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Comment by tgflynn | Hacker News Reader

I wonder if it might be possible to gain not percentages but orders of magnitude more or less just by making the targets bigger. Is it conceivable that the same basic approach and a comparable amount of input energy could be used to ignite a 100 MJ or even 1 GJ target ? Of course that would present some containment challenges but perhaps not insurmountable ones.

I'm also a bit concerned that this type of research may encounter national security related obstacles. Obviously a pure fusion bomb would be a game changer for nuclear (non-)proliferation.

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I don't think a pure fusion bomb will have any form of advantage compared to the current hydro-bombs. They wouldn't produce more energy, but will need more gear to reach ignition.

The advantage would be that you wouldn't need tightly controlled and hard to make materials like U-235 or Pu to make one.

I'm not in any way saying that using lasers would be a plausible route to such a weapon, since the NIF facility is huge, but if it turns out that the research needs to focus on how to get more output per shot, which I think it inevitably would since a typical conventional or nuclear power plant generates on the order of 1 GW thermal power (To match that with a 1 Hz repetition rate, likely a stretch for a MJ class laser, you would need to generate 1 GJ per shot, comparable to the energy in a ton of TNT.), it would probably be touching on areas that are highly classified.

Shiiiit... here I was thinking how cool it would be if they could miniaturize this, having somehow forgotten that my pet solution to the Fermi paradox is that a nigh-inevitable wrung on the ladder to interstellar presence involves discovering One Weird Trick to release a whole bunch of energy pretty easily, even on a DIY basis. Instant end of civilization. Even ant-like societies might have mutated members who'd go rogue and misuse the tech, and it wouldn't take many to ruin everything.

Basically it's a twist on the ice-9 solution to the paradox.

any sufficiently speedy spacecraft makes for a deadly kinetic kill vehicle, unfortunately.

One use for such a 'laser initiated fusion' is the LLNL (Teller) proposed X-Ray laser satellite from the Star Wars program in the 80s. The proposed solution then was to use the X-rays from an exploding bomb at the heart of a satellite and amplify and focus them through a 'lasing' material. It turns out they never found a lasing material that would work, and it would be fairly easy to confuse defeat it, so the project died. What also killed it was the end of the testing program.

A pure fusion bomb would produce less (not zero) fallout. Neutron activation would still produce some fallout, but you wouldn't have the fission byproducts like caesium-137, iodine-129 or strontium-90.

This is probably a bad thing; politicians might decide the bombs are clean enough to use.

even without actual radioactivity, pure fusion bombs would still be politically radioactive. look at the fallout (so to speak!) from the Hafnium controversy. they nixed all the research and stopped looking, after realizing that nuclear isomers would do little for energy storage (due to emitting energy as gamma radiation) but lots for bypassing restrictions on fissile materials.

To be clear, pure fusion bombs would still emit massive amounts of radiation. Gamma rays, x-rays, thermal radiation, all off that EM radiation would be emitted just like a regular fission bomb. Neutron radiation too. You'd have less (not zero) contamination of the earth itself afterwards, but everybody in the area would still be very badly irradiated.

I don't know enough about the Hafnium controversy to comment on it.

I mean almost all of the power in a nuclear bomb comes from fusion. The fission part of it is just like the detonator for the real explosion.

Almost all nuclear weapons rely heavily on fission of the tamper for yield.

Suggest "Ripple: An Investigation of the World’s Most Advanced High-Yield Thermonuclear Weapon Design" from the Journal of Cold War studies to read about a predominantly fusion device family.

This seems to say to me that D-T reactions produce neutrons, and that the kinetic energy of the neutrons is smaller than what you get by hitting U with that neutron. You already have the energy from the neutron (which will land somewhere in the system eventually), and you might as well get a multiplier by putting a blanket of U-238 in front of it.

That could be carbon-copied to a fusion power plant, and indeed, there are many proposals of hybrid fusion-fission plants in the literature that only require Q values marginally greater than 1. But if you go that route, you have radiation just like a fission plant, and one starts to question why you don't just build a fission plant (indeed, why don't we?).

My personal pet theory of the future is that, one day, we'll progress so far in fusion research that we get economic energy. But at the same time, the line blurs between both fission and weapons technology, so people are unhappy with the result. This doesn't feel particularly contrarian but no one ever seems to bring it up.

Since you asked: We don't build fission plants because they cost more than every other energy source. Fusion plants, if they could ever be made to work at all, would cost a lot more. So, there won't be any.

This is actually backwards. Fusion weapons are substantially higher yield because they result in more fission, partly by preventing the fission primary from blowing itself up before it has finished.

Wikipedia: "Fast fission of the tamper and radiation case is the main contribution to the total yield and is the dominant process that produces radioactive fission product fallout."

https://en.m.wikipedia.org/wiki/Thermonuclear_weapon

It varies quite a bit by design, apparently the USSR’s initial design was only 15-20% fusion while US designs where closer to 50% which is still apparently the most efficient option in terms of warhead size.

However it’s possible to have higher fusion ratios at the expense of a larger device for the same yield. Most notably in the case of the Tsar Bomba’s which reduced the contribution of fission and too massively reduce the amount of fallout produced.

Most of the fission energy in an H-bomb comes from the massive amounts of neutrons created by the fusion bomb initiating a fission reaction in the U-238 tamper of the secondary. The primary is used for its X-Rays, which cause the incredible pressures within the secondary by ablating the surface of the cylinder. When you look at this experiment and see it uses x-rays to ablate case containing the hydrogen, causing an implosion, the purpose of the experiment is clear.

This particular research is literally built off of weapons research (National Ignition Facility).

Yes but, as far as I know, research into achieving laser induced fusion hasn't itself run into many classification hurdles. I suspect that may not be the case with the scaling phase.

It is interesting that there hasn't been many classification hurdles on something that is pretty explicitly weapons research. My guess is that they achieved high Q values a long time ago in classified research using mechanisms that would give away some secrets.

I think it's mainly because using a laser the size of a football stadium was never seen as a practical way to make a bomb.

Well, its not so much in making the bomb, but the particular ways of focusing the X-Rays and particular efficiencies in creating the tamper for the most velocity and thus most compression in the secondary.

The key thing here is the Lasers aren't doing the immediate compression, the lasers are simulating X-Ray radiation which then is ablating the casing around the tritium. Figuring out how to create and amplify a x-ray pulse was a major sticking point in the Star Wars program.

It's intended to confirm computer simulations. It doesn't have to be viable for the end article.

The bigger the target, the more energy is needed to compress it. So it would require more laser energy to get to 100 MJ or 1 GJ, I think. But maybe only a few times more powerful. (Pity they didn't build in some head room!)

The question is whether once ignition is achieved there could be a way to design the target/geometry in such a way that the fusion reaction becomes self-sustaining/self-propagating.

The history of thermonuclear weapons leads me to think that the answer may be yes. There does not seem to be any real upper limit to how large a thermonuclear weapon can be made. In the 50's and/or 60's there were proposals to build GT devices. I don't think the fission trigger for such a weapon could have scaled by nearly as much.

So by analogy if a relatively small fission trigger can cause a fusion explosion that is many orders of magnitudes larger maybe one or more tiny laser induced fusion reactions could be used to trigger a much larger one.

If that were the case the efficiency of the laser trigger would be of little importance.

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