{"id":4308,"date":"2015-03-21T11:51:25","date_gmt":"2015-03-21T16:51:25","guid":{"rendered":"http:\/\/helian.net\/blog\/?p=4308"},"modified":"2015-03-21T22:01:56","modified_gmt":"2015-03-22T03:01:56","slug":"no-ignition-at-the-national-ignition-facility-a-post-mortem","status":"publish","type":"post","link":"https:\/\/helian.net\/blog\/2015\/03\/21\/inertial-confinement-fusion\/no-ignition-at-the-national-ignition-facility-a-post-mortem\/","title":{"rendered":"No Ignition at the National Ignition Facility: A Post Mortem"},"content":{"rendered":"

The National Ignition Facility<\/a>, or NIF,\u00a0at Lawrence Livermore National Laboratory (LLNL)\u00a0in California\u00a0was designed and built, as its name implies, to achieve fusion ignition.\u00a0 The first experimental campaign intended to achieve that goal, the National Ignition Campaign, or NIC, ended in failure<\/a>.\u00a0 Scientists at LLNL recently published a paper in the journal Physics of Plasmas<\/em><\/a> outlining, to the best of their knowledge to date, why the experiments failed.\u00a0 Entitled “Radiation hydrodynamics modeling of the highest compression inertial confinement fusion ignition experiment from the National Ignition Campaign,” the paper concedes that,<\/p>\n

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.<\/p><\/blockquote>\n

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.\u00a0 Here’s the short version:\u00a0 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.\u00a0 The average binding energy per nucleon (proton or neutron) due to the strong force is greatest for the elements\u00a0in the middle of the periodic table, and gradually decreases in the directions of both the lighter and heavier elements.\u00a0 That’s why energy is released by fissioning heavy atoms like uranium into lighter atoms, or fusing light atoms like hydrogen into heavier atoms.\u00a0 Fusion of light elements isn’t easy.\u00a0 Before the strong force that holds atomic nuclei together can take effect, two light nuclei must be brought very close to each other.\u00a0 However, atomic nuclei are all positively charged, and like charges repel.\u00a0 The closer they get, the stronger the repulsion becomes.\u00a0 The sun solves the problem with its crushing gravitational force.\u00a0 On earth, the energy of fission can also provide the necessary force in nuclear weapons.\u00a0 However, concentrating enough energy to accomplish the same thing in the laboratory has proved a great deal more difficult.<\/p>\n

The problem is to confine incredibly hot material at sufficiently high densities for a long enough time for significant fusion to take place.\u00a0 At the moment there are two mainstream approaches to solving it:\u00a0 magnetic fusion and inertial confinement fusion, or ICF.\u00a0 In the former, confinement is achieved with powerful magnetic lines of force.\u00a0 That’s the approach\u00a0at the international ITER <\/a>fusion reactor project currently under construction in France.\u00a0 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.\u00a0 That’s the approach\u00a0being pursued\u00a0at the NIF.<\/p>\n

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.\u00a0 It is much larger than the next biggest similar facility, the OMEGA laser system<\/a> at the Laboratory for Laser Energetics in Rochester, NY, which maxes out at about 40 kilojoules.\u00a0 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).\u00a0 Instead, the target was mounted inside of a tiny, cylindrical enclosure known as a hohlraum with the aid of a thin, plastic “tent.”\u00a0 The lasers were fired through holes on each end of the hohlraum, striking the walls of the cylinder, generating a pulse of x-rays.\u00a0 These x-rays then struck the target, ablating material from its surface at high speed.\u00a0 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.\u00a0 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.\u00a0 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.\u00a0 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.”\u00a0 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.\u00a0 “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.<\/p>\n

As noted above, things didn’t go as planned.\u00a0 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.\u00a0 The LLNL paper in Physics of Plasmas discusses some of the reasons for this, and describes subsequent\u00a0improvements to the codes that account for some, but not all, of the experimental discrepancies.\u00a0 According to the paper,<\/p>\n

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.\u00a0 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.<\/p><\/blockquote>\n

In a later section, the LLNL scientists also note,<\/p>\n

Since this study was undertaken, some evidence has also arisen suggesting an additional perturbation source other than the three specifically\u00a0considered here.\u00a0 That is, larger than anticipated fuel pre-heat due to energetic electrons produced from laser-plasma interactions in the hohlraum.<\/p><\/blockquote>\n

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.\u00a0 Any variation from perfectly spherical symmetry during the implosion\u00a0can 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.\u00a0 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.\u00a0\u00a0A larger than anticipated\u00a0perturbation due to the “tent” would have seeded instabilities, such as the Rayleigh-Taylor instability.\u00a0 Imagine holding a straw filled with water upside down.\u00a0 Atmospheric pressure will prevent the water from running out.\u00a0 Now imagine filling a perfectly cylindrical bucket with water to the same depth.\u00a0 If you hold it upside down, the atmospheric pressure over the surface of the water is the same.\u00a0 Based on the straw experiment, the water should stay in the bucket, just as it did in the straw.\u00a0 Nevertheless, the\u00a0water comes pouring out.\u00a0 As they say in the physics business, the straw experiment doesn’t “scale.”\u00a0 The reason for this anomaly is the Rayleigh-Taylor instability.\u00a0 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.\u00a0 Another, related instability, the Richtmeyer-Meshkov instability, leads to similar results in material where shocks are present, as in the NIF experiments.<\/p>\n

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.\u00a0 At the time, government used a “key decision” process to approve major proposed projects.\u00a0 The first key decision, known as Key Decision 0, or KD0, was approval to go forward with conceptual design.\u00a0 The second was KD1, approval of engineering design and acquisition.\u00a0 There were more “key decisions” in the process, but after passing KD1, it could safely be assumed that most projects were “in the bag.”\u00a0 In the early 90’s, a federal advisory committee<\/a>, known as the Inertial Confinement Fusion Advisory Committee<\/a>, or ICFAC, had been formed to advise the\u00a0responsible agency, the Department of Energy (DOE),\u00a0on matters relating to the national ICF program.\u00a0 Among other things, its mandate including advising the government on whether it should proceed with key decisions on the NIF project.\u00a0 The Committee’s advice was normally followed by DOE.<\/p>\n

At the time, there were six major “program elements” in the national ICF program.\u00a0 These included the three weapons laboratories, LLNL, Los Alamos National Laboratory (LANL), and Sandia National Laboratories (SNL).\u00a0 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).\u00a0 Spokespersons from all these “program elements” appeared before the ICFAC at a series of meetings in the early 90’s.\u00a0 The critical meeting as far as approval of the decision to pass through KD1 is concerned took place in May 1994.\u00a0 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.\u00a0 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.\u00a0 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.<\/p>\n

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.\u00a0 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.\u00a0 Ablated material from the\u00a0capsule itself could also absorb laser and x-ray radiation,\u00a0again destroying symmetry.\u00a0\u00a0He pointed out that codes had raised the\u00a0possibility of\u00a0pressure perturbations on the capsule surface\u00a0due to stagnation of the blow-off material\u00a0on the hohlraum\u00a0axis.\u00a0\u00a0LLNL’s response was that these problems could be successfully\u00a0addressed by filling the hohlraum with a gas such as\u00a0helium, which would hold back the blow-off from the walls and target.\u00a0\u00a0Bodner\u00a0replied that\u00a0such “solutions” had never really been tested because of the inability to do experiments on Nova with sufficient pulse length.\u00a0 In other words, it was impossible to conduct experiments that would “scale” to the NIF on existing facilities.\u00a0 In building the NIF, we might be passing from the “straw” to the “bucket.”\u00a0 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.<\/p>\n

In light of these uncertainties, Bodner suggested delaying approval of KD1 for a year or two until these issues could be more carefully studied.\u00a0 At that point, we may have gained the technological confidence to proceed.\u00a0 However, I suspect he knew that two years would never be enough to resolve the issues he had raised.\u00a0 What Bodner really wanted to do was build a much larger facility, known as the Laboratory Microfusion Facility, or LMF.\u00a0 The LMF would have a driver energy of from 5 to 10 megajoules compared to the NIF’s 1.8.\u00a0 It had been seriously discussed in the late 80’s and early 90’s.\u00a0 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.\u00a0 It would be powerful enough to erase the physics uncertainties he had raised by “brute force.”\u00a0 Bodner’s proposed approach was plausible and reasonable.\u00a0 It was also\u00a0a forlorn hope.<\/p>\n

Funding for the ICF program had been cut in the early 90’s.\u00a0 Chances of gaining approval for a beast as expensive as LMF were minimal.\u00a0 As a result, it was now officially considered a “follow-on” facility to the NIF.\u00a0 No one took this seriously at the time.\u00a0 Everyone knew that, if NIF failed, there would be no “follow-on.”\u00a0 Bodner knew this, the scientists at the other program elements knew it, and so did the members of the ICFAC.\u00a0 The ICFAC was composed of brilliant scientists.\u00a0 However, none of them had any real insight into the guts of the computer codes that were predicting ignition on the NIF.\u00a0 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.\u00a0 They chose the big\u00a0codes.\u00a0 With the exception of Tim Coffey, then\u00a0Director of NRL, they voted to approve passing through KD1 at the May meeting.<\/p>\n

In retrospect, Bodner’s objections seem prophetic.\u00a0 The NIC has failed, and he was not far off the mark concerning the reasons for the failure.\u00a0 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\u00a0of physics.\u00a0 Things aren’t that simple.\u00a0 The LLNL people, not to mention the supporters of NIF from the other program elements,\u00a0included many\u00a0responsible and brilliant scientists.\u00a0 They were not as pessimistic as Bodner, but none of them was 100% positive that the NIF would succeed.\u00a0 They decided the risk was warranted, and they may well yet prove to be\u00a0right.<\/p>\n

In the first place, as noted above, chances that an LMF\u00a0might be substituted for the NIF after another year or two of study were very slim.\u00a0 The funding just wasn’t there.\u00a0 Indeed, the number of laser beams on the NIF itself\u00a0had been reduced from the originally proposed 240 to 192, at least in part, for that very reason.\u00a0 It was basically a question of the NIF or nothing.\u00a0 Studying the problem to death, now such a typical feature of the culture at our national research laboratories, would have led nowhere.\u00a0 The NIF was never conceived as an energy project, although many scientists preferred to see it in that light.\u00a0 Rather, it was built to serve the national nuclear weapons program.\u00a0 It’s supporters were aware that it would be of great value to that program\u00a0even if it didn’t achieve ignition.\u00a0 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.\u00a0 Furthermore, LLNL and the other weapons laboratories were up against another problem – what you might call a demographic cliff.\u00a0 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.\u00a0 A facility like the NIF, capable of exploring\u00a0issues in\u00a0inertial fusion energy, astrophysics, and other non-weapons-related areas of high energy density physics, would certainly help address that problem as well.<\/p>\n

Finally, the results of the NIC in no way “proved” that ignition on the NIF is impossible.\u00a0 There are alternatives to the current indirect drive approach with frequency-tripled “blue” laser beams.\u00a0 Much more energy, up to around 4 megajoules, might be available if the known problems of using longer wavelength “green” light can be solved.\u00a0 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\u00a0instead of shooting the laser beams into a “hohlraum” can,\u00a0was also left open, using a so-called “polar” illumination approach.\u00a0 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.”\u00a0 Instead, once the target had achieved maximum density, the hot spot would be created on the outer surface using a separate driver beam.<\/p>\n

In other words, while the results of the NIC are disappointing, stay tuned.\u00a0 Pace<\/em> Dr. Bodner, the scientists at LLNL may yet pull a rabbit out of their hats.<\/p>\n

\"ICF\"<\/a><\/p>\n

\n\t\t\t\t\tTweet<\/a>\n\t\t\t\t<\/div><\/div>","protected":false},"excerpt":{"rendered":"

The National Ignition Facility, or NIF,\u00a0at Lawrence Livermore National Laboratory (LLNL)\u00a0in California\u00a0was designed and built, as its name implies, to achieve fusion ignition.\u00a0 The first experimental campaign intended to achieve that goal, the National Ignition Campaign, or NIC, ended in failure.\u00a0 Scientists at LLNL recently published a paper in the journal Physics of Plasmas outlining, … Continue reading “No Ignition at the National Ignition Facility: A Post Mortem”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[78,22,246,23,114,33],"tags":[218,219],"class_list":["post-4308","post","type-post","status-publish","format-standard","hentry","category-fusion-energy","category-inertial-confinement-fusion","category-national-ignition-facility-2","category-nuclear-energy","category-physics","category-technology","tag-inertial-fusion","tag-national-ignition-facility"],"jetpack_publicize_connections":[],"jetpack_shortlink":"https:\/\/wp.me\/pwEq0-17u","jetpack_sharing_enabled":true,"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/helian.net\/blog\/wp-json\/wp\/v2\/posts\/4308","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/helian.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/helian.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/helian.net\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/helian.net\/blog\/wp-json\/wp\/v2\/comments?post=4308"}],"version-history":[{"count":9,"href":"https:\/\/helian.net\/blog\/wp-json\/wp\/v2\/posts\/4308\/revisions"}],"predecessor-version":[{"id":4319,"href":"https:\/\/helian.net\/blog\/wp-json\/wp\/v2\/posts\/4308\/revisions\/4319"}],"wp:attachment":[{"href":"https:\/\/helian.net\/blog\/wp-json\/wp\/v2\/media?parent=4308"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/helian.net\/blog\/wp-json\/wp\/v2\/categories?post=4308"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/helian.net\/blog\/wp-json\/wp\/v2\/tags?post=4308"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}