NIF failed in its promise for ignition when driven by fully operational laser. Should this be a surprise?
The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory delivered its entire laser beam energy to its target and the fuel somehow did not come even close to ignition. This huge US$ 3+ billion facility for inertial confinement fusion (ICF) can not deliver what it promised. So where is the surprise?
The magazine LaserFocusWorld released this interesting report by Jeff Hecht based on an article in the 2012-Sep21 issue of Science Magazine (summary here) by writer Dan Clery.
ICF could work Do not misunderstand – it could probably reach breakeven later on. But the NIF scheme will need big refinements if it is to produce fusion energy. We described NIF’s real job earlier. All NIF-related pictures are from the on-line archives from LLNL.
FIg 1: Inside NIF’s target chamber. 192 laser beams focus on a 3 mm target at the very end of the cone-shaped rod (rod much closer to the camera). The holes in the walls allow the beams to come through, some are for instruments to measure how a shot works.
The NIF shot The issue with the fusion experimental result is that they generated laser power at its full output of 500 billion kilowatts (500 TW) and deposited the full 1.8 M joule (135,000 kW·hr) laser energy onto the implosion target.
For the last 20+ years this was promised to ignite the fuel (powerful enough to cause hydrogen to fuse into helium, with no additional driving source). But … no joy.
The end result of the implosion was maybe a factor of 10× below what was promised. Does this seem like a lot to you? Would you notice a 10× reduction in your salary?
Fig 2: Beam lines used to transport and amplify laser energy from its source (the oscillator) to the target chamber (Fig 1)
The world’s largest laser facility
NIF is the world-wide premier laser fusion installation, with scientists of the huge repute.
LLNL spun off from Los Alamos (New Mexico) where the plutonium bomb was developed, and became the early leader in the thermonuclear hydrogen fusion bomb. With all of this, how could it have gone wrong?
What could have gone wrong?
I wish I could give a firm answer to that. But, instead, all I can give are my presumptive ideas. This section is a bit physics-y. Click here to jump past this discussion.
Fig 3 shows the business end of the NIF implosion facility. (Again, images from LLNL)
Fig 3: LEFT: Target to implode, about 3 mm (about 1/8 inch) in diameter. This has a frozen inner layer of a mixture of deuterium and tritium (DT) that is to fuse into helium. CENTER: Holhraum chamber – the target is inserted and the assembly is mounted to the rod tip in Fig 1. RIGHT: Laser beams enter the chamber and strike the walls, not the target. The walls disintegrate into a plasma gas and strongly radiate x-rays, which strike the target, drive the implosion to shrink the target by more than 100x
The beam should be UV. NIF laser power starts out as IR beams that need “frequency tripling” to convert into the ultraviolet that provide greatest effect when impacting the hohlraum (German for radiation room). I have not found current lists of how much power is wasted in the lossy conversion process, but maybe 25% of what emerges from the amplifier beam lines is used up during conversion. Add further inefficiencies from hohlraum conversion of UV to x-ray, and further losses because only a fraction of the x-ray photons that are made actually participate in the implosion.
Simulations are the new Bible. LLNL has been running computer simulations since even before the NIF contract was awarded. The announced result is that NIF is good as gold for ICF fusion. These computations have been taken as God’s truth, which is why LLNL won the competitive bidding in the late 1980s, early 1990s. NIF was shown to be all that is needed for success.
ICF generators were not connected up to the power grid in the late 1970s because a great deal more happens than the simple discussion under Fig 3. Most implosions are highly chaotic with uncontrolled instabilities popping up everywhere during the plasma phase.
One of the last effects KMS discovered is that when a laser beam strikes a surface, jets of plasma “plumes” shoot out from the low intensity parts of the beam. This effect will disturb the intended distribution of the x-ray light. No laser beam is smooth in its cross section so all must generate this plume effect. There are 192 such beams generating chaotic plasma plumes inside the hohlraum. And this may be the least significant issue that this fusion scenario faces.
Computer codes cannot be trusted without verification. Here are 2 points to support the premiss of “Trust But Verify.” There are certainly more…
- Simulations use lists of arbitrary constants describing unmeasurable physical processes, all of them must be given a value before simulations start. You need a bunch of functioning laser-target implosions to get these values, which LLNL did using their old NOVA facility. But the NIF geometry differs from the ancient NOVA one. The beam energy is different, the beam count is different, the target shells are different. So why should these codes be expected to give accurate estimates for NIF operations?
- Simulations follow plasma instabilities pretty well, and there are myriad published simulations of known instability effects. But they can not project when processes link together and feed back to each other. These are non-linear interactions with chaotic results that could lead to mistaken dynamical simulations. NIF forms 192 hot spots, each generating x-rays and each pushing plumes into the cavity space – sounds like an interactive, cross-linking situation to me.
NIF’s performance is horrifying because its true purpose is verification of the effectiveness of our nuclear deterrence capability. As Science’s Cleary says, the failed test led the U.S. Department Of Energy to started a review of implications to our nuclear stockpile.
Should we trust LLNL? During ICF’s classified years, LLNL had a fusion solution de jour, it seemed (actually, not daily, but only each half year or so – I do so hate exaggeration!) . Basically, each was absolutely the final solution for fusion power generation, but all were different. The simulations were from LLNL, though, and therefore the best. Because of this, I developed a personal distrust of LLNL reports.
There were several other proposals … the one from Los Alamos proposed an upgrade of their Aurora KrF laser that did its original lasing in UV, no need to frequency quadruple with energy loss, as in the NIF proposal. Postscript to the story: politics forced Los Alamos to close its laser facility. KrF lasers are now ‘advanced concept’ items, and are actively supported at the U.S. Naval Laboratory and England’s Rutherford Laboratory among others. It’s hard to articulate reaction to the praise of farsighted research that duplicates 25 year old paths. (Why so? the space program does that routinely.)
Construction ran into issues. During the original build, NIF ran to many problems, that led to a budget overrun by a factor of 2 and a construction time overrun by over 50%.
- One factor was that all the glass optics for their beamlines had to be replaced. They used high purity glass, but not high enough. Metallic inclusions, even at the very rare occurrences of their glass, were expected to absorb part of the beam power, heat, and shatter the optics. The beautiful replacements were, to my knowledge, unique in ultra purity. This also delayed things and raised the total cost.
- It appears that implementation of the complex geometry also contributed to cost and schedule overruns.
- A third issue was that E. Michael Campbell, appointed NIF director in 1991, and Associate Director for LLNL 1997, did not have a Ph.D., he resigned when that info hit the news stands in 1999.
That seemed stupid to me. When I met him in the late ’80s, Campbell was a good scientist and an effective natural leader. This was a disruption that the lab did not need, and added to their problems. Other high-level science leaders at LLNL did not have that top degree, Mike seemed to have been singled out for disgrace. Not permanently, though. This very fine scientist moved to General Atomics in 2001 as one of their top leaders and has contributed well. LLNL looks like the loser here.
Finally: I have a very personal opinion on all this. I have followed LLNL’s self-aggrandizing actions over several decades – some of what they do in public seems pretty close to a scam. This was true when it started with Edward Teller’s Super Bomb (justified founding a new weapons lab costing how many $Billions?), through Teller’s ‘Star Wars’ program of the Reagan years (how many $Billions?), and seems close to truth with the NIF program during the last 20 years. But… although LLNL has always been great with public relations, only a few of their projects should be listed as political flim-flam.
Maybe LLNL will do a small tweak this week or next, and actually demonstrate an ignited fusion plasma for the first time ever. Who knows? This is not an ex-cathedra post. I am an experimentalist and (perhaps) test results will justify the investment in NIF as a fusion energy device – time will tell. But I do have doubts.
Update: 2012 Oct 2: Made changes per comment below – NIF focuses near ultraviolet beams on target and uses frequency tripling, reducing the wavelength by a factor or 3, not 4, as originally stated. Losses may be as low as only 25%, not 50%. This indicates more power on target per generated watt so the NIF facility is even more marginally designed for fusion power than estimated earlier.
Update: 2012 Oct 05: Comment today in IEEE Spectrum magazine to its professional engineers. The Spectrum quotes Richard Garwin, one of our top experts on nuclear weapons, as saying that NIF is not going to prove directly relevant to nuclear weapons testing. This has been well know for some time, I have not been sure whether it was a classified conclusion or not. NIF is funded by by the National Nuclear Security Administration, and is really too small to be effective for that, too .
……………………………….
Charles J. Armentrout, Ann Arbor
2012 Sep 28
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NIF beams are frequency tripled, not frequency quadrupled. Furthermore, your estimates of conversion loss may be high- we know conversion efficiency of 75% or higher is achievable. For starters, see http://www.lle.rochester.edu/media/publications/lle_review/documents/v75/75_01Demo.pdf
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You are right, of course. NIF starts with near IR at the oscillator and delivers near UV to target. So 1068 nm divide by 3 is 356 nm. /4 would have produced 267 nm, which is harder UV. sorry for the slip
The 75% efficiency is much better than any US power system, so it sounds great. But NIF was designed to operate at the lowest edge of feasibility using the equipment it had. If NO losses, and if the 75% figure for overall efficiency is right, how would the marginal system have performed with 2.4 MJ on target, not 1.8 MJ? I sure cannot project this, and codes are inadequate. We need a series of almost right shots to get the models right, then use the models to tweak the system to make it work. This is not the LLNL way. They trumpeted their computer excellence and drowned all opposition, including the LLE direct drive one, and the LANL KrF far UV one. This is very ancient history but the question of lab publicity vs lab competence still current. Just look at how LLNL handled NIF prior to last January. NIF operates at the lowest possible margin for success and is an experiment in process, one that needs severe experimental test and development. Lots of work before they bring out the champagne. I think Albright (NIF) knew this, I think most of the workers there were uncomfortable with the hype. But Government milestones are true mill-stones, they grind surely and they will grind anything. I met such a Laboratory milestone once. Would not want to repeat that stress, ever.
NIF uses a brute force approach and just hoped that they could deliver enough to the hammer to get ignition. I remember the early competition when Steve Bodner (NRL) challenged Bob McCrory (LLE) to drive a 1mm square piece of target shell one cm, intact. Bob just laughed. Bodner was nearly always right, IMHO. Inertial fusion requires ultra-smooth beams or stocastically churning ones that flicker at least 10x faster then the fastest hydro process. Bodner’s group produced the first simulation that I am aware of that bright points in a beam will imprint on the imploding shell and drive instabilities. LLE (and KMS when it existed) made lots of lovey pictures of targets with multiple kernels of high density, caused by beam non-uniformities. I suspect hohlraum drive non-uniformities play a part in the current NIF issue.
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The reason for the NIF failure is not Mike Campbell, who had already made his dissertation and in the meantime got a PhD, but who may have been (like myself) the target of professional jealousy, but rather physics, more specifically the RT instability for spherical implosions. As we learned by the NIF experiments, RT instability was underestimated, and direct drive is unlikely to solve this problem. RT instability is less serious for cylindrical targets. And If supported by 10^7 Ampere-Multimegavolt Ion beams, which by their magnetic field axially confine the fusion alpha particles within the cylinder, ignition is there much more likely. But inspite of the spherical shell laser implosion fiasco at the NIF, DOE program manager still continue to fund this worthless research done in university physics departments rather than supporting novel ideas. F. Winterberg, Professor of Physics, University of Nevada.
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Excellent comment, and thank you for contributing, Dr. Winterberg. The strength of the Rayleigh Taylor instability seemed to have been a surprise to early investigators. In the very early 1970s, KMS Fusion thought they would be “on line by ’79” but were naive about irregularities in their beam profile. NIF’s huge beam power is not the solution to RT. …nor would selecting a scapegoat cure the problem (as you also point out). Steve Bodner’s NRL pointed out by at least the ’80s that beam irregularities lasting fractions of a nanosecond can imprint the shell and cause RT related breakup of the high density imploded kernel. Your point about target geometry is really interesting; thanks for sharing it.
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It should be pointed out that ignition can hardly be expected even if there are no losses. According to a scaling law by R. Kidder, the ignition energy is inverse proportional to the 6th power of the ratio of the initial to the final implosions radius, or vice verse this ratio is inverse proportional to the 1/6 power of the energy needed for ignition. This means that the ignition energy is very sensitive to this ratio, but also that by raising the NIF enery input ten fold from 2 MJ to 20 MJ, for example, would only lower this ratio by 10^1/6 that is by 1.47. But the factor 10 is much less than the factor 4 by going from a 25% to 100% efficiency. To reach ignition a much smaller final radius, not more energy, is needed than RT instability seems to permit. F. Winterberg, Professor of Physics University of Nevada.
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