Submitted by i_owe_them13 t3_zpax7p in askscience
With the recent fusion breakthrough, lasers were used to produce X-rays that, in turn, compressed a tritium-deuterium fuel pellet, causing fusion. How do X-rays “compress” a material? Is this a semantics thing—as in, is “compression” actually occurring, or is it just a descriptor of how the X-rays impart energy to the pellet?
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Here's a video - the very end shows the lasers hitting the walls of the "package" holding the sphere with the fuel inside. The lasers hitting the walls raises their temperature so they produce X-rays. The X-rays ablate the outside of the fuel sphere, which flies outward. The inside of the sphere reacts to the ablation by blowing inwards, compressing the fuel.
If you do a couple google searches I'm sure you'll find an appropriate level explanation.
X-ray compression is indeed a physical compression process, just like if you submerged the fuel pellet into a tank of (very high pressure!) water. It is not immediately obvious why X-rays should do this to a solid object, though, and I don't think any of the major news articles on the recent NIF shot explain it very well.
The pressure responsible for the fuel compression is called the X-ray ablation pressure. When X-rays interact with matter, they deposit their energy into the material. Most of this energy goes into heating the material. X-rays do not penetrate especially deep into the material, which means that they dump all of their energy into a very thin (several microns, or less than 1/100th of a millimeter) surface layer. The x-ray pulse is also very short, usually shorter than 10 nanoseconds. The energy density in this surface layer rises very, very fast as a result. This produces a two step compression in the target.
For more detail on the physics, A.T. Anderson's PhD thesis "X-Ray Ablation Measurements and
Modeling for ICF Applications" is a pretty good and non-paywalled option.
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It is actually not, or at least is mostly not, the radiation pressure. The radiation pressure from the x-rays in ICF is not nearly sufficient to ignite the fuel. Rather, it is the ablation pressure from the outer "shell" of the target, which is very rapidly heated by the x-rays. The rapid heating, of course, produces a very large change in internal energy and therefore a large pressure; the outer layer of the target rapidly expands outwards, in the direction of least resistance, producing "recoil" motion in the form of a shockwave directed towards the center of the target.
ablation provides much higher pressures than direct radiation pressure
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Is this pulse - expand - recoil compression a cycle that repeats multiple times? Hence the pulse, or is it a one off ?
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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So how would one go about keeping the reaction going to keep producing energy?
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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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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.
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.
Excellent explanation. Extremely grateful for the sources as well. Thank you!
So why is this the most effective way of compressing fusion fuel?
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?
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?
Technically it's not! The most effective known way of compressing fusion fuel is to generate an insanely high x-ray pressure using a fission primary. We've been doing that since the 50s! But these have, ahem, other issues for energy purposes.
To achieve ignition temperatures in the fuel pellet, you need really large pressures. Stagnation pressures in previous NIF shots are on the order of 1-10 gigabar, or 800 terapascal; similar attempts at OMEGA, a similar laser facility, have achieved lower but still very high pressures of about 200 gigapascal using direct laser ablation rather than using the laser to produce x-rays. Laser and x-ray ablation are well suited to producing such high pressures, because they can dump a lot of energy into the target very very fast; this allows the ablated layer to reach high energy densities before anything can really start moving.
There are other ways to do it, maybe! For instance there's a startup called First Light which is trying to use light gas guns to produce the requisite pressures using physical impacts. They may have gotten this idea from a somewhat infamous nuclear physicist named Friedwardt Winterberg, who proposed a number of interesting mechanisms for compressing fusion fuel. Like this idea to use a hypervelocity projectile to adiabatically compress a high-atomic-weight gas, which will then get hot enough to ignite the fuel pellet!
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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.
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.
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?
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.
Do you need to perform this for every pellet of fuel, or once you've got ignition, can you keep dumping them in there and they'll ignite off the first pellet?
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.
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.
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.
In other words, heat the gass in a sealed chamber get immense pressure?
Is ablation pressure just a special case of radiation pressure then? Utilising wavelengths with poor penetrative ability for higher efficiency in applying the pressure?
No. Radiation pressure is the pressure exerted by the radiation itself. The ablation pressure is a material response to the radiation heating.
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Not the gas, the outer layer of a small sphere containing the fuel.
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.
I see. In this application, is the radiation pressure comparable in significance to the ablation pressure or is it negligible?
Sounds like that Helion process seems to be the most immediately viable.
"...the beams pass through the final optical assembly, which converts the infrared light into ultraviolet light..."
Um, what??!
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Light has momentum correct, is the compression equal to a relative pressure from this momentum or is the process of heating the only cause?
Ablation pressure is basically the rocket equation. Radiation boils off the outermost layer, pushing that layer away from the pellet as a gas with some thermal energy. Equal and opposite reaction pushes the pellet in the opposite direction. Now make this evenly around the pellet and all the pellet can do is compress into a higher density.
There is a process that often gets used in handheld lasers called second harmonic generation (or frequency doubling) it's basically black magic though.
Gotcha, thanks for clarifying.
Yes! I was lucky enough to get a tour of the NIF, and I asked the guide what the conversion efficiency was of the UV conversion (as those can be pretty inefficient), and he said 50%. Think about that - you go through the whole multi-building-sized process of building hundreds of terawatts of laser beams, and the last step - like 25 feet from the target it looked like - you lose HALF of the energy to UV-upconversion. Then 90% of the X-ray energy is lost. So you've got to start with a LOT of laser power. See https://lasers.llnl.gov/about/how-nif-works/final-optics.
Wow. Very nice, very clear response. Thank you.
So going by number 2, the laser basically explodes the skin of the pellet and the shockwave travels inward, compressing what remains.
So it's not just force exerted by the photons, but instead a different mechanic is going on?
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.
In this application the radiation pressure is pretty minimal yeah. I haven't seen numbers for it myself, but in some other settings where you care about direct radiation pressure & ablation pressure, you usually discard the radiation pressure term entirely unless you are very close to the source or it's an incredibly potent source. For instance, in Teller-Ulam type thermonuclear bombs, the radiation pressure from the fission stage is assumed to provide virtually all of the implosion pressure for the fusion stage [going by unclassified sources only ofc, e.g Winterberg "The Physical Principles of Thermonuclear Explosive Devices"].
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Right. It's true that photons have momentum, but not much as these things go. It's rather more efficient that the photons boil the outer layer, and the reaction force from the gases boiling off push the pellet inward radially.
This sets up a situation where a light fluid is pushing against a heavy fluid (not unlike putting vinegar on top of oil in a salad dressing) so a slight nonuniformity amplifies because of the Rayleigh–Taylor instability, so some of the fuel squirts out, rather than compress uniformly and your target won't fuse. This is why the targets have to be so smooth and the radiation needs to be uniform.
There are some so called fast ignition schemes that aim to relax these requirements, but they haven't been demonstrated yet. We're on the path.
The ablation pressure is much higher, making the radiation pressure mostly or entirely irrelevant.
It’s the reaction force (Newton’s) resulting from material flying away at extremely high acceleration and thus force.
A similar principle is thought to ignite the secondary in a thermonuclear weapon.
> Technically it’s not! The most effective known way of compressing fusion fuel is to generate an insanely high x-ray pressure using a fission primary. We’ve been doing that since the 50s!
It’s believed that ablation pressure is responsible for the compression of the secondary here as well, and not radiation pressure, as I understand it. Ablation pressure seems unavoidable and is much higher.
To ablation pressure (indirectly resulting from heating) will be much higher than the direct radiation pressure.
This answer is perfectly crafted. Detailed, yet understandable explanation with references for deeper explanation. Well done.
I would certainly believe that either is possible, but good ol' Winterberg seems to think that radiation pressure dominates, since it scales so strongly with temperature - to the order of 5000 TPa on the surface of the tamper! Meanwhile the ablation pressure should scale "only" with the P-T EOS of the tamper material.
Nice, thank you very much for this explanation! You seem to be knowing quite a lot about this.
What about alternating chambers? While one is fusing you place another pallet at the right spot on the other(s). Would it work?
The articles I read analyses that ablation pressure is so dominant as to render radiation pressure irrelevant. But since it’s not publicly known…
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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Not really following too closely, because 1) it is just an incremental step towards fusion, and 2) all the discussions have been professionals talking to professionals rather than laymen. Having said that:
Are they discussing “greater energy output than input” in terms of measurement, or useful energy production? I mean, they can do the first in terms of using fission to produce fusion, but the only practical use is making large explosions for warfare. (Limited usefulness.) Or are they at the stage of recovering energy for self sustaining energy production?
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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.
This sounds similar to opto-acoustic techniques used for thin film measurement.
Was the idea of bomb testing ever a serious idea outside selling congress for money?
Is that package where the fuel pellet is held what is called a hohlraum?
No. The fuel pellet is inside the hohlraum. Details and image here: https://lasers.llnl.gov/news/frustraum-hohlraum-design-is-shaping-up
And note that this was carried out by Livermore, which was founded as and still largely is a nuclear weapons lab. Not a coincidence at all.
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In a real power plant using this technology there would be a stream of target beads. Every reaction is independent of each other.
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?
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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Thanks very much for the correction.
u\labroid, your video sounds interesting but unfortunately the link doesn't work anymore. Do you have an alternative link?
It looks like the site is under maintenance. I'd give it a couple days and try again....
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