The problem with ion convergence

Discuss how polywell fusion works; share theoretical questions and answers.

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drmike
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Post by drmike »

I think asking fundamental questions is always a good thing. If the basic fundamentals don't look good, there is no way to make things work. Like Art said, it's easy to make mistakes and I hope I'm off by 3 or 4 orders of magnitude.

Here's and interesting paper on p-B fusion using particle beams. Even in principle it's pretty darn close to impossible. 1 MW needs 10^18 interactions per second. He needed 10000 revolutions of his beams to do that, and space charge would blow things up before that. The number of interactions is the same order of magnitude for a polywell. You need 10000 bounces, or a hell of a lot more current. But lots of current blows out your magnetic confinement.

We're all after the same thing - a good understanding of reality. "Victory" comes with the "Ah-Ha!" or "ah-shit!" It's the understanding that matters.

scareduck
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Post by scareduck »

I have to admit that Art's comment about Dr. Nebel's response upthread ("'it will be difficult to draw accurate quantitative conclusions' is pretty weak") is a good one; if this turns out to be a fundamental limitation, we'd better find out now.

kcdodd
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Post by kcdodd »

drmike wrote:There is another way to look at this calculation. It is the number of deflections that occur driving ions off center due to coulomb interactions. In that sense it's really good - on any one pass there are very few deflections and ions should stay radial for a good number of passes.

The cross section for p-B is about .1 barns for V ~ 100 kV. If we take the core of radius r_c, the probability of fusion is ~ i*n_0*sigma*r_c with various geometric factors ignored. i is the flux of ions flowing radially into the center.
For r_c ~ .1 m, n_0 ~ 10^20 m^-3, sigma ~ 10^-29 m^2 we get a fusion
rate of 10^-10 * i fusions/sec.

At 8.7MeV per fusion this gives ~ 10^-22 * i Watts. To generate megawatts takes huge flux, or huge density, or both.

But in terms of ion convergence, I'm thinking there's not much of a problem. It's just not likely to have enough fusions to generate net power.

But I need to think about it a lot more.....
Is n_0 the density? Of which species? Don't you need both or something?
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MSimon
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Post by MSimon »

If we can get D-D working we will eventually figure out how to make pB11 work.
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Aero
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Post by Aero »

drmike
Is that consistent with the experimental results from WB-6?
Aero

Art Carlson
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Post by Art Carlson »

MSimon wrote:If we can get D-D working we will eventually figure out how to make pB11 work.
"Blessed are they that have not seen, and yet have believed." Improving the triple product n*T*tau by a factor of 500 (OK, the numbers compare D-T to p-B11, not D-D to p-B11) won't be a walk in the park. It might even be impossible no matter how small you make your conduction and convection losses: Bremsstrahlung is a rock-bottom limit that is only a few percent of D-T fusion power density but approximately equal to p-B11 power density. And even if you could do it, I don't know why you would want to run on p-B11 when in the same machine you could get 2000 times more power out by running on D-T. OK, you have some neutrons to deal with, but I always think of neutrons as an elegant way to absorb power volumetrically, so you don't blast away your first wall. On the other hand, if you only produce piddling power, then your first wall isn't in danger.

MSimon
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Post by MSimon »

Art Carlson wrote:
MSimon wrote:If we can get D-D working we will eventually figure out how to make pB11 work.
"Blessed are they that have not seen, and yet have believed." Improving the triple product n*T*tau by a factor of 500 (OK, the numbers compare D-T to p-B11, not D-D to p-B11) won't be a walk in the park. It might even be impossible no matter how small you make your conduction and convection losses: Bremsstrahlung is a rock-bottom limit that is only a few percent of D-T fusion power density but approximately equal to p-B11 power density. And even if you could do it, I don't know why you would want to run on p-B11 when in the same machine you could get 2000 times more power out by running on D-T. OK, you have some neutrons to deal with, but I always think of neutrons as an elegant way to absorb power volumetrically, so you don't blast away your first wall. On the other hand, if you only produce piddling power, then your first wall isn't in danger.
The advantages of pB11 - direct conversion - should more than offset the factor of 2,000 increase in volume (a factor of 13 increase in linear size). Yes the reactor is more expensive. However, eliminating the steam plant is a huge gain in terms of plant manufacturability and operations. Steam plants require a LOT of maintenance. They are also long lead time items. Typically 3 to 4 years.

BTW due to the alphas produced in D-D fusion the first wall problem is not eliminated. Just reduced.

As you point out pB11 may not be feasible. However, with a working D-D machine the question will get a lot more attention than our, so far, piddling attempts to figure out how it might work. There will also be the advantage of real data to work with as opposed to our hopeful (pessimistic) guesses.

I am in agreement that the neutron problems (re: superconductors) can be worked around. It is not a show stopper.

I would avoid D-T machines due to the problems of T availability (manufacture).
Last edited by MSimon on Sun Jul 20, 2008 3:22 am, edited 1 time in total.
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zbarlici
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Post by zbarlici »

Blessed are they that have not seen, and yet have believed....

That|s pretty much how i feel about polywell right about now seeing as how Mr. Carlson|s concern regarding ion convergence has not been answered.

Regardless, ... i have to hope theres something qwirky within this polywell device which helps with ion convergence...

...and should the fall peer review deem the polywell as a viable fusion system(only by a miracle it seems to me as of now), then it would be a sure sign for Mr. Carlson to devote his time to its implemetation...

TallDave
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Post by TallDave »

"Blessed are they that have not seen, and yet have believed."
Heh, even Jesus had his Doubting Thomas. Of course, he had a higher burden of proof. He had to turn water into wine; we only have to turn hydrogen into helium.
And even if you could do it, I don't know why you would want to run on p-B11 when in the same machine you could get 2000 times more power out by running on D-T.
That's an interesting notion. How do you arrive at that? EDIT: Ah, the power density relation.

Of course, producing 2000x as much energy probably wouldn't be a good thing; that would probably destroy any reactor that was producing power. This would seem to come down to how much more expensive it is to burn p-B11 than D-D/D-T versus how much cheaper it is to convert alphas to electricity than neutrons.

What's the thermal loss from neutrons > electricity? 80% or so?
Last edited by TallDave on Sun Jul 20, 2008 3:36 am, edited 3 times in total.

MSimon
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Post by MSimon »

Art,

Let me add that 80% of a nuke plant's material cost is in the steam portion of the plant.
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cuddihy
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Post by cuddihy »

drmike wrote:There is another way to look at this calculation. It is the number of deflections that occur driving ions off center due to coulomb interactions. In that sense it's really good - on any one pass there are very few deflections and ions should stay radial for a good number of passes.

The cross section for p-B is about .1 barns for V ~ 100 kV. If we take the core of radius r_c, the probability of fusion is ~ i*n_0*sigma*r_c with various geometric factors ignored. i is the flux of ions flowing radially into the center.
For r_c ~ .1 m, n_0 ~ 10^20 m^-3, sigma ~ 10^-29 m^2 we get a fusion
rate of 10^-10 * i fusions/sec.

At 8.7MeV per fusion this gives ~ 10^-22 * i Watts. To generate megawatts takes huge flux, or huge density, or both.

But in terms of ion convergence, I'm thinking there's not much of a problem. It's just not likely to have enough fusions to generate net power.

But I need to think about it a lot more.....
you're using prob(fus) =i*n_0*sigma*r_c; geometry independent. Implies the central 10% of the device (0.1 m?) is an undifferentiated, isotropic thermal distribution. sigma is geometry independent, per Art's equation above.

I'm just confused about what you're calculating. Are you trying to show that if you have an isotropic, low density, thermal plasma in the machine that you don't get much fusion? I thought we already knew that.

Farther up, I'm trying to figure out, if the "lumpiness" of the field, that is the imperfect geometry, is really a potential problem, then how the equation for sigma can be geometry independent.

Can't have it both ways for collisionality-- geometry dependent where it hurts, not where it matters? Or am i confused on how you're calculating sigma?
Tom.Cuddihy

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blaisepascal
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Post by blaisepascal »

zbarlici wrote:Blessed are they that have not seen, and yet have believed....

That|s pretty much how i feel about polywell right about now seeing as how Mr. Carlson|s concern regarding ion convergence has not been answered.
I haven't answered his concern because I freely admit I don't have a clue about the physics involved. Physics is an interest, but not my vocation or training. I suspect many here are in the same boat.

As far as I am aware, few people here have the knowledge and skills necessary to properly address Dr. Carlson's concerns. We all lack good insight into what Dr. Bussard thought theoretically, and what he achieved experimentally.

At this point, it seems the only person posting here who has a very good understanding of the capabilities and limitations of the devices is Dr. Nebel -- and he isn't talking in detail until peer review happens. He does sound optimistic, though.

It seems the data embargos are the curse of polywell. The 10-year data embargo prevented any published peer review of the previous run of experiments until after the project shut down, and it is preventing us now from hearing about what's going on. I think, if I had the money to fund this research, I would ask for just the opposite: near contemporaneous public posting of data (even before analysis) and weekly status reports/blogging of what's going on, in terms of experimentation and analysis. The level of detail used by Armadillo Aerospace would be fine for the status reports. I'm sure there are good reasons to not do it that way (and can't be done that way under the current contract), but I'd like to try.

MSimon
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Post by MSimon »

A little birdie told me that there is news forthcoming and we will get it soon. The definition of soon is undetermined.
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choff
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Post by choff »

I don't understand the math either. My understanding comes from being exposed briefly to klystrons, magnetrons,and microwave guides during electronics training 35 some years ago, and reading scientific american articles on physics about as long.

To my way of thinking electron flows have been precisely controlled for the last century. If they weren't, my old vacuum tube TV would look fuzzy, and every time I used a microwave oven to heat up a cheese sandwich I'd need to replace the magnetron tube. As for using radar to land in the fog, forget it. I see the polywell as a large vacuum tube, probably less than a milligram of fuel is ever inside, hardly enough to destablize it. Trash me if I'm wrong.
CHoff

Art Carlson
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Post by Art Carlson »

I wanted to recall to mind the disadvantages, difficulties, and possible (probable) impossibility of aneutronic fusion, but I don't intend to get into a general discussion here. I would, however, like to challenge some statements by MSimon.
MSimon wrote:The advantages of pB11 - direct conversion - should more than offset the factor of 2,000 increase in volume (a factor of 13 increase in linear size). Yes the reactor is more expensive. However, eliminating the steam plant is a huge gain in terms of plant manufacturability and operations. Steam plants require a LOT of maintenance. They are also long lead time items. Typically 3 to 4 years.
MSimon wrote:Let me add that 80% of a nuke plant's material cost is in the steam portion of the plant.
This is the first time I have ever heard the claim that the steam portion of a nuclear plant accounts for 80% of the total. (Or is there a misleading formulation by referring to the "material" cost?) I would have put it the other way around. Of course the exact figure depends on what kind of plant you are talking about and how you do your accounting. I couldn't find any really satisfying figures in a hurry on-line, but what I found pointed toward something like 5-15% for the fraction of the capital costs attributable to the balance of plant.

It is also amusing to that MSimon thinks these costs could be reduced by going to direct conversion. In some studies it is hard to find the plasma because it is dwarfed by the direct conversion system. Direct conversion is in principle simple, potentially very efficient, and certainly elegant, but it is not easy and probably isn't going to be cheap.

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