fusion energy – Helian Unbound

Fisking a Fusion Fata Morgana

Why is it that popular science articles about fusion energy are always so cringe-worthy? Is scientific illiteracy a prerequisite for writing them? Take the latest one to hit the streets, for example. Entitled Lockheed Martin Now Has a Patent For Its Potentially World Changing Fusion Reactor, it had all the familiar “unlimited energy is just around the corner” hubris we’ve come to expect in articles about fusion. When I finished reading it I wondered whether the author imagined all that nonsense on his own, or some devilish plasma physicist put him up to it as a practical joke. The fun starts in the first paragraph, where we are assured that,

If this project has been progressing on schedule, the company could debut a prototype system that size of shipping container, but capable of powering a Nimitz-class aircraft carrier or 80,000 homes, sometime in the next year or so.

Trust me, dear reader, barring divine intervention no such prototype system, capable of both generating electric energy and fitting within a volume anywhere near that of a shipping container, will debut in the next year, or the next five years, or the next ten years. Reading on, we learn that,

Unlike in nuclear fission, where atoms hit each other release energy, a fusion reaction involves heating up a gaseous fuel to the point where its atomic structure gets disrupted from the pressure and some of the particles fuse into a heavier nucleus.

Well, not really. Fission is caused by free neutrons, not by “atoms hitting each other.” It would actually be more accurate to say that fusion takes place when “atoms hit each other,” although it’s really the atomic nuclei that “hit” each other. Fusion doesn’t involve “atomic structure getting disrupted from pressure.” Rather, it happens when atoms acquire enough energy to overcome the Coulomb repulsion between two positively charged atomic nuclei (remember, like charges repel), and come within a sufficiently short distance of each other for the much greater strong nuclear force of attraction to take over. According to the author,

But to do this you need to be able to hold the gas, which is eventually in a highly energized plasma state, for a protracted period of time at a temperature of hundreds of millions of degrees Fahrenheit.

This is like claiming that a solid can be in a liquid state. A plasma is not a gas. It is a fourth state of matter quite unlike the three (solid, liquid, gas) that most of us are familiar with. Shortly thereafter we are assured that,

Running on approximately 25 pounds of fuel – a mixture of hydrogen isotopes deuterium and tritium – Lockheed Martin estimated the notional reactor would be able to run for an entire year without stopping. The device would be able to generate a constant 100 megawatts of power during that period.

25 pounds of fuel would include about 15 pounds of tritium, a radioactive isotope of hydrogen with a half-life of just over 12 years. In other words, its atoms decay about 2000 times faster than those of the plutonium 239 found in nuclear weapons. It’s true that the beta particle (electron) emitted in tritium decay is quite low energy by nuclear standards but, as noted in Wiki, “Tritium is an isotope of hydrogen, which allows it to readily bind to hydroxyl radicals, forming tritiated water (HTO), and to carbon atoms. Since tritium is a low energy beta emitter, it is not dangerous externally (its beta particles are unable to penetrate the skin), but it can be a radiation hazard when inhaled, ingested via food or water, or absorbed through the skin.” Obviously, water and many carbon compounds can be easily inhaled or ingested. Tritium is anything but benign if released into the environment. Here we will charitably assume that the author didn’t mean to say that 25 pounds of fuel would be available all at once, but would be bred gradually and then consumed as fuel in the reactor during operation. The amount present at any given time would more appropriately be measured in grams than in pounds. The article continues with rosy scenarios that might have been lifted from a “Back to the Future” movie:

Those same benefits could apply to vehicles on land, ships at sea, or craft in space, providing nearly unlimited power in compact form allowing for operations across large areas, effectively eliminating the tyranny of distance in many cases. Again, for military applications, unmanned ground vehicles or ships could patrol indefinitely far removed from traditional logistics chains and satellites could conduct long-term, resource intensive activities without the need for large and potentially dangerous fission reactors.

Great shades of “Dr. Fusion!” Let’s just say that “vehicles on land” is a bit of a stretch. I can only hope that no Lockheed engineer was mean-spirited enough to feed the author such nonsense. Moving right along, we read,

Therein lies perhaps the biggest potential benefits of nuclear fusion over fission. It’s produces no emissions dangerous to the ozone layer and if the system fails it doesn’t pose nearly the same threat of a large scale radiological incident. Both deuterium and tritium are commonly found in a number of regular commercial applications and are relatively harmless in low doses.

I have no idea what “emission” of the fission process the author thinks is “dangerous to the ozone layer.” Again, as noted above, tritium is anything but “relatively harmless” if ingested. Next we find perhaps the worst piece of disinformation of all:

And since a fusion reactor doesn’t need refined fissile material, its much harder for it to serve as a starting place for a nuclear weapons program.

Good grief, the highly energetic neutrons produced in a fusion reactor are not only capable of breeding tritium, but plutonium 239 and uranium 233 from naturally occurring uranium and thorium as well. Both are superb explosive fuels for nuclear weapons. And tritium? It is used in a process known as “boosting” to improve the performance of nuclear weapons. Finally, we run into what might be called the Achilles heel of all tritium-based fusion reactor designs:

Fuel would also be abundant and relatively easy to source, since sea water provides a nearly unlimited source of deuterium, while there are ready sources of lithium to provide the starting place for scientists to “breed” tritium.

I think not. Breeding tritium will be anything but a piece of cake. The process will involve capturing the neutrons produced by the fusion reactions in a lithium blanket surrounding the reactor, doing so efficiently enough to generate more tritium from the resulting reactions than the reactor consumes as fuel, and then extracting the tritium and recycling it into the reactor without releasing any of the slippery stuff into the environment. Do you think the same caliber of engineers who brought us Chernobyl, Fukushima, and Three Mile Island will be able to pull that rabbit out of their hats without a hitch? If so, you’re more optimistic than I am.

Hey, I like to be as optimistic about fusion as it’s reasonable to be. I think it’s certainly possible that some startup company with a bright idea will find the magic bullet that makes fusion reactors feasible, preferably involving fusion reactions that don’t involve tritium. It’s also quite possible that the guys at Lockheed will achieve breakeven, although getting a high enough gain of energy in versus energy out to enable efficient generation of electric power is another matter. There’s a difference between optimism and scientifically illiterate hubris, though. Is it too much to ask that people who write articles about fusion at least run them by somebody who actually knows something about the subject to see if they pass the “ho, ho” test before publishing? What’s that you say? What about me? Please read the story about the Little Red Hen.

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.

Another Fusion White Elephant Sighted in Germany

According to an article that just appeared in Science magazine, scientists in Germany have completed building a stellarator by the name of Wendelstein 7-X (W7-X), and are seeking regulatory permission to turn the facility on in November. If you can’t get past the Science paywall, here’s an article in the popular media with some links. Like the much bigger ITER facility now under construction at Cadarache in France, W7-X is a magnetic fusion device. In other words, its goal is to confine a plasma of heavy hydrogen isotopes at temperatures much hotter than the center of the sun with powerful magnetic fields in order to get them to fuse, releasing energy in the process. There are significant differences between stellarators and the tokamak design used for ITER, but in both approaches the idea is to hold the plasma in place long enough to get significantly more fusion energy out than was necessary to confine and heat the plasma. Both approaches are probably scientifically feasible. Both are also white elephants, and a waste of scarce research dollars.

The problem is that both designs have an Achilles heel. Its name is tritium. Tritium is a heavy isotope of hydrogen with a nucleus containing a proton and two neutrons instead of the usual lone proton. Fusion reactions between tritium and deuterium, another heavy isotope of hydrogen with a single neutron in addition to the usual proton, begin to occur fast enough to be attractive as an energy source at plasma temperatures and densities much less than would be necessary for any alternative reaction. The deuterium-tritium, or DT, reaction will remain the only feasible one for both stellarator and tokamak fusion reactors for the foreseeable future. Unfortunately, tritium occurs in nature in only tiny trace amounts.

The question is, then, where do you get the tritium fuel to keep the fusion reactions going? Well, in addition to a helium nucleus, the DT fusion reaction produces a fast neutron. These can react with lithium to produce tritium. If a lithium-containing blanket could be built surrounding the reaction chamber in such a way as to avoid interfering with the magnetic fields, and yet thick enough and close enough to capture enough of the neutrons, then it should be possible to generate enough tritium to replace that burned up in the fusion process. It sounds complicated but, again, it appears to be at least scientifically feasible. However, it is by no means as certain that it is economically feasible.

Consider what we’re dealing with here. Tritium is an extremely slippery material that can pass right through walls of some types of metal. It is also highly radioactive, with a half-life of about 12.3 years. It will be necessary to find some way to efficiently extract it from the lithium blanket, allowing none of it to leak into the surrounding environment. If any of it gets away, it will be easily detectable. The neighbors are sure to complain and, probably, lawyer up. Again, all this might be doable. The problem is that it will never be doable at a low enough cost to make fusion reactor designs based on these approaches even remotely economically competitive with the non-fossil alternative sources of energy that will be available for, at the very least, the next several centuries.

What’s that? Reactor design studies by large and prestigious universities and corporations have all come to the conclusion that these magnetic fusion beasts will be able to produce electricity at least as cheaply as the competition? I don’t think so. I’ve participated in just such a government-funded study, conducted by a major corporation as prime contractor, with several other prominent universities and corporations participating as subcontractors. I’m familiar with the methodology used in several others. In general, it’s possible to make the cost electricity come out at whatever figure you choose, within reason, using the most approved methods and the most sound project management and financial software. If the government is funding the work, it can be safely assumed that they don’t want to hear something like, “Fuggedaboudit, this thing will be way too expensive to build and run.” That would make the office that funded the work look silly, and the fusion researchers involved in the design look like welfare queens in white coats. The “right” cost numbers will always come out of these studies in the end.

I submit that a better way to come up with a cost estimate is to use a little common sense. Do you really think that a commercial power company will be able to master the intricacies of tritium production and extraction from the vicinity of a highly radioactive reaction chamber at anywhere near the cost of, say, wind and solar combined with next generation nuclear reactors for baseload power? If you do, you’re a great deal more optimistic than me. W7-X cost a billion euros. ITER is slated to cost 13 billion, and will likely come in at well over that. With research money hard to come by in Europe for much worthier projects, throwing amounts like that down a rat hole doesn’t seem like a good plan.

All this may come as a disappointment to fusion enthusiasts. On the other hand, you may want to consider the fact that, if fusion had been easy, we would probably have managed to blow ourselves up with pure fusion weapons by now. Beyond that, you never know when some obscure genius might succeed in pulling a rabbit out of their hat in the form of some novel confinement scheme. Several companies claim they have sure-fire approaches that are so good they will be able to dispense with tritium entirely in favor of more plentiful, naturally occurring isotopes. See, for example, here, here, and here, and the summary at the Next Big Future website. I’m not optimistic about any of them, either, but you never know.

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.

Fusion Follies at Der Spiegel

Who says there’s no such thing as German humor? Take, for example, some of the comments left by Teutonic wags after an article about the recent fusion “breakthrough” reported by scientists at Lawrence Livermore National Laboratory working on the National Ignition Facility (NIF). One of the first was left by one of Germany’s famous “Greens,” who was worried about the long term effects of fusion energy. Very long term. Here’s what he had to say:

So nuclear fusion is green energy, is it? The opposite is true. Nuclear fusion is the form of energy that guarantees that any form of Green will be forever out of the question. In comparison, Chernobyl is a short-lived joke! Why? Have you ever actually considered what will be “burned” with fusion energy? Hydrogen, one of the two components of water, (and a material without which life is simply impossible)! Nuclear fusion? I can already see the wars over water coming. And, by the way, the process is irreversible. Once hydrogen is fused, it’s gone forever. Nothing and no one will ever be able to make water out of it ever again!

I’m not kidding! The guy was dead serious. Of course, this drew a multitude of comments from typical German Besserwisser (better knowers), such as, “If you don’t have a clue, you should shut your trap.” However, some of the other commenters were more light-hearted. for example,

No, no, no. What eu-fan (the first commenter) doesn’t seem to understand is that this should be seen as a measure against the rise in sea level that will result from global warming. Less hydrogen -> less water -> reduced sea level -> everything will be OK.

Another hopeful commenter adds,

…if it ever actually does succeed, this green fusion, can we have our old-fashioned light bulbs back?

Noting that the fusion of hydrogen produces helium, another commenter chimes in,

So, in other words, if a fusion reactor blows up, the result will be a global bird cage: The helium released will make us all talk like Mickey Mouse!

In all seriousness, the article in Der Spiegel about the “breakthrough” wasn’t at all bad. The author actually bothered to ask a local fusion expert, Sibylle Günter, Scientific Director of the Max Planck Institute for Plasma Physics, about Livermore’s “breakthrough.” She replied,

The success of our colleagues (at Livermore) is remarkable, and I don’t want to belittle it. However, when one speaks of a “breakeven point” in the classical sense, in which the fusion energy out equals the total energy in, they still have a long way to go.

That, of course, is entirely true. The only way one can speak of a “breakthough” in the recent NIF experiments is by dumbing down the accepted definition of “ignition” from “fusion energy out equals laser energy in” to “fusion energy out equals energy absorbed by the target,” a much lower amount. That didn’t deter many writers of English-language reports, who couldn’t be troubled to fact check Livermore’s claims with the likes of Dr. Günter. In some cases the level of fusion wowserism was extreme. For example, according to the account at Yahoo News,

After fifty years of research, scientists at the National Ignition Facility (NIF) in Livermore, have made a breakthrough in harnessing and controlling fusion.

and,

According to the BBC, NIF conducted an experiment where the amount of energy released through the fusion reaction was more than the amount of energy being absorbed by it. This process is known as “ignition” and is the first time it has successfully been done anywhere in the world.

I’m afraid not. The definition of “ignition” that has been explicitly accepted by scientists at Livermore is “fusion energy out equals laser energy in.” That definition puts them on a level playing field with their magnetic fusion competitors. It’s hardly out of the question that the NIF will reach that goal, but it isn’t there yet. Not by a long shot.

Another Fusion Tease?

It has always seemed plausible to me that some clever scientist(s) might find a shortcut to fusion that would finally usher in the age of fusion energy, rendering the two “mainstream” approaches, inertial confinement fusion (ICF) and magnetic fusion, obsolete in the process. It would be nice if it happened sooner rather than later, if only to put a stop to the ITER madness. For those unfamiliar with the field, the International Thermonuclear Experimental Reactor, or ITER, is a gigantic, hopeless, and incredibly expensive white elephant and welfare project for fusion scientists currently being built in France. In terms of pure, unabashed wastefulness, think of it as a clone of the International Space Station. It has always been peddled as a future source of inexhaustible energy. Trust me, nothing like ITER will ever be economically competitive with alternative energy sources. Forget all your platitudes about naysayers and “they said it couldn’t be done.” If you don’t believe me, leave a note to your descendants to fact check me 200 years from now. They can write a gloating refutation to my blog if I’m wrong, but I doubt that it will be necessary.

In any case, candidates for the hoped for end run around magnetic and ICF keep turning up, all decked out in the appropriate hype. So far, at least, none of them has ever panned out. Enter two stage laser fusion, the latest pretender, introduced over at NextBigFuture with the assurance that it can achieve “10x higher fusion output than using the laser directly and thousands of times better output than hitting a solid target with a laser.” Not only that, but it actually achieved the fusion of boron and normal hydrogen nuclei, which produces only stable helium atoms. That’s much harder to achieve than the usual deuterium-tritium fusion between two heavy isotopes of hydrogen, one of which, tritium, is radioactive and found only in tiny traces in nature. That means it wouldn’t be necessary to breed tritium from the fusion reactions just to keep them going, one of the reasons that ITER will never be practical.

Well, I’d love to believe this is finally the ONE, but I’m not so sure. The paper describing the results NBF refers to was published by the journal Nature Communications. Even if you don’t subscribe, you can click on the figures in the abstract and get the gist of what’s going on. In the first place, one of the lasers has to accelerate protons to high enough energies to overcome the Coulomb repulsion of the stripped (of electrons) boron nuclei produced by the other laser. Such laser particle accelerators are certainly practical, but they only work at extremely high power levels. In other words, they require what’s known in the business as petawatt lasers, capable of achieving powers in excess of a quadrillion (10 to the 15th power) watts. Power comes in units of energy per unit time, and such lasers generally reach the petawatt threshold by producing a lot of energy in a very, very short time. Often, we’re talking picoseconds (trillionths of a second).

Now, you can do really, really cool things with petawatt lasers, such as pulling electron positron pairs right out of the vacuum. However, their practicality as drivers for fusion power plants, at least in their current incarnation, is virtually nil. The few currently available, for example, at the University of Rochester’s Laboratory for Laser Energetics, the University of Texas at Austin, the University of Nevada at Reno, etc., are glass lasers. There’s no way they could achieve the “rep rates” (shot frequency) necessary for useful energy generation. Achieving lots of fusions, but only for a few picoseconds, isn’t going to solve the world’s energy problems.

As it happens, conventional accelerators can also be used for fusion. As a matter of fact, it’s a common way of generating neutrons for such purposes as neutron radiography. Unfortunately, none of the many fancy accelerator-driven schemes for producing energy that people have come up with over the years has ever worked. There’s a good physical reason for that. Instead of using their energy to overcome the Coulomb repulsion of other nuclei (like charges repel, and atomic nuclei are all positively charged), and fuse with them, the accelerated particles prefer to uselessly dump that energy into the electrons surrounding those nuclei. As a result, it has always taken more energy to drive the accelerators than could be generated in the fusion reactions. That’s where the “clever” part of this scheme comes in. In theory, at least, all those pesky electrons are gone, swept away by the second laser. However, that, too, is an energy drain. So the question becomes, can both lasers be run efficiently enough and with high enough rep rates and with enough energy output to strip enough boron atoms to get enough of energy out to be worth bothering about, in amounts greater than that needed to drive the lasers? I don’t think so. Still, it was a very cool experiment.

Fusion Update: The NIF Inches Closer to Ignition

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.

The NIF: Lots of Power and Energy, but No Ignition

According to a recent press release from Lawrence Livermore National Laboratory (LLNL) in California, the 192-beam National Ignition Facility (NIF) fired a 500 terawatt shot on July 5. The world record power followed a world record energy shot of 1.89 Megajoules on July 3. As news, this doesn’t rise above the “meh” category. A shot at the NIF’s design energy of 1.8 Megajoules was already recorded back in March. It’s quite true that, as NIF Director Ed Moses puts it, “NIF is becoming everything scientists planned when it was conceived over two decades ago.” The NIF is a remarkable achievement in its own right, capable of achieving energies 50 times greater than any other laboratory facility, with pulses shaped and timed to pinpoint precision. The NIF team in general and Ed Moses in particular deserve great credit, and the nation’s gratitude, for that achievement after turning things around following a very shaky start.

The problem is that, while the facility works as well, and even better than planned, the goal it was built to achieve continues to elude us. As its name implies, the news everyone is actually waiting for is the announcement that ignition (defined as fusion energy out greater than laser energy in) has been achieved. As noted in the article, Moses said back in March that “We have all the capability to make it happen in fiscal year 2012.” At this point, he probably wishes his tone had been a mite less optimistic. To reach their goal in the two months remaining, the NIF team will need to pull a rabbit out of their collective hat. A slim chance remains. Apparently the NIF’s 192 laser beams were aimed at a real ignition target with a depleted uranium capsule and deuterium-tritium fuel on July 5, and not a surrogate. The data from that shot may prove to be a great deal more interesting than the 500 terawatt power announcement.

Meanwhile, the Russians are apparently forging ahead with plans for their own superlaser, to be capable of a whopping 2.8 Megajoules, and the Chinese are planning another about half that size, to be operational at about the same time (around 2020). That, in itself, speaks volumes about the real significance of ignition. It may be huge for the fusion energy community, but not that great as far as the weaponeers who actually fund these projects are concerned. Many weapons designers at LLNL and Los Alamos were notably unenthusiastic about ignition when NIF was still in the planning stages. What attracted them more was the extreme conditions, approaching those in an exploding nuke, that could be achieved by the lasers without ignition. They thought, not without reason, that it would be much easier to collect useful information from such experiments than from chaotic ignition plasmas. Apparently the Russian bomb designers agree. They announced their laser project back in February even though LLNL’s difficulties in achieving ignition were well known at the time.

The same can be said of some of the academic types in the NIF “user community.” It’s noteworthy that two of them, Rick Petrasso of MIT and Ray Jeanloz of UC Berkeley, whose enthusiastic comments about the 500 terawatt shot where quoted in the latest press release, are both key players in the field of high energy density physics. Ignition isn’t a sine qua non for them either. They will be able to harvest scores of papers from the NIF whether it achieves ignition or not.

The greatest liability of not achieving early ignition may be the evaporation of political support for the NIF. The natives are already becoming restless. As noted in the Livermore Independent,

In early May, sounding as if it were discussing an engineering project rather than advanced research, the House Appropriations Committee worried that NIF’s “considerable costs will not have been warranted” if it does not achieve ignition by September 30, the end of the federal fiscal year.

and,

Later that month, in a tone that seemed to demand that research breakthroughs take place according to schedule, the House Armed Services Committee recommended that NIF’s ignition research budget for next year be cut by $30 million from the requested $84 million budget unless NIF achieves ignition by September 30.

Funding cuts at this point, after we have come so far, and are so close to the goal, would be short-sighted indeed. One must hope that a Congress capable of squandering billions on white elephants like the International Space Station will not become penny-wise and pound-foolish about funding a project that really matters.