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BalderSion t1_j0tgpko wrote

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

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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