Why is polywell supposed to be better than cusp confinement?

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

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Art Carlson
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Why is polywell supposed to be better than cusp confinement?

Post by Art Carlson »

Greetings! After some interesting though inconclusive exchanges with some of you on cosmiclog.msnbc.msn.com, I thought it would be fun to drop by here. Allow me to introduce myself. I have a Ph.D. in plasma physics from the University of Washington, where I worked on Field-Reversed Configurations. After that spent 16 years working on tokamaks at the Max Planck Institute for Plasma Physics in Germany. The last time I worked as a plasma physicist was 5 years ago. I am currently working on making a catalog of galaxy clusters using grid computing technology.
With this biography, it should be clear that my interest in the polywell arises out of a long-standing interest in fusion in general and alternate concepts in particular. I believe that the tokamak is a long shot as a practical reactor, but it's the best game in town. If I can, I will uncover the soft underbelly of the polywell concept and eviscerate it. Then you will all thank me kindly for stopping your waste of time, and we will all move on. If, on the other hand, you are able to convince me using solid physics that the polywell has a real potential, I will meekly withdraw my arrogance and claim that I said that all along.
It is not always clear what the polywell is supposed to look like, and some of the proposals contradict each other. This is legitimate, but makes it difficult to know where to start with a critique. I think my biggest difficulties are with the non-thermal ion velocity distribution, but it may not be as important to the concept as sometimes claimed. More basically, it looks like the polywell should be classed as high beta cusp confinement. I'm not familiar with the details, but I know that this idea is as old as the hills and was rejected very early. Why is the polywell supposed to be better? Possibly it is the idea of using point cusps instead of line cusps. If that is the case, then I will amplify the reasons I have already indicated for believing that line cusps are unavoidable.
There. I have staked my ground. Let the games begin.

tonybarry
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Post by tonybarry »

Hello Dr. Carlson,
Welcome to Talk-Polywell. I hope your visits here are as productive for you as I am sure they will be for me.
Art Carlson wrote: If I can, I will uncover the soft underbelly of the polywell concept and eviscerate it. Then you will all thank me kindly for stopping your waste of time, and we will all move on.
May I say that this is our goal too. If the polywell is to work, then it must withstand all criticism. If it is impractical, then the sooner we find out, the better. If it is the Real Deal, then we all benefit.

Regards,
Tony Barry

MSimon
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Post by MSimon »

As you may (or may not) know I am one of the strongest proponents of the BFR/Polywell on the 'net. When I read your statement:
If I can, I will uncover the soft underbelly of the polywell concept and eviscerate it. Then you will all thank me kindly for stopping your waste of time, and we will all move on.
I was most pleased.

So far I have done as much as I have been able to find holes (and fill them). If you can find some holes we haven't it would be most helpful.

I think you will find the technical ability to sycophant ratio rather good here. Not to mention that rnebel shows up here from time to time.

Welcome aboard.
Engineering is the art of making what you want from what you can get at a profit.

rcain
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Post by rcain »

Welcome Dr Carlson :)

I am fairly new here myself but have been following your discourses with Dr Nebel with great interest.

My background is cybernetics, not physics, but I can just about follow the science. I hope to learn and to be there when they hand out the t-shirts.

Essential to solving any problem is posing the problem right in the first place - I sincerely believe your experience and thinking can only help us all on our way here.

Looking forward to what lies ahead...

drmike
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Post by drmike »

Howdy Art! Welcome aboard, there's plenty of physics to discuss, and lots of experiments to do. Hopefully we can prove this will or won't work with some pretty simple equipment.

To start off with, I'd like to point out Krall's description of a stable plasma with magnetic field interface. Basically, the Polywell (or BFR == Bussard Fusion Reactor) is a 3D stable plasma-magnetic field structure. So to zeroth order, it is at least a stable plasma configuration.

It is also a standard IEC configuration, which has proven to generate fusion neutrons when fed with deuterium. So again, to zeroth order, the BFR is an IEC fusion reactor.

The game is on! Your move. :D

And I hope you are successful, either way. Plasma physics is fun!

scareduck
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Re: Why is polywell supposed to be better than cusp confinem

Post by scareduck »

Art Carlson wrote:I'm not familiar with the details, but I know that this idea is as old as the hills and was rejected very early. Why is the polywell supposed to be better? Possibly it is the idea of using point cusps instead of line cusps. If that is the case, then I will amplify the reasons I have already indicated for believing that line cusps are unavoidable.
Expand on this, please.

rnebel
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Post by rnebel »

Hi Art. (I hope we can forgo titles here). Anybody from UW can't be all bad! I've also done some work on FRCs, but mostly at LANL I worked on Reversed-Field Pinches and fiber Z pinches. I know most of the UW FRC crew. I've worked on electrostatics off and on for over 15 years.
What I think you will find here is that there aren't obvious fatal flaws in the Polywell. It has been vetted by some pretty good plasma physicists (Nick Krall, for instance worked on it for about 20 years) and nobody has been able to shoot it down, although some people think they have. However, there are some things that are going to be "challenging" and they may or may not work. This isn't a slam dunk. However, if we can make this work it is a very attractive system.
When we talk about fusion there are three important parameters: density, Temperature (or energy) and confinement. For conventional magnetic devices density is easy, temperature and confinement are difficult. For electrostatics, temperature is easy, density is difficult, and confinement may or may not be difficult. The problems faced are almost orthogonal to magnetic devices. That's not to say that they are easier, just different. I've found that a lot of times the intuition I developed working on magnetic confinement devices fails for electrostatic devices.

Art Carlson
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Post by Art Carlson »

Then you would be Rick. Seems appropriate to use first names among scientists and also for an internet forum. Call me Ishmael. My thesis advisor was George Vlases. The LANL folks are more likely to know Fred Ribe. My actual thesis experiment was done at Mathematical Sciences Northwest, whatever it may be called now.
I realize that intuition is developed in a context and, while it can still be useful outside that context, is no longer reliable there. If you permit, I will barge ahead anyway, and let you explain what went wrong when I make unwarranted assumptions.
Can you start by doing a "compare and contrast" between polywell and cusp confinement?

TallDave
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Post by TallDave »

Let me also say welcome Art! Great to have more knowledgable people here, esp. people who are skeptics.

My take on Art's question (if I'm understanding it correctly) is that Polywell confinement is enhanced cusp confinement plus recirculation plus a magnetically shielded grid. The cusp confinement works mainly to amplify the ratio of density inside the coils versus outside (Bussard says this ratio can reach a value into the tens of thousands; he estimated a value of around a thousand to one is necessary for fusion iirc) while the magnetic shielding and conformal coil shapes eliminate grid losses. The electron pushback against the B fields also enhances cusp confinement by "squeezing" the cusps nearly closed (or at least making the angle less hospitable to electrons trying to bounce out).

That's what I get from the Valencia paper, anyway.
Last edited by TallDave on Thu Jun 26, 2008 5:14 pm, edited 1 time in total.

MSimon
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Post by MSimon »

Art Carlson wrote:Then you would be Rick. Seems appropriate to use first names among scientists and also for an internet forum. Call me Ishmael. My thesis advisor was George Vlases. The LANL folks are more likely to know Fred Ribe. My actual thesis experiment was done at Mathematical Sciences Northwest, whatever it may be called now.
I realize that intuition is developed in a context and, while it can still be useful outside that context, is no longer reliable there. If you permit, I will barge ahead anyway, and let you explain what went wrong when I make unwarranted assumptions.
Neural network programming is very good at "feeling" patterns, if you have fed the network useful patterns for the problem. If not everything feels strange and "wrong".

One of the worst problems I have seen in that respect is the lock the idea of temperature has on the tokamak trained folks. I have seen it lead many otherwise knowledgeable folks astray when it comes to discussing BFRs. It may be "true" when dealing with non-Maxwellian distributions but it is not useful.
Engineering is the art of making what you want from what you can get at a profit.

rnebel
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Post by rnebel »

Art:

Two comments on cusp confinement:

1. Just like with FRCs, confinement times in a Polywell don't have to be comparable to those in a Tokamak since it is envsioned that the system will operate in the 1e16/cm**-3 (average density) range. The parameters for these machines tend to be between ICF and magnetic confinement. These machines will require 10s of msec for breakeven, if the focussing works. More if it doesn't.

2. At high beta, these systems operate in the "wiffleball" mode. "Mirror-like" confinement is terrible in both Polywells and cusps. It's not much better than ballistic confinement. The field null effectively works like a scattering mechanism everytime the electron passes through the system. If the gradiant scale length of the magnetic field is greater than the Larmor radius, then the adiabatic invariants hold over most of the plasma radius and you get mirror-like confinement. I suspect that most cusps operate in this regime (correct me if I'm wrong) and it's likely that the early Polywells such as the HEPS device were in this regime as well. Furthermore, mirror confinement scales like B (mirror ratio) and wiffleball confinement scale like B**2 (Larmor radius **2). If you would like more explanation of that, I’ll supply more details later. The data shows that the Polywell is clearly in the wiffleball mode.

Rick Nebel

pstudier
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Re: Why is polywell supposed to be better than cusp confinem

Post by pstudier »

Art Carlson wrote:Greetings! After some interesting though inconclusive exchanges with some of you on cosmiclog.msnbc.msn.com, I thought it would be fun to drop by here.
If you haven't already done this, go to http://tech.groups.yahoo.com/group/IEC_Fusion/ and read the recent posts by Indrek. He has done a lot of simulations, and generated interesting pictures, but has turned into a skeptic.
Fusion is easy, but break even is horrendous.

drmike
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Post by drmike »

Interesting coincidences. Fred Ribe got me into plasma physics and grad school because I invited him to give a talk at Boulder. I finished at UW under Conrad.

To start with, check out Bussard's comparison of "Multi-cusp confinement" with cusp confinement: http://stinet.dtic.mil/cgi-bin/GetTRDoc ... tTRDoc.pdf

The main difference is presence of electrostatic fields.

rcain
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Post by rcain »

rnebel wrote:Art:
...
The data shows that the Polywell is clearly in the wiffleball mode.

Rick Nebel
Wow! you mean WB mode works already? I didn't realize it was out of the modelling stage. Sounds really positive.

To pstudier: On the other hand, worrying to hear Indrek has turned septic, what has turned him do you suppose? (Maybe we should ask him).

scareduck
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Post by scareduck »

rcain wrote:To pstudier: On the other hand, worrying to hear Indrek has turned septic, what has turned him do you suppose? (Maybe we should ask him).
http://tech.groups.yahoo.com/group/IEC_ ... ssage/1407

Edit: If indrek has turned septic, better send him to the hospital right away.
Last edited by scareduck on Thu Jun 26, 2008 11:03 pm, edited 1 time in total.

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