How can a hohlraum prevent laser fusion instability?

[spurserh]

Hey everyone,

I am not knowledgeable in the field of physics, so please excuse me if my question is obvious or poorly worded. I have found myself reading recently about various Nuclear Fusion research, and just now I came across an overview of fusion contained by lasers, specifically at the National Ignition Facility. There seems to be an attempt to focus and calibrate the lasers perfectly enough to contain a plasma for long enough to "burn" a meaningful amount of fuel.

My question pertains to the "problem" with containing the plasma this way as I have perceived it from reading: apparently, the calibration of the lasers so precisely is very difficult and usually the plasma does not stay contained due to some tiny imperfection. That leads me to wonder: since people are willing to build enormous facilities to run these tests, why don't we accept the imperfections by doing something to distribute them evenly around the plasma? A simple conceptual example would be a sphere of laser sources that spins around the plasma in a randomized way, such that the imperfections in the laser calibration are distributed more or less evenly around. Is it just that redirecting the lasers quickly enough would be too large of a technical challenge?

Thanks for your answers, Sean

 

[mathman]

The NIF uses very short duration laser pulses, so your suggestion (spinning source) can't apply.

 

[spurserh]

I don't actually mean a spinning source; that's just a way to think about it. I have no idea how it could actually be done - or are you saying that it simply can't be done theoretically for such a short time?

 

[Astronuc]

The fuel target in inertial confinement is compressed and momentarily confined by the pressure induced by the momentum of the ablation layer. The ablation layer is the outer solid surface that absorbs the laser light.

With asymmetries in the surface pressure field, the fuel doesn't get compressed in those spots and then can be blown out in the low pressure spots.

Ideally the ablation is spherically symmetric over the surface, that is done with the spatial targeting and timing of the lasers and laser pulse shape.

 

[spurserh]

I understand that, but I am under the impression that that sort of perfect "spatial targeting and timing of the lasers and laser pulse shape" has yet to be achieved. My question, really just a thought experiment to help my own understanding, is why we don't just accept the imperfection and work around it by scattering the lasers at random somehow (so that the distribution is even, despite the inherent imperfection of the source)?

This idea stems from my background in computer graphics, where this kind of thinking is pretty common. For example, in generating a realistic wood grain using only a "pseudo-random" number generator. Of course, in the case of computers, one is working at the problem from the opposite side: computers are too perfect, and to make realistic simulations with them, one must introduce a modicum of imperfection. In some sense I am wondering why we don't work in the opposite direction on this problem.

 

[J Makepeace]

Once "First Ignition" of a single pellet of Deuterium/Tritium fuel has taken place at NIF (with an extremely perfect spherical pellet) the next step for laser fusion is to make that process work on a continuous basis. That means (among other things) mass-producing the fuel pellets, which will burn in a laser fusion plant at a rate of about 5 per second.

To make pellets at this rate means they cannot be so perfectly spherical... and that leads to the solution of "Fast Ignition" in which a second, much shorter and more intense laser shot, which will "tunnel" through the plasma first set of laser shots and coincide as the initial converging shockwave reaches the core, thus reaching the density and tempterature necessary to trigger fusion, even in a relatively asymetrical pellet.

This work is going foward under the HiPER Laser project, a UK-led European initiative with partners worldwide. Google "HiPER Laser" for more information on this.

 

[spurserh]

That's interesting, but again, unless I'm missing something, it doesn't seem to have much to do with my question?

In other words, what I'm wondering is why we don't scatter the lasers into an even probability distribution so that imperfections are smoothed out. Is it because imperfections in the target itself would make it moot?

 

[hagopbul]

you are asking about a specific information that could be use in weaponry right ,
you need numbers and blue prints

and i don't buy the idea that it is "I am not knowledgeable in the field of physics, so please excuse me if my question is obvious or poorly worded. I have found myself reading recently about various Nuclear Fusion research, and just now I came across an overview of fusion contained by lasers, specifically at the National Ignition Facility"

this kind of reading is not a chance , and not a hobby , you are asking like you are from...

 

[spurserh]

Oh! You got me! I am really from Iran and I want to destroy your homeland. Muaaahahahaha

All I need to know is how to scatter the lasers in my football-field sized fusion facility and my plan for world domination will be complete! You see, I want to build a new Ignition Facility and drop it on New York City, where its sheer size and uselessness will crush everyone!

 

[mathman]

That's interesting, but again, unless I'm missing something, it doesn't seem to have much to do with my question?

In other words, what I'm wondering is why we don't scatter the lasers into an even probability distribution so that imperfections are smoothed out. Is it because imperfections in the target itself would make it moot?

Your method doesn't give the results you are trying to get. Once the lasers are in place they are fixed for the given shot so their placement is worse than distributing them as evenly as possible.

 

[spurserh]

"
Your method doesn't give the results you are trying to get. Once the lasers are in place they are fixed for the given shot so their placement is worse than distributing them as evenly as possible.
"

I was talking about distributing them evenly, and asking why we keep them fixed as opposed to that. I think you've got it backwards?

 

[mathman]

Your expression was "even probability distribution " which is quite different from making the distribution as even as possible.

 

[spurserh]

How so?

 

[berkeman]

you are asking about a specific information that could be use in weaponry right ,
you need numbers and blue prints

and i don't buy the idea that it is "I am not knowledgeable in the field of physics, so please excuse me if my question is obvious or poorly worded. I have found myself reading recently about various Nuclear Fusion research, and just now I came across an overview of fusion contained by lasers, specifically at the National Ignition Facility"

this kind of reading is not a chance , and not a hobby , you are asking like you are from...

I was inclined to delete your post, hagopbul. But spurserh's response is just too darned funny. I have to leave the pair of posts.

 

[berkeman]

That's interesting, but again, unless I'm missing something, it doesn't seem to have much to do with my question?

In other words, what I'm wondering is why we don't scatter the lasers into an even probability distribution so that imperfections are smoothed out. Is it because imperfections in the target itself would make it moot?

Actually, I think J Makepeace's post is pretty good at addressing your question. It looks like the HiPER folks are asking the same questions you are, and their potential solution (after looking at many possibilities I'm sure) sounds interesting.

Your original post (OP) asked "why don't we accept the imperfections, and do something..." after which you suggested some possible ways. But given the physics of the very short time duration of the imploding shock wave, the best we can do is make it as symmetrical and simultaneous as possible, and look for tricks like HiPER to augment whatever we have.

LLNL is just over the hill from me... Need to see if I can get on a tour sometime.

https://lasers.llnl.gov/

.

 

[spurserh]

I should have realized where the confusion was - I was talking about a probability distribution over time, not fixed per "ignition". But it sounds like the likely conclusion is right - that there is no material that can scatter the lasers [into different arrangements] that quickly? For a conceptual example, some crystal with imperfections in it which could be vibrated very quickly?

 

[berkeman]

I should have realized where the confusion was - I was talking about a probability distribution over time, not fixed per "ignition". But it sounds like the likely conclusion is right - that there is no material that can scatter the lasers [into different arrangements] that quickly? For a conceptual example, some crystal with imperfections in it which could be vibrated very quickly?

For low-power applications, there probably could be some neat adaptive mirror arrangement. In fact that would be a really cool high-speed feedback arrangement -- fun to work on.

But for these laser ignition setups, AFAIK, the powers are so high in the final stages of amplification that you are not going to put an adaptive mirror in the way. Just stand back and let her fire.

Hey, maybe we could organize a PF Field Trip to the NIF. That would be fun!

 

[hagopbul]

good one :)

 

[DoYouKnow]

Laser fusion instability is alleviated by a hohlraum, which is used in indirect drive fusion. By shooting lasers or particles at a metal cylinder surrounding the fusion fuel, the heating and subsequent blackbody x-ray emission of the hohlraum will evenly heat the fuel capsule. By making sure the hohlraum is uniform and the laser positioning is good, you can avoid Rayleigh-Taylor instabilities during implosion.

DoYouKnow
(Michael)