Inertial confinement fusion, or ICF, is one of the two “mainstream” approaches to harnessing nuclear fusion in the laboratory.\u00a0 As its name would imply, it involves\u00a0dumping energy into nuclear material, commonly consisting of heavy isotopes of hydrogen,\u00a0so fast that its own inertia hold it in place long enough for significant thermonuclear fusion to occur.\u00a0 “Fast” means times on the order of billionths of a second.\u00a0 There are, in turn, two main approaches to supplying the necessary energy; direct drive and indirect drive.\u00a0 In direct drive the “target” of fuel material is hit directly by laser or some other type of energetic beams.\u00a0 In indirect drive, the target is mounted inside of a “can,” referred to as a “hohlraum.”\u00a0 The beams are\u00a0aimed through holes in the hohlraum at the inner walls.\u00a0 There they are absorbed, producing x-rays, which supply the actual energy to the target.<\/p>\n
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.\u00a0 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.\u00a0 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.\u00a0 They predicted ignition, but LLNL missed it by over a factor of 10.\u00a0 That doesn’t necessarily mean that the indirect drive approach will never work.\u00a0 However, the prospects of that happening are becoming increasingly dim.<\/p>\n
Enter direct drive.\u00a0 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.\u00a0 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.\u00a0 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\u00a0moved into a perfectly symmetric arrangement for direct drive.\u00a0 However, by carefully tailoring the length and power in each of the 192 laser beams, and\u00a0delicately adjusting\u00a0the thickness of the target at different locations, it is still theoretically possible to get a symmetric implosion.\u00a0 That is the idea behind polar direct drive.<\/p>\n
With indirect drive on the ropes, there are signs that direct drive may finally have its turn.\u00a0 One such sign was the recent appearance in the prestigious journal, Physics of Plasmas<\/a><\/em>, of a paper entitled Direct-drive inertial confinement fusion: A review<\/a><\/em>.\u00a0 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.\u00a0 The article is more than 100 pages long, and contains no less than 912 references to work by other scientists.\u00a0 However, look at the list of authors.\u00a0 They include familiar direct drive stalwarts like Bob McCrory, John Sethian, and Dave Meyerhofer.\u00a0 However, one can tell which way the wind is blowing by looking at some of the other names.\u00a0 They include some that haven’t been connected so closely with direct drive in the past.\u00a0 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.<\/p>\n Will direct drive ignition experiments happen on the NIF?\u00a0 Not only science, but politics is involved, and not just on Capitol Hill.\u00a0 Money is a factor, as operating the NIF isn’t cheap.\u00a0 There has always been a give and take, or tug of war, if you will, between the weapons guys and the fusion energy guys.\u00a0 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.\u00a0 There is enough energy in the NIF beams to create conditions sufficiently close to those that occur in nuclear weapons without it.\u00a0 Finally, many in the indirect drive camp are far from being ready to throw in the towel.<\/p>\n In spite of that, some tantalizing signs of a change in direction are starting to turn up.\u00a0 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<\/em><\/a> 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<\/em><\/a>.\u00a0 An exploding pusher target is basically a little glass shell filled with fusion fuel, usually in gaseous form.\u00a0 For various reasons, such targets are incapable of reaching ignition\/breakeven.\u00a0 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.\u00a0 According to the paper, all of the NIF’s 192 beams were used to implode\u00a0such a\u00a0target, and they were, in fact, tuned for polar direct drive.\u00a0 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!\u00a0 Intriguingly enough, that happens to be just about the same energy that can be delivered by OMEGA.\u00a0 The target was filled with a mixture of deuterium (hydrogen with an extra neutron), and helium 3.\u00a0 Fusion of those two elements produces a highly energetic proton at 14.7 MeV.\u00a0 According to the paper copious amounts of these mono-energetic protons were detected.\u00a0 Ostensibly, the idea was to use the protons as a “backlighter.”\u00a0 In other words, they would be used merely as a diagnostic, shining through some other target to record its behavior at very high densities.\u00a0 That all sounds a bit odd to me.\u00a0 If all 192 beams are used for the backlighter, what’s left to hit the target that’s supposed to be backlighted?\u00a0 My guess is that the real goal here was to try out polar direct drive for later attempts at direct drive ignition.<\/p>\n All I can say is, stay tuned.\u00a0 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.\u00a0 Regardless, I hope the guys at LLNL manage to pull a rabbit out of their hat and get ignition one way or another.\u00a0 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.\u00a0 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.\u00a0 Meanwhile, I can but echo the words of Edgar Allan Poe:<\/p>\n Over the Mountains
\nOf the Moon,
\nDown the Valley of the Shadow,
\nRide, boldly ride,
\nThe shade replied —
\nIf you seek for El Dorado.<\/p>\n