Carlson and Nebel

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

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TallDave
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Postby TallDave » Wed Jul 02, 2008 4:34 am

Ah, thank you drmike!

I should have looked harder.

Art Carlson
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Postby Art Carlson » Wed Jul 02, 2008 6:03 am



Yeah. I got the assignment and finished it before writing my latest tirade. Most of what I got out of that paper was confirmation of the gyroradius loss width and the renewed impression that Bussard erroneously thought he could turn a bi-conical cusp into a configuration with only point cusps by adding more coils. Is there something in particular you think I'm overlooking in there?

TallDave
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Postby TallDave » Wed Jul 02, 2008 4:52 pm

Art,

I've never worried much about this, because it seems impossible WB-6 could have gotten the neutron counts at those drive depths if WB trapping didn't happen.

It looks like the SCIF device referenced in the paper did provide some experimental data in this regard, but was relatively low-power. I would guess there is better WB trapping data from future models, but I haven't seen it. Maybe someone will share it, if it's not under lock and key.

Solo
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Postby Solo » Wed Jul 02, 2008 6:32 pm

Art Carlson wrote:impression that Bussard erroneously thought he could turn a bi-conical cusp into a configuration with only point cusps by adding more coils


That seems like a good summary of the concept. So I gather your main contention (at the moment) is that this won't actually produce all point cusps: it will leave line cusps where the coils are nearly tangent to each other. Is that a good summary of what you are saying?

Art Carlson
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Postby Art Carlson » Wed Jul 02, 2008 7:10 pm

Solo wrote:
Art Carlson wrote:impression that Bussard erroneously thought he could turn a bi-conical cusp into a configuration with only point cusps by adding more coils


That seems like a good summary of the concept. So I gather your main contention (at the moment) is that this won't actually produce all point cusps: it will leave line cusps where the coils are nearly tangent to each other. Is that a good summary of what you are saying?


Yes. That's the jist of it.

Aero
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Yes, line cusps exist but they are not insurmountable

Postby Aero » Thu Jul 03, 2008 1:51 am

Gently speaking, have you read Bussard's 2006-9 IAC paper? In which he freely acknowledges the existence of line cusps in WB-6? He points out some other things regarding them and WB-6 but I don't want to misquote him so I point to the paper, its an easy read.
Aero

scareduck
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Re: Yes, line cusps exist but they are not insurmountable

Postby scareduck » Thu Jul 03, 2008 2:16 am

Aero wrote:Gently speaking, have you read Bussard's 2006-9 IAC paper? In which he freely acknowledges the existence of line cusps in WB-6? He points out some other things regarding them and WB-6 but I don't want to misquote him so I point to the paper, its an easy read.


Art has already said the IAC paper won't fly because it amounts to a kind of Fermat's Last Theorem if you are feeling charitable ("I have a truly marvellous proof of this proposition which this margin is too narrow to contain"), or "the dog ate my homework" if you aren't.

Aero
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Re: Yes, line cusps exist but they are not insurmountable

Postby Aero » Thu Jul 03, 2008 6:35 pm

scareduck wrote:
Aero wrote:Gently speaking, have you read Bussard's 2006-9 IAC paper? In which he freely acknowledges the existence of line cusps in WB-6? He points out some other things regarding them and WB-6 but I don't want to misquote him so I point to the paper, its an easy read.


Art has already said the IAC paper won't fly because it amounts to a kind of Fermat's Last Theorem if you are feeling charitable ("I have a truly marvellous proof of this proposition which this margin is too narrow to contain"), or "the dog ate my homework" if you aren't.


Well, thats very interesting. Dr. Bussard makes a point and Dr. Carlson agrees with him, now it is up to us to prove that they are both right. Do I correctly understand the issue here?
Aero

93143
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Re: Yes, line cusps exist but they are not insurmountable

Postby 93143 » Thu Jul 03, 2008 7:55 pm

Aero wrote:Well, thats very interesting. Dr. Bussard makes a point and Dr. Carlson agrees with him, now it is up to us to prove that they are both right. Do I correctly understand the issue here?


Well, that's the starting point. The actual issue appears to be that Dr. Bussard thought the Polywell would still work even with line cusps, provided recirculation was allowed for, whereas Dr. Carlson thinks it won't, at least once you scale it up.

If we can reduce this to a disagreement about whether or not recirculation happens, we can call the question closed (pending experiment) and move on to recirculation.

I made an attempt here to demonstrate through physical reasoning that Bussard was correct. I'm still waiting for a response. I mention the Valencia paper a few times, but my argument doesn't depend on it.

drmike
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Postby drmike » Tue Jul 08, 2008 1:36 pm

Art - your question was "what is the difference between regular cusp devices and polywell." If you'd have read Bussard's paper, you would have seen this:
Thus much, if not all, of the historical work on "cusp confinement" is simply not of relevance to the problem in Polywell systems. Conclusion, perseptions and "understanding" drawn from this body of work are thus often either irrelevant, valueless or wrong.


The positions of the "cusps" have nothing to do with his fundamental argument. I think a better way to question this work is to ask how he expects the counter currents to build up into the "wiffle-ball" mode. All the
assumptions are 1-D but to build up these currents requires 3-D analysis.

Art Carlson
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Postby Art Carlson » Tue Jul 08, 2008 1:53 pm

drmike wrote:Art - your question was "what is the difference between regular cusp devices and polywell." If you'd have read Bussard's paper, you would have seen this:
Thus much, if not all, of the historical work on "cusp confinement" is simply not of relevance to the problem in Polywell systems. Conclusion, perseptions and "understanding" drawn from this body of work are thus often either irrelevant, valueless or wrong.

The positions of the "cusps" have nothing to do with his fundamental argument. I think a better way to question this work is to ask how he expects the counter currents to build up into the "wiffle-ball" mode. All the
assumptions are 1-D but to build up these currents requires 3-D analysis.

Oh, good! Have I finally found someone who can explain to me what Bussard was saying? I saw this statement and continued reading in anticipation, but I was not able to figure out why he thought the polywell was so different from (other) cusps. BTW, as I understand Bussard, the reduction of losses from line cusps and point cusps due to high beta effects - i.e., the whiffle ball mode - is also seen in the bi-conical cusp (i.e. 2-d).

drmike
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Postby drmike » Tue Jul 08, 2008 2:10 pm

Bussard felt that the addition of electrostatics made the polywell sufficiently different from the other cusp confinement systems. I think you need 4 coils in a tetrahedron as a minimum rather than a biconical form because you won't get the pseudo spherical E field portion.

If you try to chop it down from a 3D problem, I don't see how it can work. With just 2 coils, you have a 2D problem and there is no cross current. With "corners" you can have currents flowing opposite of the coil currents, but also coupling the different coils.

The problem I have with Bussard's analysis is that he never states how these currents arise, nor does he really describe where they flow.

Also, when he talks about "recirculation" he means within the coils. My reading of his stuff indicates that anything that gets past the coil radius is "lost". In that sense, it seems very similar to all other cusp systems.

Art Carlson
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Postby Art Carlson » Tue Jul 08, 2008 3:46 pm

drmike wrote:Bussard felt that the addition of electrostatics made the polywell sufficiently different from the other cusp confinement systems. I think you need 4 coils in a tetrahedron as a minimum rather than a biconical form because you won't get the pseudo spherical E field portion.

By "electrostatics" do you mean having a plasma with a negative potential? Why shouldn't that be equally possible in a bi-conical cusp machine?
If you try to chop it down from a 3D problem, I don't see how it can work. With just 2 coils, you have a 2D problem and there is no cross current. With "corners" you can have currents flowing opposite of the coil currents, but also coupling the different coils.

I don't understand. What do you mean by "cross current".
The problem I have with Bussard's analysis is that he never states how these currents arise, nor does he really describe where they flow.

Also, when he talks about "recirculation" he means within the coils. My reading of his stuff indicates that anything that gets past the coil radius is "lost". In that sense, it seems very similar to all other cusp systems.

I think your interpretation of what Bussard said is untenable. In the Valencia paper, e.g., he says (my emphasis)
This means that the ONLY Polywell
systems that can be made to work are those in which there is
NO metal surface exposed - this requires open cusp,
recirculating electron flow, around B field coils that are
spatially conformable to the magnetic fields surfaces that
they produce.

and
Cross-field transport constitutes an
unavoidable loss to coil structure, while cusp axis flow need
not be a “loss“ if the device is open and the electrons can
recirculate along the cusp axes to the outside of the machine,
thence to return along cusp axes field lines
.

and
WB-5 was a closed box
machine, like HEPS, with its coils outside - so that it could
not allow e- recirculation out and back through its magnetic
cusps
.

I think it is clear that Bussard thought the machines operated in a way we just decided was impossible. In fact, he thought they will not work any other way:
Thus, in order for a Polywell to be driven in the mode
described for the basic concept, open, recirculating MaGrid
(MG) machines are essential.


Side note: I was feeling a little embarrassed for thinking Bussard was concerned about the electron density outside the machine, when I knew that arcing depended on neutral density. While responding to your post, I found this in the Valencia paper (again, my emphasis):
In a recirculating MG machine, this factor is important since
it sets the minimum density that can be maintained outside
the machine, for any given interior edge density, as required
for sufficient fusion production. It is desired to keep this
outside density low, in order to avoid exterior Paschen curve
arcing
, which can prevent machine operation. To have low
exterior density of electrons, and high interior density
requires large Gwb factors, thus, good Wiffle Ball
confinement is essential to system operation at net power.

I was confused about what denstiy was meant because Bussard was confused about it, too.

Personal opinion: Between the fact that we have decided that electrons lost through the cusps cannot recirculate outside the coils, and the fact that Bussard was confused about the dramatic difference between electron density and neutral density, we can pretty much forget everything Bussard thought about polywell physics.

MSimon
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Postby MSimon » Tue Jul 08, 2008 3:48 pm

The problem I have with Bussard's analysis is that he never states how these currents arise, nor does he really describe where they flow.


Yeah. I'd love to see the current flow diagram of the plasma. Esp. the electrons. I don't think static analysis can get you the answer. It is a 3-D real time problem. I think that is all implied by the WB effect. It is quite possible that there is no way to do useful simulations until we actually know how the device works. i.e. we know what shortcuts are valid.

There is also the question of the difficulty of getting into the WB state. Is it path dependent? What are the parameters that give you the highest probability of entering the WB state? Or is it "natural"?
Engineering is the art of making what you want from what you can get at a profit.
http://protonboron.com/
THE OPEN POLYWELL FUSION CONSORTIUM

MSimon
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Postby MSimon » Tue Jul 08, 2008 3:59 pm

Personal opinion: Between the fact that we have decided that electrons lost through the cusps cannot recirculate outside the coils, and the fact that Bussard was confused about the dramatic difference between electron density and neutral density, we can pretty much forget everything Bussard thought about polywell physics.


I tend to be of a somewhat similar opinion. I don't think you get "recirculation". I think you get oscillation.

It is an oscillating beam machine. The Dietrich paper (MIT) and his lab partner McGuire did some interesting work that tend to point in that direction.

However, the wrong explanation for something interesting is not unusual.

The question at this point is: How good was Dr. B. in the lab? That question will be answered soon.
Engineering is the art of making what you want from what you can get at a profit.
http://protonboron.com/
THE OPEN POLYWELL FUSION CONSORTIUM


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