Internet joy – laser fusion at breakeven! um…NOT. BBC reported, copycats ignited. Smoke only, no flame.
Big buzz in Technology news is the rumor that the NIF (National Ignition Facility) laboratory in California made a spectacular step forward. Did they generate as much fusion energy out as was supplied by the laser light? Or, what? LastTechAge became aware of the noise via an email … Isn’t this great? Aren’t we on our way to fusion energy? Would that we were.
Daniel Clery is a knowledgeable science reporter for the journal Science. According to his post on the ScienceInsider blog, somehow the BBC News got hold of an NIF internal report.
Box A
“The BBC understands that during an experiment in late September, the amount of energy released through the fusion reaction exceeded the amount of energy being absorbed by the fuel – the first time this had been achieved at any fusion facility in the world.”
The internal report was dated 2013-Sep29 and was a discussion of the preliminary data from a shot performed late the previous evening. Reality: if the full 1.82 MJ of laser energy were used, input was 130 times more than output. The box (right) use the values reported to have been in the NIF memo; oops.
Their own interest seems so have been the early estimate that the shot emitted about 5×1015 neutrons (n0) [US: 5 quadrillion n0, metric: 5 peta n0] 75% more than any other shot to date, indicating a higher level of fusion activity than usual. Accurate measurements require days of checking error modes and cross comparing diagnostic sets. These numbers must be the preliminary “back of the envelope” projections always sent to managers to keep them happy.
Why the buzz? Somehow BBC ran with the NIF rumor and it caught the public imagination. Even LastTechAge has comments by people who dream fusion dreams.
Comments on this Great Puppy-Tail Chase
NIF was born in strife. As is typical with achievements by any powerful group, less connected labs were closed to justify centralizing laser experiments. Fast forward to Now. 21st Century America has few world leadership fusion efforts except NIF, assuming NIF is one. We whittled our magnetic fusion leader-labs down to 3, and – although it took a real effort – finally eliminated one of those, the Alcator C-Mod at MIT that does/did modern plasma fusion studies and trains leadership-level student scientists.
NIF is missing several key requirements for successful laser fusion.
1. Low power facility laser. NIF yields 1.8 MJ onto target, not the 10 MJ originally proposed. Part of this was clumsy burning of its funds by poor planning and failure mode analysis, part the historic loss of fusion support by Washington. Nothing at NIF seemed to work right the first time, for example NIF guys started with impure glass optics that cracked and exploded in the 1990s when tested with high power beams. Used up the money, this way. If I had been so negligent with my own experiment on the General Atomic tokamak, ignored so many known issues, did not probe for unforeseen gotchas, my boss would have fired me.
2. 192 different laser beams. Although the key to success is driver uniformity (known since the beginning) each beam has its own slightly different start time and ramp-up profile to full power on target. The result is 192 different sledgehammers separately whacking at the target. It has to split into many sub-knots as it implodes. Guess what! New calculations show we need highly symmetrical target illumination. New discovery? Maybe in the 1960s.
Fig 1 NIF Hohlraum
3. Incomplete hohlraum technique. This is a “radiation room” that encapsulates the target in the uniform bath of x-rays that drives the implosion. A German lab director once told me Planck radiation (what we call blackbody radiation) is the success key. We should see an enclosed chamber, but NIF’s is more like a cylinder with open ends. When does a short open cylinder cease to model an enclosed chamber?
So NIF shines its 1.8 MJ of laser light through the end gaps and onto the inner walls to generate the x-rays which hammer the target. Will the x-ray wave front hit the target as a solid hammer, or will it arrive irregularly over a period of time?
Assuming of course laser light makes it into the chamber. If a small bit shines on the entrance opening, the generated plasma might well self-shield the interior from the laser.
Box B
4. Low energy IR primary beams. It has been understood for 3+ decades that IR causes fuel preheating and will not work. NIF down-converts to efficient UV but with a huge waste of power, see box (right).
Inefficient. Any other reason for this choice other than that is what they knew? Why did Livermore advocate closing the high intensity Los Alamos Aurora KrF laser? (KrF’s emit high intensity UV, directly.) To my understanding, only Naval Research Labs are still working with a small KrF. This was a very poor success strategy.
Box C
5. Constant power beam pulse. By the early 1980s, physicists understood that neither strategy in the box would lead to success. The innovative engineers at KMS demonstrated the first ramped laser beam in the late ’80s, then used them to demonstrate improvement with shots. How could the LLNL crew have ignored this? Guess what! New calculations have shown the way! ICF needs a ramped turn-on for the beam (adiabatic beam growth) and much shorter pulses to force the compression in times very short compared to disassembly times.
Box D
What about the targets, themselves? Somehow the PR does not include discussions of them. I do not have access to current publications, but cryogenic fuel inside lightweight shells are/were the way to success. This is what we were doing when KMS closed. I would have expected huge improvements in the shell strategies during the pasts 2 decades.
The cause of this latest round of excitement is suggested in Box D. Your choice. Based on my past contact with control efforts by LLNL management, my personal favorite is (B).
LTA discussed the national funding history behind our loss of fusion world-leadership by discussing the sad story of MFTF-B at LLNL. We plan to continue following NIF results with great interest.
……………………………….
Charles J. Armentrout, Ann Arbor
2013 Oct 15
Listed under Technology … Technology > ICF/IFE
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The Last Tech Age 
I think it’s useful to illustrate the full energy path, end to end…
422 MJ in the capacitors
perhaps 40 MJ of light from the flash lamps
about 4 MJ of IR laser light
1.8 MJ after conversion to UV, delivered to the hohlraum
maybe 750 kJ after reflections off the hohlraum and heating it up
around 170 to 250 kJ of x-rays delivered to the fuel capsule
So the Q is actually much worse than your article states – it’s not 14 / 1800, it’s 14 / 422000.
That said there are some obvious answers to the questions you raise. The reason for using IR glass lasers rather than KrF is due to pulse timing control and inter-beam granularity. As hard as these things are in NIF, they’re even harder in KrF. Finally, NIF is definitely ramped, and chirped, pulsed etc. The beam is definitely not linear in time!
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I like your analogy of hitting a target with 192 sledgehammers. Is this a timed, coordinated assault that can be seen as putting more and more energy into the target in such a way as to take advantage of resonances (assuming there are any), or just an uncontrollable artifact of the impossibility of timing them to be simultaneous?
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Actually the lasers don’t hit the target directly in NIF.
Instead, they heat up gold foil on the canister surrounding it. This gets so hot it starts radiating x-rays, which fill the area inside the can. Those are what ends up pushing on the fuel pellet.
It’s all very rube goldberg. But it’s x-rays from an atom bomb that push on the fuel in a hydrogen bomb, so you can see why NIF is interested in this approach… :-)
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Yes. Atom bombs work. Compared to NIF geometery, bombs are really big and their time scales of fusion interactions are really long. ICF has gone through classic start phase. Positive belief of instant succes, magical results, followed by painstaking investigation of all the various things that have to be just right to work. They are still in this phase.
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Your synchronized strikes are the ideal, ABE. Targets are so small, shots so short, and laser distances so long that it is difficult to manage. Maybe not impossible. (?)
Simultaneous would mean that a single wavefront from the parent oscillator strikes all inside surfaces of the hohlraum in a synchronized timing so that the x-rays generated by that front reach the target, all at the same time. The geometry makes complicated logistics, and normally one ends up with a target imploded into numerous clumps. bad.
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Herding (very tiny) cats – an impossibility.
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