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Jon_Beveryman t1_j0t43d9 wrote

Reply to comment by twinkletoes987 in How do X-rays “compress” a nuclear fusion pellet? by i_owe_them13

My understanding is that in the recent NIF shot it was a single event, since after you've done it [assuming it works], you have ignition and there's no need to keep compressing the target. X-ray pulse isn't meant to imply repetition here, it's just the term used in the literature.

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Captain-Barracuda t1_j0t5u6p wrote

So how would one go about keeping the reaction going to keep producing energy?

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vegiimite t1_j0t98ql wrote

It is essentially impossible for several reasons.

You need to position the target very precisely otherwise the shockwave is not symmetric and you get a fizzle instead of full power.

You also need to zap a target every few seconds to get a continuous output of energy. So perhaps dropping a frozen ball of DT ice every couple seconds and zapping when it reaches the right spot might work.

But try to imagine what the inside of the reactor would be like once burning started. It will be filled with hot plasma and hard radiation from a bunch of fusion reactions in the center. So there is no way to get a new pellet into the right spot. It will vaporize long before it can be ignited.

Even if you solve that you will have to fire your lasers into this hot plasma which will distort the incoming pulses in unpredictable ways. And if the lasers don't hit perfectly you will get a fizzle.

Next the targets that the lasers hit that produce the x-rays that compress the full need to be precisely machined and made of gold. They cost about $5,000 each to make. So operating costs will be an issue.

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BalderSion t1_j0tgpko wrote

So I was in the fusion technology field in grad school 10 years ago, but there are a couple of things here I'd like to address.

In the conceptual ICF reactor studies we and other groups put out, the rep rate was 10 Hz, not less than 1 Hz. For a less than 1 Hz rep rate you'd need much bigger pellets, that are driven much higher beam energy to maintain the power output. Also plant efficiency goes up with rep rate.

The good news is you can inject the pellet at 10's of metres per second. A compression and fusion burn wave will be over in nano seconds and still maintain their center of mass velocity, so the resulting expanding plasma can clear the chamber in time for the next shot, if the engineering is done right.

Also, in the field, for a fusion powerplant it is well recognized the plant will need to be direct drive, that is the driver (particle beam or laser) will need to be incident on the pellet directly, rather than use the hohlraum, because the cost per shot needs to be on the order of 25¢ per shot to be cost effective. NIF used a hohlraum to relax the driver requirements, but direct drive is another hurdle to overcome on the way to ICF fusion.

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Jon_Beveryman t1_j0thgz5 wrote

Hey, thanks for chiming in! I did not realize anyone had gone that far in the engineering studies. That makes a lot more sense. I was dimly aware of developments in direct drive in the last few years, do you think direct drive is likely to hit the required pressures?

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BalderSion t1_j0tpoqf wrote

It's funny, because I have high confidence they can, and low confidence how. Just exposing my bias. Of course, I expected this result from NIF 10 years ago.

The challenge is likely to be uniformity rather than pressure. Presumably this can be addressed, but again I don't know much about the how.

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vegiimite t1_j0ucrt7 wrote

Thanks, I really appreciate this. I had a fundamental misconception about how it would work and the time scales involved. I pictured the interior of the reactor being a continuous hot plasma, not having time to cool between shots.

I guess that changes my opinion to not actually impossible. I still think it is an unlikely path to commercial power.

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BalderSion t1_j0ugo13 wrote

Yeah, and that's probably a fair assessment. Fusion is an optimist's game. For non optimists, the promise is too great to ignore, but it took decades just to get our arms around how difficult it was going to be; hence the fusion is 50 years away and always will be reputation.

I would take this result as proof ICF can generate power, not that it's ready to. I mean, we knew from hydrogen bombs it was possible to get Q>1 from inertial confinement, but not if it could be done with beams like this. Similarly, if ITER gets their Q>10 result in the next couple of years, I would take that as evidence that magnetically confined burning plasmas can be stable, so we'll know MFE can generate power, not that it's ready to.

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EBtwopoint3 t1_j12d356 wrote

How do you get the energy out? My understanding is that fission plants are essentially fancy steam boats, heating water to turn a turbine that powers a generator. How does this work in theory for a system like this?

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BalderSion t1_j12ukvz wrote

A fusion plant would be the same steam generator. The engineering is mature, and it's the most efficient way to turn hot into electricity. The D+T fusion reaction produced puts 80% of its energy into a neuron and 20% into a helium. Both will strike the wall of the chamber and that will heat the chamber. Cooling channels running through the wall carry the heat to a heat exchanger which makes steam for the turbine. Any other mechanism would be less efficient than steam generator.

There are the p+Boron 11 schemes that produce energetic charged particles (no neutrons), which could be, magnetically funneled into collectors to create a very high voltage DC current, however the physics challenges with that fusion reaction are higher.

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Jon_Beveryman t1_j0ta3ku wrote

I do think it's fairly telling that, despite the explosion in commercial fusion start-up companies in the last decade, I can only think of one doing ICF, and none that are doing NIF-type ICF. First Light is doing an admittedly kind of far-out projectile based ICF.

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[deleted] t1_j0tfy7a wrote

In my layman interpretation (I mean, I did do the majority of classes needed to do NMM work for the Navy, but that doesn't equate to the understanding of the fine process that actual particle physics degress/doctorates would grant someone, ofc) wouldn't that just be wise anyway, to have a lab essentially volunteer to do the crazy, one-off experiments that nobody really puts a lot of stock in that have a vanishingly small chance of actually working, just to check to see if that is actually on the wrong track?

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Jon_Beveryman t1_j0th4sk wrote

So...yes, there is a big role for government labs and government-funded academic groups to do that kind of work. and the Department of Energy supports a lot of that work! But there's a wrinkle here, which is that NIF is "owned" by Lawrence Livermore National Laboratory. LLNL is one of the Department of Energy's 3 "weapons" labs. See, for historical reasons [which you probably know already] the DOE owns the nuclear weapons design mission instead of Defense being in charge. 3 of the DOE national labs [Livermore, Los Alamos, and Sandia] are considered the weapons labs. Livermore and Los Alamos are each responsible for nuclear weapons science and design, while Sandia is responsible for the engineering side. The US also does not test live nuclear weapons since the end of the Cold War, so the weapons labs acquired a new mission - "stockpile stewardship and management". Essentially, "go do a bunch of science to make sure that the nuclear arsenal will still work every time we need it to". A big part of this was figuring out how to experimentally replicate the conditions of a thermonuclear explosion, aka fusion. NIF is first and foremost in support of that effort, and not the energy job.

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[deleted] t1_j0thvg7 wrote

Outstanding, thanks for the reply, at least I was right in a way! Just not what I initially expected, but good, least I learned something neat.

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[deleted] t1_j0tfosc wrote

Wouldn't the heat then be what overwhelms the coulomb forces that would normally keep nuclei apart at that point? Would you even need anything else aside from a magnetic field to act in lieu of intense gravity to maintain the fusion at that point, or am I wildly misunderstanding?

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[deleted] t1_j0tgm9d wrote

[removed]

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[deleted] t1_j0thpwr wrote

Oh cool! Well, not literally (in that case, hah, I know, lame physics joke.) Thank you! I only have a superficial knowledge of things because I was going in as an NMM and couldn't complete all of my classes, just the 60% or so that kept insisting it was the basics. I wish I coulda gone further in college, but the math just broke my mind.

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Fredasa t1_j0tt93c wrote

Sounds like that Helion process seems to be the most immediately viable.

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Jon_Beveryman t1_j0u2zgl wrote

To be blunt: Helion smells like grift to me. Their recent media blitz on youtube and reddit adds to this impression, for me at least. They have a really unorthodox method, and their claims about radiation safety in their design are at best incredibly optimistic, if not outright misleading. For instance, in a past life I did some work on plasma facing materials for ITER. Anything you expose to a burning fusion plasma is going to suffer a lot of neutron damage, including neutron activation -- i.e the neutrons turn your nice non-radioactive wall material into something quite radioactive. Helion's claims about "low activation" materials for this setting don't really pass my sniff test, professionally.

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Branpri t1_j0u9g5b wrote

What about alternating chambers? While one is fusing you place another pallet at the right spot on the other(s). Would it work?

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hegbork t1_j0tb4k8 wrote

You don't, it's basically impossible with this kind of setup and it was never something they designed for. The mission of NIF is to perform fusion experiments so that they can replace nuclear bomb testing. This whole talk about power generation is probably just marketing to get more funding.

It was pretty clear during the press conference. Everyone involved was talking about "stewardship" as the first part of their statements. There was some handwavey stuff about private actors taking this experiment and running with it for power generation, but it should be pretty clear what the market thinks about this technology by observing that fusion startups that claim to want to shoot things with lasers have almost no funding.

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Colddigger t1_j0tjjn1 wrote

Funny, most tech plays up their usefulness in military use for funding, while this steers away from that obvious path in favor of the vague fusion dream.

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toastar-phone t1_j0ujhad wrote

Was the idea of bomb testing ever a serious idea outside selling congress for money?

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antonivs t1_j0tkr0n wrote

As other comments have pointed out, you can’t.

It’s important to notice that this means that the term “ignition” here is misleading. The term is being used to imply that this means some physically important threshold has been reached, but that’s not true.

“Ignition” in this context is simply an arbitrary name for a symbolic point on the reaction efficiency chart. It has no physical meaning. No actual breakthrough in fusion physics has occurred, simply an improvement in efficiency.

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Octavus t1_j0xg43h wrote

In a real power plant using this technology there would be a stream of target beads. Every reaction is independent of each other.

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