Cusp leakage in non-hexahedral configuration

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

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Schneibster
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Cusp leakage in non-hexahedral configuration

Postby Schneibster » Sun Mar 20, 2016 2:05 am

Having just read the paper posted on arXiv by the Polywell team I think that electron leakage from the cusps might be reduced in the octahedral or higher-order configurations; does anyone have any insight into this? I'm kinda throwing this out there for discussion. Is the reason higher-order polygons haven't been tried only monetary?
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krenshala
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Re: Cusp leakage in non-hexahedral configuration

Postby krenshala » Wed Mar 23, 2016 1:28 pm

My understanding is that it is both the money to make the 'extra' pieces, and the time to get them made. Why spend +33% (for the octohedral) or +100% (for the dodec) the time on making parts when you can stick with the cube and start testing sooner.

And yes, I really want to see a working dodecahedral polywell. ;)

hanelyp
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Re: Cusp leakage in non-hexahedral configuration

Postby hanelyp » Wed Mar 23, 2016 3:42 pm

The 6 and 8 coil configurations are duals of each other, the physical coil on one being a virtual coil on the other. So long as circular coils are used the 6 coil version appears to have better symmetry. Same for the 12 vs. 20 coil configurations.
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KitemanSA
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Re: Cusp leakage in non-hexahedral configuration

Postby KitemanSA » Sun Mar 27, 2016 3:07 am

As Dr. Bussard said, it would be wise to test out a square plan-form magnet for the cube-octahedron deignthey have been working on. Personally, I suspect that a bowsided square plan-form would do best.

D Tibbets
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Re: Cusp leakage in non-hexahedral configuration

Postby D Tibbets » Sun Mar 27, 2016 7:48 pm

The rounded square magnet shape may be the best compromise, provided the corners abut against each other, not the long sides. This long side paralleling the next side provides to much surface area close to the cusps (within several gyroradii) so ExB losses may become dominate. This is in line with the earlier (prior to WB6) flawed modeling where "Funny Cusps" were a given and desired condition.

With square magnets arranged with the corners abutting, the point cusps and corner cusps will be closer in size. The symmetry through the center will also be maintained. Cusp losses through the corners may be somewhat increased, and point cusp losses may be decreased. But this six magnet arrangement would be more spherical- quasispherical. When asked why 'Mini B' had such wide spacing between the magnets, Dr Parks did not elaborate on the effect on increased corner cusp losses, but he did say it was an effort to increase the sphericity of the machine. How this related to confinement issues was not addressed. As there was little potential well, any anticipated ion convergence to a common center would (I think) be insignificant.

With increased spacing between the round magnets in 'Mini B' the truncated corners of the cube would be larger and the 14 real and virtual magnets would form a more spherical shape. Near square magnets may increase this effect. Going to larger numbers of real magnets could do this better, but at the cost of more complexity, and more (admittedly smaller) point cups.

Dan Tibbets
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Schneibster
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Re: Cusp leakage in non-hexahedral configuration

Postby Schneibster » Sat Apr 02, 2016 1:02 am

Ahhh, so increasing the number of cusps might increase the loss, and the number of edges that approach the cusps doesn't mitigate this effect.

That makes sense. Thanks Dan!
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Re: Cusp leakage in non-hexahedral configuration

Postby KitemanSA » Sat Apr 02, 2016 10:02 am

D Tibbets wrote:The rounded square magnet shape may be the best compromise, provided the corners abut against each other, not the long sides. This long side paralleling the next side provides to much surface area close to the cusps (within several gyroradii) so ExB losses may become dominate. This is in line with the earlier (prior to WB6) flawed modeling where "Funny Cusps" were a given and desired condition.

The side to side configuration would be a cube, not a cube-octahedron. And in the cube-octahedron configuration, the abutting corners create the "funny cusp" as well as a physical construct can.

KitemanSA
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Re: Cusp leakage in non-hexahedral configuration

Postby KitemanSA » Sat Apr 02, 2016 10:05 am

D Tibbets wrote: Going to larger numbers of real magnets could do this better, but at the cost of more complexity, and more (admittedly smaller) point cups.
More magnets with fewer windings each.


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