won’t do. In reality what they do is generate massive stacks of paper studies and reports on advanced systems that have no chance of being built. Enough must have accumulated since the last research reactor was actually built at any of the national labs to stretch back and forth to the moon several times. Oh, well, we can take comfort in the knowledge that at least some people at DOE are thinking about the possibilities.

Moving right along, as most of my readers are aware, the National Ignition Facility, or NIF, did not live up to its name. It failed to achieve inertial confinement fusion (ICF) ignition in the most recent round of experiments, missing that elusive goal by nearly two orders of magnitude. The NIF is a giant, 192 beam laser system at Lawrence Livermore National Laboratory (LLNL) that focuses all of its 1.8 megajoules of laser energy on a tiny target containing deuterium and tritium, two heavy isotopes of hydrogen. Instead of generating energy by splitting or “fissioning” heavy atoms, the goal is to get these light elements to “fuse,” releasing massive amounts of energy.

Actually, the beams don’t hit the target itself. Instead they’re focused through two holes in the ends of a tiny cylinder, known as a hohlraum, that holds a “capsule” of fuel material mounted in its center. It’s what’s known as the indirect drive approach to ICF, as opposed to direct drive, in which the beams are focused directly on a target containing the fuel material. When the beams hit the inside walls of the cylinder they generate a burst of x-rays. These are what actually illuminate the target, causing it to implode to extremely high densities. At just the right moment a “hot spot” is created in the very center of this dense, imploded fuel material, where fusion ignition begins. The fusion reactions create alpha particles, helium nuclei containing two neutrons and two protons, which then smash into the surrounding “cold” fuel material, causing it to ignite as well, resulting in a “burn wave,” which spreads outward, igniting the rest of the fuel. For this to happen, everything has to be just right. The most important thing is that the implosion be almost perfectly symmetric, so that the capsule isn’t squished into a “pancake,” or squashed into a “sausage,” but is very nearly spherical at the point of highest density.

Obviously, everything wasn’t just right in the recently concluded ignition experiments. There are many potential reasons for this. Material blowing off the hohlraum walls could expand into the interior in unforeseen ways, intercepting some of the laser light and/or x-rays, resulting in asymmetric illumination of the capsule. So-called laser/plasma interactions with abstruse names like stimulated Raman scattering, stimulated Brillouin scattering, and two plasmon decay, could be more significant than expected, absorbing laser light so as to prevent symmetric illumination and at the same time generating hot electrons that could potentially preheat the fuel, making it much more difficult to implode and ignite. There are several other potential failure mechanisms, all of which are extremely difficult to model on even the most powerful computers, especially in all three dimensions.

LLNL isn’t throwing in the towel, though. In fact, there are several promising alternatives to indirect drive with cylindrical hohlraums. One that recently showed promise in experiments on the much smaller OMEGA laser system at the University of Rochester’s Laboratory for Laser Energetics (LLE) is substitution of “rugby ball” shaped targets in place of the “traditional” cylinders. According to the paper cited in the link, these “exhibit advantages over cylinders, in terms of temperature and of symmetry control of the capsule implosion.” LLNL could also try hitting the targets with “green” laser light instead of the current “blue.” The laser light is initially “red,” but is currently doubled and then tripled in frequency by passing it through slabs of a special crystal material, shortening its wavelength to the shorter “blue” wavelength, which is absorbed more efficiently. However, each time the wavelength is shortened, energy is lost. If “green” light were used, as much as 4 megajoules of energy could be focused on the target instead of the current maximum of around 1.8. If “green” is absorbed well enough and doesn’t set off excessive laser/plasma interactions, the additional energy just might be enough to do the trick. Other possible approaches include direct drive, hitting the fuel containing target directly with the laser beams, and “fast ignitor,” in which a separate laser beam is used to ignite a hot spot on the outside of the “cold,” imploded fuel material instead of relying on the complicated central hot spot approach.

Regardless of whether ignition is achieved on the NIF or not, it will remain an extremely valuable experimental facility. The reason? Even without ignition it can generate extreme material conditions that are relevant to those that exist in exploding nuclear weapons. As a result, it gives us a significant leg up over other nuclear weapons states in an era of no nuclear testing by enabling us to field experiments relevant to the effects of aging on the weapons in our stockpile, and suggesting ways to insure they remain safe and reliable. Which brings us to the final topic of this post, the Advanced Hydrodynamic Facility, or AHF.

The possibility of building such a beast was actively discussed and studied back in the 90’s, but Google it now and you’ll turn up very little. It would behoove us to start thinking seriously about it again. In modern nuclear weapons, conventional explosives are used to implode a “pit” of fissile material to supercritical conditions. The implosion must be highly symmetric or the pit will “fizzle,” failing to produce enough energy to set off the thermonuclear “secondary” of the weapon that produces most of the yield. The biggest uncertainty we face in maintaining the safety and reliability of our stockpile is the degree to which the possible deterioration of explosives, fusing systems, etc., will impair the implosion of the pit. Basically, an AHF would be a system of massive particle accelerators capable of generating bursts of hard x-rays, or, alternatively, protons, powerful enough to image the implosion of the fission “pit” of a nuclear weapon at multiple points in time and in three dimensions. Currently we have facilities such as the Dual-Axis Radiographic Hydrodynamic Test Facility (DARHT) at Los Alamos National Laboratory (LANL), but it only enables us study the implosion of small samples of surrogate pit material. An AHF would be able to image the implosion of actual pits, with physically similar surrogate materials replacing the fissile material.

Obviously, such experiments could not be conducted in a conventional laboratory. Ideally, the facility would be built at a place like the Nevada Test Site (NTS) north of Las Vegas. Experiments would be fielded underground in the same way that actual nuclear tests were once conducted there. That would have the advantage of keeping us prepared to conduct actual nuclear tests within a reasonably short time if we should ever be forced to do so, for example, by the resumption of testing by other nuclear powers. With an AHF we could be virtually certain that the pits of the weapons in our arsenal will work for an indefinite time into the future. If the pit works, we will also be virtually certain that the secondary will work as well, and the reliability of the weapons in our stockpile will be assured.

Isn’t the AHF just a weaponeer’s wet dream? Why is it really necessary? Mainly because it would remove once and for all any credible argument for the resumption of nuclear testing. Resumption of testing would certainly increase the nuclear danger to mankind, and, IMHO, is to be avoided at all costs. Not everyone in the military and weapons communities agrees. Some are champing at the bit for a resumption of testing. They argue that our stockpile cannot be a reliable deterrent if we are not even sure if our weapons will still work. With an AHF, we can be sure. It’s high time for us to dust off those old studies and give some serious thought to building it.

No Ignition at the National Ignition Facility: A Post Mortem

The National Ignition Facility, or NIF, at Lawrence Livermore National Laboratory (LLNL) in California was designed and built, as its name implies, to achieve fusion ignition. The first experimental campaign intended to achieve that goal, the National Ignition Campaign, or NIC, ended in failure. Scientists at LLNL recently published a paper in the journal Physics of Plasmas outlining, to the best of their knowledge to date, why the experiments failed. Entitled “Radiation hydrodynamics modeling of the highest compression inertial confinement fusion ignition experiment from the National Ignition Campaign,” the paper concedes that,

The recently completed National Ignition Campaign (NIC) on the National Ignition Facility (NIF) showed significant discrepancies between post-shot simulations of implosion performance and experimentally measured performance, particularly in thermonuclear yield.

To understand what went wrong, it’s necessary to know some facts about the fusion process and the nature of scientific attempts to achieve fusion in the laboratory. Here’s the short version: The neutrons and protons in an atomic nucleus are held together by the strong force, which is about 100 times stronger than the electromagnetic force, and operates only over tiny distances measured in femtometers. The average binding energy per nucleon (proton or neutron) due to the strong force is greatest for the elements in the middle of the periodic table, and gradually decreases in the directions of both the lighter and heavier elements. That’s why energy is released by fissioning heavy atoms like uranium into lighter atoms, or fusing light atoms like hydrogen into heavier atoms. Fusion of light elements isn’t easy. Before the strong force that holds atomic nuclei together can take effect, two light nuclei must be brought very close to each other. However, atomic nuclei are all positively charged, and like charges repel. The closer they get, the stronger the repulsion becomes. The sun solves the problem with its crushing gravitational force. On earth, the energy of fission can also provide the necessary force in nuclear weapons. However, concentrating enough energy to accomplish the same thing in the laboratory has proved a great deal more difficult.

The problem is to confine incredibly hot material at sufficiently high densities for a long enough time for significant fusion to take place. At the moment there are two mainstream approaches to solving it: magnetic fusion and inertial confinement fusion, or ICF. In the former, confinement is achieved with powerful magnetic lines of force. That’s the approach at the international ITER fusion reactor project currently under construction in France. In ICF, the idea is to first implode a small target of fuel material to extremely high density, and then heat it to the necessary high temperature so quickly that its own inertia holds it in place long enough for fusion to happen. That’s the approach being pursued at the NIF.

The NIF consists of 192 powerful laser beams, which can concentrate about 1.8 megajoules of light on a tiny spot, delivering all that energy in a time of only a few nanoseconds. It is much larger than the next biggest similar facility, the OMEGA laser system at the Laboratory for Laser Energetics in Rochester, NY, which maxes out at about 40 kilojoules. The NIC experiments were indirect drive experiments, meaning that the lasers weren’t aimed directly at the BB-sized, spherical target, or “capsule,” containing the fuel material (a mixture of deuterium and tritium, two heavy isotopes of hydrogen). Instead, the target was mounted inside of a tiny, cylindrical enclosure known as a hohlraum with the aid of a thin, plastic “tent.” The lasers were fired through holes on each end of the hohlraum, striking the walls of the cylinder, generating a pulse of x-rays. These x-rays then struck the target, ablating material from its surface at high speed. In a manner similar to a rocket exhaust, this drove the remaining target material inward, causing it to implode to extremely high densities, about 40 times heavier than the heaviest naturally occurring elements. As it implodes, the material must be kept as “cold” as possible, because it’s easier to squeeze and compress things that are cold than those that are hot. However, when it reaches maximum density, a way must be found to heat a small fraction of this “cold” material to the very high temperatures needed for significant fusion to occur. This is accomplished by setting off a series of shocks during the implosion process that converge at the center of the target at just the right time, generating the necessary “hot spot.” The resulting fusion reactions release highly energetic alpha particles, which spread out into the surrounding “cold” material, heating it and causing it to fuse as well, in a “burn wave” that propagates outward. “Ignition” occurs when the amount of fusion energy released in this way is equal to the energy in the laser beams that drove the target.

As noted above, things didn’t go as planned. The actual fusion yield achieved in the best experiment was less than that predicted by the best radiation hydrodynamics computer codes available at the time by a factor of about 50, give or take. The LLNL paper in Physics of Plasmas discusses some of the reasons for this, and describes subsequent improvements to the codes that account for some, but not all, of the experimental discrepancies. According to the paper,

Since these simulation studies were completed, experiments have continued on NIF and have identified several important effects – absent in the previous simulations – that have the potential to resolve at least some of the large discrepancies between simulated and experimental yields. Briefly, these effects include larger than anticipated low-mode distortions of the imploded core – due primarily to asymmetries in the x-ray flux incident on the capsule, – a larger than anticipated perturbation to the implosion caused by the thin plastic membrane or “tent” used to support the capsule in the hohlraum prior to the shot, and the presence, in some cases, of larger than expected amounts of ablator material mixed into the hot spot.

In a later section, the LLNL scientists also note,

Since this study was undertaken, some evidence has also arisen suggesting an additional perturbation source other than the three specifically considered here. That is, larger than anticipated fuel pre-heat due to energetic electrons produced from laser-plasma interactions in the hohlraum.

In simple terms, the first of these passages means that the implosions weren’t symmetric enough, and the second means that the fuel may not have been “cold” enough during the implosion process. Any variation from perfectly spherical symmetry during the implosion can rob energy from the central hot spot, allow material to escape before fusion can occur, mix cold fuel material into the hot spot, quenching it, etc., potentially causing the experiment to fail. The asymmetries in the x-ray flux mentioned in the paper mean that the target surface would have been pushed harder in some places than in others, resulting in asymmetries to the implosion itself. A larger than anticipated perturbation due to the “tent” would have seeded instabilities, such as the Rayleigh-Taylor instability. Imagine holding a straw filled with water upside down. Atmospheric pressure will prevent the water from running out. Now imagine filling a perfectly cylindrical bucket with water to the same depth. If you hold it upside down, the atmospheric pressure over the surface of the water is the same. Based on the straw experiment, the water should stay in the bucket, just as it did in the straw. Nevertheless, the water comes pouring out. As they say in the physics business, the straw experiment doesn’t “scale.” The reason for this anomaly is the Rayleigh-Taylor instability. Over such a large surface, small variations from perfect smoothness are gradually amplified, growing to the point that the surface becomes “unstable,” and the water comes splashing out. Another, related instability, the Richtmeyer-Meshkov instability, leads to similar results in material where shocks are present, as in the NIF experiments.

Now, with the benefit of hindsight, it’s interesting to look back at some of the events leading up to the decision to build the NIF. At the time, government used a “key decision” process to approve major proposed projects. The first key decision, known as Key Decision 0, or KD0, was approval to go forward with conceptual design. The second was KD1, approval of engineering design and acquisition. There were more “key decisions” in the process, but after passing KD1, it could safely be assumed that most projects were “in the bag.” In the early 90’s, a federal advisory committee, known as the Inertial Confinement Fusion Advisory Committee, or ICFAC, had been formed to advise the responsible agency, the Department of Energy (DOE), on matters relating to the national ICF program. Among other things, its mandate including advising the government on whether it should proceed with key decisions on the NIF project. The Committee’s advice was normally followed by DOE.

At the time, there were six major “program elements” in the national ICF program. These included the three weapons laboratories, LLNL, Los Alamos National Laboratory (LANL), and Sandia National Laboratories (SNL). The remaining three included the Laboratory for Laser Energetics at the University of Rochester (UR/LLE), the Naval Research Laboratory (NRL), and General Atomics (GA). Spokespersons from all these “program elements” appeared before the ICFAC at a series of meetings in the early 90’s. The critical meeting as far as approval of the decision to pass through KD1 is concerned took place in May 1994. Prior to that time, extensive experimental programs at LLNL’s Nova laser, UR/LLE’s OMEGA, and a host of other facilities had been conducted to address potential uncertainties concerning whether the NIF could achieve ignition. The best computer codes available at the time had modeled proposed ignition targets, and predicted that several different designs would ignite, typically producing “gains,” the ratio of the fusion energy out to the laser energy in, of from 1 to 10. There was just one major fly in the ointment – a brilliant physicist named Steve Bodner, who directed the ICF program at NRL at the time.

Bodner told the ICFAC that the chances of achieving ignition on the NIF were minimal, providing his reasons in the form of a detailed physics analysis. Among other things, he noted that there was no way of controlling the symmetry because of blow-off of material from the hohlraum wall, which could absorb both laser light and x-rays. Ablated material from the capsule itself could also absorb laser and x-ray radiation, again destroying symmetry. He pointed out that codes had raised the possibility of pressure perturbations on the capsule surface due to stagnation of the blow-off material on the hohlraum axis. LLNL’s response was that these problems could be successfully addressed by filling the hohlraum with a gas such as helium, which would hold back the blow-off from the walls and target. Bodner replied that such “solutions” had never really been tested because of the inability to do experiments on Nova with sufficient pulse length. In other words, it was impossible to conduct experiments that would “scale” to the NIF on existing facilities. In building the NIF, we might be passing from the “straw” to the “bucket.” He noted several other areas of major uncertainty with NIF-scale targets, such as the possibility of unaccounted for reflection of the laser light, and the possibility of major perturbations due to so-called laser-plasma instabilities.

In light of these uncertainties, Bodner suggested delaying approval of KD1 for a year or two until these issues could be more carefully studied. At that point, we may have gained the technological confidence to proceed. However, I suspect he knew that two years would never be enough to resolve the issues he had raised. What Bodner really wanted to do was build a much larger facility, known as the Laboratory Microfusion Facility, or LMF. The LMF would have a driver energy of from 5 to 10 megajoules compared to the NIF’s 1.8. It had been seriously discussed in the late 80’s and early 90’s. Potentially, such a facility could be built with Bodner’s favored KrF laser drivers, the kind used on the Nike laser system at NRL, instead of the glass lasers that had been chosen for NIF. It would be powerful enough to erase the physics uncertainties he had raised by “brute force.” Bodner’s proposed approach was plausible and reasonable. It was also a forlorn hope.

Funding for the ICF program had been cut in the early 90’s. Chances of gaining approval for a beast as expensive as LMF were minimal. As a result, it was now officially considered a “follow-on” facility to the NIF. No one took this seriously at the time. Everyone knew that, if NIF failed, there would be no “follow-on.” Bodner knew this, the scientists at the other program elements knew it, and so did the members of the ICFAC. The ICFAC was composed of brilliant scientists. However, none of them had any real insight into the guts of the computer codes that were predicting ignition on the NIF. Still, they had to choose between the results of the big codes, and Bodner’s physical insight bolstered by what were, in comparison, “back of the envelope” calculations. They chose the big codes. With the exception of Tim Coffey, then Director of NRL, they voted to approve passing through KD1 at the May meeting.

In retrospect, Bodner’s objections seem prophetic. The NIC has failed, and he was not far off the mark concerning the reasons for the failure. It’s easy to construe the whole affair as a morality tale, with Bodner playing the role of neglected Cassandra, and the LLNL scientists villains whose overweening technological hubris finally collided with the grim realities of physics. Things aren’t that simple. The LLNL people, not to mention the supporters of NIF from the other program elements, included many responsible and brilliant scientists. They were not as pessimistic as Bodner, but none of them was 100% positive that the NIF would succeed. They decided the risk was warranted, and they may well yet prove to be right.

In the first place, as noted above, chances that an LMF might be substituted for the NIF after another year or two of study were very slim. The funding just wasn’t there. Indeed, the number of laser beams on the NIF itself had been reduced from the originally proposed 240 to 192, at least in part, for that very reason. It was basically a question of the NIF or nothing. Studying the problem to death, now such a typical feature of the culture at our national research laboratories, would have led nowhere. The NIF was never conceived as an energy project, although many scientists preferred to see it in that light. Rather, it was built to serve the national nuclear weapons program. It’s supporters were aware that it would be of great value to that program even if it didn’t achieve ignition. In fact, it is, and is now providing us with a technological advantage that rival nuclear powers can’t match in this post-testing era. Furthermore, LLNL and the other weapons laboratories were up against another problem – what you might call a demographic cliff. The old, testing-era weapons designers were getting decidedly long in the tooth, and it was necessary to find some way to attract new talent. A facility like the NIF, capable of exploring issues in inertial fusion energy, astrophysics, and other non-weapons-related areas of high energy density physics, would certainly help address that problem as well.

Finally, the results of the NIC in no way “proved” that ignition on the NIF is impossible. There are alternatives to the current indirect drive approach with frequency-tripled “blue” laser beams. Much more energy, up to around 4 megajoules, might be available if the known problems of using longer wavelength “green” light can be solved. Thanks to theoretical and experimental work done by the ICF team at UR/LLE under the leadership of Dr. Robert McCrory, the possibility of direct drive experiments on the NIF, hitting the target directly instead of shooting the laser beams into a “hohlraum” can, was also left open, using a so-called “polar” illumination approach. Another possibility is the “fast ignitor” approach to ICF, which would dispense with the need for complicated converging shocks to produce a central “hot spot.” Instead, once the target had achieved maximum density, the hot spot would be created on the outer surface using a separate driver beam.

In other words, while the results of the NIC are disappointing, stay tuned. Pace Dr. Bodner, the scientists at LLNL may yet pull a rabbit out of their hats.

Do We Really Need New Nukes?

If an article that just appeared in the LA Times is any indication, the agitation for jump-starting the nuclear weapons program at the Department of Energy (DOE) and the three nuclear weapons laboratories (Lawrence Livermore, Los Alamos, and Sandia National Laboratories) continues unabated. Entitled “New nuclear weapons needed, many experts say, pointing to aged arsenal,” it cites all the usual talking points of the weaponeers. For example,

Warheads in the nation’s stockpile are an average of 27 years old, which raises serious concerns about their reliability, they say. Provocative nuclear threats by Russian President Vladimir Putin have added to the pressure to not only design new weapons but conduct underground tests for the first time since 1992.

“It seems like common sense to me if you’re trying to keep an aging machine alive that’s well past its design life, then you’re treading on thin ice,” said Rep. Mac Thornberry (R-Texas), chairman-elect of the House Armed Services Committee. “Not to mention, we’re spending more and more to keep these things going.”

Thornbury also offered support for renewed testing, saying, “You don’t know how a car performs unless you turn the key over. Why would we accept anything less from a weapon that provides the foundation for which all our national security is based on?”

Such comments are entirely typical. They would make a lot of sense if the U.S. nuclear weapons program existed in a vacuum. However, it doesn’t. It exists in a world with several other major nuclear powers, and they all have the same problems. Under the circumstances, the fact that such problems exist and are shared by all the nuclear powers is less significant than the question of which nuclear power is best equipped to deal with them. The question of who will benefit by the building of new weapons and a resumption of nuclear testing depends on the answer to that question. If one country has a significant advantage over its rivals in dealing with a common problem as long as the status quo is maintained, then it would be very ill-advised to initiate a change to the status quo that would allow them to catch up. At the moment, the United States is the country with an advantage. As noted in the article,

The U.S. has by far the greatest archive of test data, having conducted 1,032 nuclear tests. Russia conducted 715 and China only 45.

Beyond that, we have the ability to conduct tests with conventional explosives that mimic what goes on in the initial stages of a nuclear explosion, and superb diagnostics to extract a maximum of data from those tests. Perhaps more importantly, we have an unrivaled above ground experimental, or AGEX, capability. I refer to machines like Z at Sandia National Laboratories, or the NIF at Livermore, which are far more capable and powerful than similar facilities anywhere else in the world. Those who say they can’t access physical conditions relevant to those that occur in exploding nuclear weapons, or that they are useless for weapon effects or weapon physics experiments, either don’t know what they’re talking about or are attempting to deceive.

As far as the NIF is concerned, it is quite true that it has so far failed to achieve its fusion ignition milestone, but that by no means rules out the possibility that it ever will. More importantly, the NIF will remain a highly useful AGEX facility whether it achieves ignition or not. Indeed, before it was built, many of the weapons designers showed little interest in ignition. It would merely “muddy the waters,” making it more difficult for the diagnostics to precisely record the results of an experiment. The NIF could access weapons-relevant conditions without it. In fact, in spite of its failure to achieve ignition to date, the NIF has been a spectacular success as far as achieving its specifications are concerned. It is more than an order of magnitude more powerful than any previously existing laser system, its 192 laser beams are highly accurate, and its diagnostic suite is superb.

Another problem with the resumption of testing is that it will lead to the development of weapons that are much more likely to be used. Once the nuclear genie is out of the bottle, it will likely prove very difficult to put it back in. For example, again quoting the article,

John S. Foster Jr., former director of Lawrence Livermore National Laboratory and chief of Pentagon research during the Cold War, said the labs should design, develop and build prototype weapons that may be needed by the military in the future, including a very low-yield nuclear weapon that could be used with precision delivery systems, an electromagnetic pulse weapon that could destroy an enemy’s communications systems and a penetrating weapon to destroy deeply buried targets.

The commonly heard narrative at DOE goes something like this: “We need to develop small, precise, penetrating nuclear weapons because they will be a much better deterrent than the existing ones. Potential enemies are unlikely to believe that we would ever use one of the high yield weapons that are all that remain in the current arsenal. They would be far more likely to believe that we might use a small bunker buster that would minimize the possibility of significant collateral damage.” The problem with that narrative is that it’s true. We would be far more likely to use such a weapon than the ones in the current arsenal, and there would be no lack of voices within DOE and DoD calling for its use if an appropriate opportunity ever arose.

I can understand the agitation for a resumption of testing. It’s a lot sexier to make things that go boom than to serve as custodians for an aging pile of existing nukes. Unfortunately, the latter course is the wiser one. By resuming nuclear testing we would really be unilaterally surrendering a huge advantage, playing into the hands of our enemies and destabilizing the nuclear landscape at the same time.

Comments on Some Comments on the National Ignition Facility

We live in a dauntingly complex world. Progress in the world of science is relevant to all of us, yet it is extremely difficult, although certainly not impossible, for the intelligent layperson to gain a useful understanding of what is actually going on. I say “not impossible” because I believe it’s possible for non-experts to gain enough knowledge to usefully contribute to the conversation about the technological and social relevance of a given scientific specialty, if not of its abstruse details, assuming they are willing to put in the effort. Indeed, when it comes to social relevance it’s not out of the question for them to become more knowledgeable than the scientists themselves, narrowly focused as they often are on a particular specialty.

To illustrate my point, I invite my readers to take a look at a post that recently appeared on the blog LLNL – The True Story. LLNL, or Lawrence Livermore National Laboratory, is one of the nation’s three major nuclear weapons research laboratories. It is also home of the National Ignition Facility, which, as its name implies, was designed to achieve fusion “ignition” by focusing a giant assembly of 192 powerful laser beams on tiny targets containing a mixture of deuterium and tritium fuel. The process itself is called inertial confinement fusion, or ICF. Ignition is variously defined, but as far as the NIF is concerned LLNL officially accepted the definition as fusion energy out equal to total laser energy in, in the presence of members of a National Academy of Sciences oversight committee. This is a definition that puts it on a level playing field with the competing magnetic confinement approach to fusion.

According to the blurb that appears on the home page of LLNL – The True Story, its purpose is “for LLNL present and past employees, friends of LLNL and anyone impacted by the privatization of the Lab to express their opinions and expose the waste, wrongdoing and any kind of injustice against employees and taxpayers by LLNS/DOE/NNSA.” The post in question is entitled ICF Program is now Officially Owned by WCI (Weapons and Concepts Integration). It’s certainly harmless enough as it stands, consisting only of the line,

ICF program is now officially owned by WCI. A step forward or an attempt to bury it out of sight?

This is followed by an apparently broken link to the story referred to. This gist can probably be found here. Presumably the author suspects LLNL might want to “bury it out of sight” because the first attempt to achieve ignition, known as the National Ignition Campaign, or NIC, failed to achieve its goal. What’s really of interest is not the post itself, but the comments following it. The commenters are all listed as “anonymous,” but given the nature of the blog we can probably assume that most of them are scientists of one tribe or another. Let’s take a look at what they have to say. According to the first “anonymous,”

If (takeover of NIF by WCI) is an attempt to keep funding flowing by switching milestones from energy independence to weapons research. “Contingency Plan B”.

Another “anonymous” writes in a similar vein:

Reading between the lines it is clear that the new energy source mission of the NIF is over and now it’s time to justify the unjustifiable costs by claiming it’s a great too for weapons research.

Perhaps the second commenter would have done better to read the lines as they stand rather than between them. In that case he would have noticed that energy independence was never an official NIF milestone, not to mention its “mission.” NIF was funded for the purpose of weapons research from the start. This fact was never in any way a deep, dark secret, and has long been obvious to anyone willing to take the trouble to consult the relevant publicly accessible documents. The Inertial Confinement Fusion Advisory Committee, a Federal Advisory Committee that met intermittently in the early to mid-90’s, and whose member included a bevy of heavyweights in plasma physics and related specialties, was certainly aware of the fact, and recommended funding of the facility with the single dissenting vote of Tim Coffey, then Director of the Naval Research Laboratory, based on that awareness.

Be that as it may, the claim that the technology could also end our dependence on fossil fuel, often made by the NIF’s defenders, is credible. By “credible” I mean that many highly capable scientists have long held and continue to hold that opinion. As it happens, I don’t. Assuming we find a way to achieve ignition and high gain in the laboratory, it will certainly become scientifically feasible to generate energy with ICF power plants. However, IMHO it will never be economically feasible, for reasons I have outline in earlier posts. Regardless, from a public relations standpoint, it was obviously preferable to evoke the potential of the NIF as a clean source of energy rather than a weapons project designed to maintain the safety and reliability of our nuclear arsenal, as essential as that capability may actually be. In spite of my own personal opinion on the subject, these claims were neither disingenuous nor mere “hype.”

Another “anonymous” writes,

What’s this user facility bullshit about? Only Livermore uses the facility. Cost recovery demands that a university would have to pay $1 million for a shot. How can it be a user facility if it’s run by the weapons program? This isn’t exactly SLAC we’re talking about.

Here, again, the commenter is simply wrong. Livermore is not the only user of NIF, and it is, in fact, a user facility. Users to date include a team from MIT headed by Prof. Richard Petrasso. I’m not sure how the users are currently funded, but in the past funds for experiments on similar facilities were allocated through a proposal process, similar to that used to fund other government-funded academic research. The commenter continues,

By the way, let’s assume NIF wants to be a “user facility” for stockpile stewardship. Since ignition is impossible, the EOS (Equation of State, relevant to the physics of nuclear weapons, ed.) work is garbage, and the temperatures are not relevant to anything that goes bang, what use is this machine?

NIF does not “want to be a user facility for stockpile stewardship.” Stress has always been on high energy density physics (HEDP), which has many other potential applications besides stockpile stewardship. I was not surprised that NIF did not achieve ignition immediately. In fact I predicted as much in a post on this blog two years before the facility became operational. However, many highly competent scientists disagreed with me, and for credible scientific reasons. The idea that ignition is “impossible” just because it wasn’t achieved in the first ignition campaign using the indirect drive approach is nonsense. Several other credible approaches have not yet even been tried, including polar direct drive, fast ignitor, and hitting the targets with green (frequency doubled) rather than blue (frequency tripled) light. The latter approach would enable a substantial increase in the available laser energy on target. The EOS work is not garbage, as any competent weapons designer will confirm as long as they are not determined to force the resumption of nuclear testing by hook or by crook, and some of the best scientists at Livermore confirmed long ago that the temperatures achievable on the NIF are indeed relevant to things that go bang, whether it achieves ignition or not. In fact, the facility allows us to access physical conditions that can be approached in the laboratory nowhere else on earth, giving us a significant leg up over the international competition in maintaining a safe and reliable arsenal, as long as testing is not resumed.

Anonymous number 4 chimes in,

I love this quote (apparently from the linked article, ed.):

“the demonstration of laboratory ignition and its use to support the Stockpile Stewardship Program (SSP) is a major goal for this program”

Hey guys, this has already failed. Why are we still spending money on this? A lot of other laboratories could use the $$. You’re done.

The quote this “anonymous” loves is a simple statement of fact. For the reasons already cited, the idea that ignition on the NIF is hopeless is nonsense. The (very good) reason we’re still spending money on the project is that NIF is and will continue into the foreseeable future to be one of the most capable and effective above ground experimental (AGEX) facilities in the world. It can access physical conditions relevant to nuclear weapons regardless of whether it achieves ignition or not. For that reason it is an invaluable tool for maintaining our arsenal unless one’s agenda happens to be the resumption of nuclear testing. Hint: The idea that no one in DOE, NNSA, or the national weapons laboratories wants to resume testing belongs in the realm of fantasy. Consider, for example, what the next “anonymous” is actually suggesting:

Attempting to get funding for NIF and computations’s big machines was made easier by claiming dual purposes but I always felt that the real down and dirty main purpose was weapons research. If you want to get support from the anti-weapon Feinstein/Boxer/Pelosi contingent you need to put the “energy” lipstick on the pig. Or we could go back to testing. Our cessation of testing doesn’t seem to have deterred North Korea and Iran that much.

Yes, Virginia, even scientists occasionally do have agendas of their own. What can I say? To begin, I suppose, that one should never be intimidated by the pontifications of scientists. The specimens on display here clearly don’t have a clue what they’re talking about. Any non-technical observer of middling intelligence could become more knowledgeable than they are on the topics they’re discussing by devoting a few hours to researching them on the web. As to how the non-technical observer is to acquire enough knowledge to actually know that he knows more than the scientific specialists, I can offer no advice, other than to head to your local university and acquire a Ph.D. I am, BTW, neither employed by nor connected in any other way with LLNL.

Latest from the National Ignition Facility: More Neutrons, Less Hope

A paper with some recent ignition experiment results from the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) in California just turned up at Physical Review Letters. The good news is that they’re seeing more of the neutrons that are released in fusion reactions than ever before, and the yield is in good agreement with computer code predictions. The bad news is that they improved things by doing something that’s supposed to make things worse. Specifically, they increasing the energy in the laser “foot” pulse that’s supposed to get the target implosion started.

The NIF was designed to achieve ignition via inertial confinement fusion (ICF), a process in which the fuel material is compressed and heated to fusion conditions so quickly that its own inertia holds it in place long enough for significant fusion to occur. Scientists at LLNL are currently using “indirect drive” in their experiments. In other words, instead of hitting the BB-sized target directly, they mount it inside of a tiny, cylindrical can, or “hohlraum,” with holes at each end for the laser beams to pass through. When the beams hit the inside of the hohlraum, they produce a powerful pulse of x-rays, which then hit the target, imploding it to extremely high density. It’s harder to squeeze and implode hot objects than cold ones, so the laser beams are tailored to keep the target as “cold” as possible during the implosion process. However, the fuel material must be very hot for fusion to occur. According to theory, this can be achieved by launching a series of shocks into the imploding target, which must converge in the center at the moment of greatest compression, creating a central “hot spot.” When fusion reactions start in the hot spot, they produce highly energetic helium atom nuclei (alpha particles), which then slam into the surrounding, still cold, fuel material, heating it to fusion conditions, producing more alpha particles, resulting in an alpha-driven “burn wave,” which moves out through the target, consuming the fuel.

So far, it hasn’t worked. Apparently, hydrodynamic instabilities, such as the Rayleigh-Taylor and Richtmyer-Meshkov instabilities, are a big part of the problem. They amplify tiny target surface imperfections during the implosion process, destroying the symmetry of the implosion, and quenching the fusion process. There are some interesting simulations of the Rayleigh-Taylor instability on Youtube. In the latest experiments, the LLNL team managed to control the growth of instabilities by using a bigger target “aspect ratio,” that is, increasing the thickness of the outer shell compared to the target radius, and driving it by dumping more energy into the “foot” pulse. As a result, they drove the implosion process along a higher “adiabat,” which basically means that the fuel was hotter during the implosion. Of course, absent instabilities, making the fuel hotter during the implosion is exactly what you don’t want to do. In spite of that, LLNL is seeing more neutrons.

What this all boils down to is that LLNL has confirmed that the NIF has a big, potentially fatal problem with hydrodynamic instabilities using the current indirect drive approach to fusion ignition. That doesn’t mean the situation is hopeless. There are other approaches. Examples include direct drive, in which the laser beams are aimed directly at the target, and fast ignitor, in which the cold, compressed fuel material is ignited on the outside, by another laser beam designed specifically for that purpose, rather than in a central “hot spot.” In fact, the biggest potential problem here is probably more political than scientific. You certainly have to get ignition if you plan to use inertial fusion as a source of energy, but, in spite of occasional hype to the contrary, the NIF was never intended as an energy project. It was funded to support the weapons program in general, and to insure the continuing safety and reliability of the weapons in our arsenal in the absence of nuclear testing in particular. It can do that extremely well, whether we get ignition or not. The politicians whose support is needed to fund continued operation of the project need to realize that.

Regardless of whether it achieves ignition or not, the NIF is performing as well as or better than its design specs called for. The symmetry and synchronization of its 192 laser beams is outstanding, and it has a remarkable and highly capable suite of diagnostics for recording what happens during the experiments. The NIF can dump so much energy in a small space in a short time that it can generate physical conditions that can be reproduced in the laboratory no where else on earth. Those conditions approach those that occur inside of an exploding nuclear device. As a result, such experimental facilities give us a major leg up on the competition as long as there is no resumption of nuclear testing. In other words, with the NIF and facilities like it we have a strong, positive incentive not to resume testing, potentially losing our advantage. Without such facilities, the pressure to resume testing may become irresistible. It’s really an easy choice.

The simulation of the Rayleigh-Taylor instability below was done by Frederik Brasz.