Magrid Vs electrically biased grid

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

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MSimon
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Post by MSimon »

Concerning what M. Simon said, I assume he meant having conductive coatings on the nubs
I meant what I said. Standoffs. Nubs are a non-starter.
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KitemanSA
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Post by KitemanSA »

Remember, the mantra is that all metalic connection to the positive grid should be shielded by magnetic fields. The nubs were not. Well, slightly, but not a whole bunch. Move them away from the line-like cusp and they would be. Or, pass a whole lot more current thru them and they would be.

ladajo
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Post by ladajo »

I seem to recall Bussard thinking early on that the nubs would be shielded enough by the currents running in the magrid feed wires. however, it seems that was not the case based on the WB7.1 effort.
The standoffs could be shielded better by spiraling the feed wires inside. Being in the shadows of the coil and positioning them away from the cusps may not be enough. We won't know until we know something more concrete about how WB8 is put together as well as powered up.

edit: corrected my REALLY bad typing
Last edited by ladajo on Sun Jul 18, 2010 1:30 pm, edited 3 times in total.

D Tibbets
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Post by D Tibbets »

MSimon wrote:
Concerning what M. Simon said, I assume he meant having conductive coatings on the nubs
I meant what I said. Standoffs. Nubs are a non-starter.
Sorry, I misread :oops:
Dan Tibbets
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D Tibbets
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Post by D Tibbets »

ladajo wrote:I semm to recall Bussard thinking early on that the nubs would be shielded enough by the currents runnung in the magrid feed wires. however, it seems that was not the case based on the WB7.1 effort.
The standoffs could be shielded better by spiraling the feed wires inside. Being in the shadows of the coil and positioning them away from the cusps may not be enough. We won't know until we something more concrete about how WB8 is put together as well as powered up.
In his Google talk, I think Bussard implied that the nubs could be shielded if needed. But, in WB6 I believe there was only one wire traveling from one coil to the next through the nubs. The coils had ~ 200 windings, and produced ~ 1000 Gauss. The single wire would have produced ~ 5 Gauss in the nubs. Bunching perhaps 10 wires into a bundle and then using this bundle to wind the coils and pass through the nubs would shield them better, but the volume of the nubs would have to increase ~ 10 fold to do this- not good. With superconductors a better job could presumably be done, but considering the volume needed for insulating and cooling...

Once they appreciated the apparent losses to the nubs, I suspect they did not have to think long before deciding to abandom them.

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ladajo
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Post by ladajo »

I've alwasy thought it was kind of a no-brainer to switch to standoffs, but one never really knows until you try for real.

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Post by KitemanSA »

ladajo wrote:I've alwasy thought it was kind of a no-brainer to switch to standoffs, but one never really knows until you try for real.
for a commercial plant that only has to withstand earthquake type external loads, maybe. For Navy reactors that have to withstand weapons effects, such a complex support system may be untenable.

To me, a monocoque MaGrid seems the way to go.

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Post by ladajo »

Fair point. Shock rating does tend to throw in a monkey wrench or two, and a dollar or ten.

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Post by zapkitty »

Could the standoffs have intermediate supports without them getting in the way in turn? Supports that would reduce the cantilevering by creating load paths connecting the standoffs with each other and the chamber walls?

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Post by D Tibbets »

I suspect standoffs supporting each magnet from a ridged wall would withstand shocks better than a magrid assembly that is anchored to the wall on only one side- think whiplash. The shock absorbing buffers (rubber joints, etc. could be outside the vacuum vessel, between the reactor and the floor and various couplings to pumps, and pipes.

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Post by WizWom »

ladajo wrote:Fair point. Shock rating does tend to throw in a monkey wrench or two, and a dollar or ten.
Actually, a jiggling would not be as bad as all that... the core would flex. The precision of the placement affect the whiffleball factor, and that dropping in a production machine a few percent would be seen as a drop in fusion rate during the stress event.

As far as it goes, the design shown in the picture has the 4 supports going in at what looks like a 45 degree angle, which with 4 supports would be quite rigid.

And remember that a high vacuum chamber is going to have to be pretty rigid itself. All in all, I'd expect a Polywell to continue to fuse unless the vacuum vessel is breached.
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KitemanSA
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Post by KitemanSA »

zapkitty wrote:Could the standoffs have intermediate supports without them getting in the way in turn? Supports that would reduce the cantilevering by creating load paths connecting the standoffs with each other and the chamber walls?
I've never subscribed to the "standoffs are better" hocus pocus.
The elecrtons leaving the cups will travel almost out to whatever conducting surface is providing ground state potential. Those that have been upscattered will reach the ground state barrier and be "lost" without much loss of energy. Those that have not will ALMOST reach it. In the doing of that, they will tend to follow the mag field lines which take them either around and back in a differenct cusp, or partially around and back in the one they exited. In either case, the "around" will intersect a lot of that forest of standoffs.
If instead the unit were designed with an absolute minimum of connections tothe ground potential barrier (vacuum chamber wall?), and the "nubs" were moved away from the center of the line-like cusps and made to pass more current, then there would be an absolute minimum loss path for electrons To me, this make better sense.

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Post by KitemanSA »

D Tibbets wrote:I suspect standoffs supporting each magnet from a ridged wall would withstand shocks better than a magrid assembly that is anchored to the wall on only one side- think whiplash. The shock absorbing buffers (rubber joints, etc. could be outside the vacuum vessel, between the reactor and the floor and various couplings to pumps, and pipes.
We are now talking my area of expertice. Designing a system that has small gaps to maintain to withstand shock by attaching the components to widely seperated areas of a structure is a monumemtally dumb undertaking. IMHO, the reactor should be as monocoque as possible and as isolated from the chamber as possible. True, under shock, the MaGrid as a whole would move relative to the chamber wall, but it could easily be prevented from impacting it. And that is a lot better than having the coils moving relative to each other and smashing into each other.
As the design guidance section of MIL-S-901D says, nothing is rigid under shock.

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Post by D Tibbets »

Without working the numbers , any supports that are eg: 1/2 the minor diamiter of the magrid casing when the casings have a minor diameter of perhaps 40 to 50 cm, provides a lot of volume for very strong structure, insulating layers, etc. Another advantage of supports for each magnet casing is that the system is isolated. Coolent flow only has to cool one magnet( this may be very significant), not 6 or more in series. A single magnet can be changed out without having to dismantle the rest of the magrid, conversion grids, etc.
Also, at least while operating, the magnets are magnetically insulated from each other. Not only in the sense of charged particle insulation, but also structural insulation. Any movement of one magnet towards another neighboring magnet will be increasingly opposed by the magnetic fields as the distance shortens. Also, I'm not sure how the repulsive forces will compare to gravity, but I suspect the strongest force vector will be directly into the supports as the magnets will be trying very hard to get away from each other. I don't know the numbers, but I suspect there will be more than a few tons of force pushing out on each magnet. That is a lot of force that a necessarily small nub would have to resist.

Then there are considerations of nub losses. If they are greater than standoffs, then a larger and heavier magrid would be needed to make up the fusion gain differential.

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Post by MSimon »

The "around" idea conflicts with the oscillation idea. The consensus (worth almost nothing) is that oscillation is more likely in a beam machine with a positively charged grid.

IF we do have oscillating electron beams - the standoffs should capture very few electrons.
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