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Robo-Connery OP t1_j03jxui wrote

Reply to comment by dudaspl in National Ignition Facility (NIF) announces net positive energy fusion experiment by Robo-Connery

So in one sense, using only the 2MJ they use for NIF as a goal makes sense because that is the energy that is available to the pellet. In comparison the "goal" that MCF fusion uses is the external heating which consists of a bunch of different mechanisms (neutral beams, large plasma currents, radio stimulation of ions called cyclotron resonance heating are the big 3). They are both about as equivalent a number as you can get for their respective machines (unlike the comparisons of energy output which I already have griped about).

Both regimes are ignoring all the other energy costs of running the machines as well as the efficiencies of supplying that energy to the fuel.

It turns out though that the efficiencies of tokamaks (only with superconducting magnets) are just a couple of order of magnitudes better (than ICF) so including the other energy sinks wouldn't make the results look as bad. There are still efficiencies to gain for ICF both in the hohlraum design, the frequency doubling and the pumping of the lasing material for ICF machines and probably countless other places so we will see this record be smashed over and over again both by NIF and future machines.

In terms of gaming the system, it doesn't really make sense. JET could probably smash their own record if they designed the pulse differently but they are concerned with their own experimental program (which is heavily focused on understanding how ITER will perform and how to optimise ITER). It has made many sacrifices both in the machine design (e.g. using Be-W walls instead of carbon) and operation (going for long burns) which make peak power output lower but have other benefits.

The true gaming of the system for ITER would be to turn off the external heating during a shot once, that way the q-factor would be infinite but the external heating remains important for reasons other than just providing heat. For example the external heating makes it far easier to access and maintain something called H-mode or high confinement mode which makes the plasma more stable and has a far higher core temperature. As such, it isn't that sensible to design experiments along this line of thinking.

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Asleep-Emu-7977 t1_j03l9sd wrote

You seem like someone that I would love to drink some beers with. Thanks for your comments ^^ super informative!

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Robo-Connery OP t1_j03lp0k wrote

A very nice thing to say, thanks. I am glad it was informative.

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zbobet2012 t1_j055r4s wrote

This is really informative thank you. I've read a few counterpoints (I am not an expert) that ICF has over MCF and am curious as to your take:

  1. In the presser today and on the Wikipedia article for laser inertial fusion energy it's mentioned the lasers could be "upgraded" from ~0.5% efficiency to ~20% efficiency which certainly helps the case for "LIFE" systems.
    In general efficient laser generation is something which receives tens of billions per year in investment due to demands in semiconductor lithography (literally more than the entirety of investment in fusion research worldwide). While not directed at LIFE, much of the possible gains none the less come from it. (see commercial products such as https://www.trumpf.com/en_US/products/laser/euv-drive-laser/ )
  2. The relative size of the chamber significantly reduces problems with neutron embrittlement and the relative simplicity of the chamber means costs around lifecycle maintenance may be lower. (See https://web.archive.org/web/20160506011449/https://life.llnl.gov/why_life/life_advantages.php )
  3. ICF requires a lot less tritium for startup. Tritium is a rare resource, largely made from nuclear breeder reactors today. It's also useful for making fission-fusion-fission bombs (the "H-bomb"), so we don't like having a ton of it lying around for proliferation reasons.
  4. I feel that there is "no clear pathway to the next step (a demonstration power plant)" somewhat contradicted by the work layed out in the laser inertial fusion energy project. What do you feel is missing from this versus a MCF approach?
  5. I think the most interesting article I've read recently suggested combining the two: https://medium.com/fusion-energy-league/the-fundamental-parameter-space-of-controlled-thermonuclear-fusion-1c1e34206ed8. So in my very uninformed opinion folks shouldn't disregard either approach. It's quite possible a future fusion power-plant could be built on the results of ICF and MCF. Would funding both of them make sense?
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Robo-Connery OP t1_j06psgk wrote

I can try to answer these follow ups:

  1. For sure there is efficiency to be gained, 20% is definitely achievable but not for a long time (maybe only when using a non frequency doubled laser), currently the most efficient diode pumped lasers are about 10% efficient. At 20% this means a 20x improvement (if you can deliver the 4MJ rather than 2MJ to target. It is hard to imagine much more improvements over that, but there is a huge ceiling for improvements in the fusion yield. They need both to get the 5 or so orders of magnitude they need between both. I would point out though that the demands on the laser are different for the different purposes, technology translates but not without adaptation.

  2. I 100% agree that one of the few (maybe only significant?) advantages of ICF is the target chamber being very simple. They actually also benefit from a smaller neutron blanket being necessary but it isn't all positives, the number of laser beam paths needed to evenly heat the holhraum makes fitting the blanket around the outside comparatively tricky.

  3. I don't believe this is either true or an advantage. NIF uses less tritium because it does tiny shots once a day. Jet pulses every 20-30 minutes with much higher amounts of tritium in each shot because it generates far more energy (50-100x more roughly) so requires far more fuel on site. Fundamentally I disagree there is a proliferation problem, tritium is not the limiting step in making a hydrogen bomb (the fission warhead is far harder) nor is it essential, DD is sufficient. Lastly, we don't like having it around because it is hard to handle, and extremely radioactive, I don't think it is in particular due to proliferation fears.

  4. So NIF is a weapons lab but it is somewhat supported by the fusion-for-power cause too. It does what it is designed to do very well (test equation of state of high density matter, test x-ray ablation of hydrogen targets, test compression and fusion of hydrogen targets). It does fusion for power reasonably badly. Without going off on paragraphs of text, MCF problems are numerous but they are mostly understood, we know we need different divertor designs and what they should be, we know we need better ELM control, we know we need to conquer tritium breeding and material science under neutron bombardment. ICF has similar problems and then 100 other ones - in the context of fusion for power. There is no sensible plan to get a 2-10Hz repeat rate on it (versus the 0.000001Hz of NIF), there is no sensible plan to get fabrication costs down by a factor of 1000. And on top of all of that MCF machines built in the 90's are about 1-1.5 order of magnitude away from our goal in terms of raw power output. (JET at 30MW versus ITER at 500MW with the same heating). ICF is 3 or 4 away (Again alongside the 6 orders in repeat rate). The disclaimer here is that ICF is extremely new science and MCF is established so there is plenty if time for ICF to mature. I'll leave it with saying that the steady state nature of a tokamak (maybe 1000s flat top burns not being out of the realms of possibility for ITER) just makes so much more sense as a power plant than pulsed explosions.

  5. Funding both of them makes 100% sense to me, I have no issues with ICF or with laser plasma physics in general and as I've said all over the place, NIF is an incredible feat of plasma physics and engineering. I doubt a commercial reactor will ever use both in my lifetime (in fact I doubt one will use ICF in my lifetime) but I am 100% certain there will be a tokamak power plant.

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Vertigofrost t1_j06lq67 wrote

I'm not an expert either but I think I can provide some limited responses for your questions.

  1. That represents a 40x reduction in power requirements for the laser, which is only a single order of magnitude and realistically not a massive change in the energy balance.

  2. While this could be true, the current cost of a shot is over 100,000x more expensive than it would need to be for that to be true. You need shots costing pennies when they currently cost >$10,000.

  3. Tritium is naturally rare but not hard to make. Bombarding lithium with neutrons is not a super complex process, though capturing the resulting tritium requires a good set-up. Not something that's really a worry for nuclear proliferation because anyone could do it. It's currently produced in 10-20kg per year from reactors and can be stored for future use. The industrial production would need to occur for future fusion needs but its not such a big problem.

  4. The LIFE project has clear goals but not really steps to get to commercial production.

  5. I will leave this one to an expert.

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ackermann t1_j05hyvx wrote

Thanks! So, a basic question. Would a hypothetical large scale, commercial reactor work with a series of discrete pulses, or “shots”? Eg, fusion wouldn’t be happening continuously, but in pulses?

I imagine this would make them safer than fission reactors. If the reaction isn’t continuous, then it can’t “runaway” out of control, or melt down, right?

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Robo-Connery OP t1_j06qeok wrote

That is the design that they are working towards. Well NIF isn't really about a pathway to fusion power but if you are following a NIF-like design for power than it needs to be pulsed. It needs to be pulsed a couple of times a second before designs start to make sense (nif is about once a day).

Modern fission designs can also not melt down (search for Gen IV or gen V reactor designs), they are completely passively controlled and cooled.

In addition magnetic fusion devices can not melt down either, they might damage the machine if you turned them off (also JET makes a horrible bang if it is turned off early) but there is no risk to anyone, reaction stops the instant you turn it off.

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ackermann t1_j08b62v wrote

Cool thanks! So magnetic confinement devices do have continuous fusion then, unlike NIF style devices?

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Robo-Connery OP t1_j08ddiu wrote

The best ones are only matter of tens of seconds. ITER is meant to be 1000s but the ultimate goal is continuous operation. Nif and other icf machines are necessarily pulsed (because they are explosions).

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