Magrid Vs electrically biased grid

[DeltaV]

Regarding the nubs that connect the magnets; If the magnets are supported via the flanges don't you still have the supports in the field lines?

If by flanges you mean supports going to the vacuum chamber walls that hold each coil separately (no nubs), then field line density and electron density is much lower there than it would be at the nubs. Fewer electron losses.

Judging from the WB-7.1/8 picture at EMC2's web site, the supports connect slightly to the inside of each torus and are aligned with the magrid shadow. But that's all inference since they don't actually show the supports.

[sd_matt]

I forgot to mention that I am a layman with only a basic understanding of magnetism

If I understand things correctly you have a single wire going into each magnet, it winds many times, and then leaves (back to the flange).

First a couple prelim. question. Could or is each coil a single circuit? Is it a single wire winding many times through each magnet?

If so then..

Could you strengthen the field surrounding the supports by adding extra windings through the supports, lets say, alternating between the windings going just through the coil and then going through the coil and supports?

Ie. You have a coil with a wire that loops fifty times ( just throwing numbers ) through the coil itself and about five times through the supports.

[DeltaV]

If I understand things correctly you have a single wire going into each magnet, it winds many times, and then leaves (back to the flange).

Yes. It doesn't have to return through the same support. Minimum 3 supports per coil for structural stability, but more likely 4 per coil to keep everything symmetrical and allow more cooling system connections for continuous operation. Could use only two supports as conduits or use all four with a bifilar winding. Think superconductor surrounded by coolant layers in nested pipes (LHe, LN, H20). Unless diamond RTS pans out.

Could or is each coil a single circuit?

They can be separate circuits if the per-coil supports are used, and I think they are in WB-7.1+. Independent control of coil current and coil case potential is then possible (either for perfecting the overall field symmetry or for intentionally weakening one or more coil fields, but the latter is probably way downstream). The default nubs configuration with only one coil anchored wires coils in series and would be extremely difficult to cool.

Is it a single wire winding many times through each magnet?

Yes. Unless you go bifilar.

Could you strengthen the field surrounding the supports by adding extra windings through the supports, lets say, alternating between the windings going just through the coil and then going through the coil and supports?

Ie. You have a coil with a wire that loops fifty times ( just throwing numbers ) through the coil itself and about five times through the supports.

The supports are insulated and are not as big a draw for electrons (which are fewer outside the magrid anyway) compared to the metallic, positively charged nubs. Not necessary.

[ladajo]

Maybe you could spiral the Magrid power and return lines in the supports instead of adding "loops". I think I talked about this somewhere else not so long ago for the Magrid itself.

[TallDave]

There was some talk about shielding the nubs. I wonder if they tried that in 7.1?

Like DeltaV, I suspect it doesn't matter with mounts because of the shape of the field lines.

[sd_matt]

Would the coolant routes through the supports have to be metal? If so then would the single wire and it's field be enough to keep the field lines away?

Or is there a non-conducting material that can handle cryogenic temps?

Or does the shielding simply take care of that all?

[D Tibbets]

Would the coolant routes through the supports have to be metal? If so then would the single wire and it's field be enough to keep the field lines away?

Or is there a non-conducting material that can handle cryogenic temps?

Or does the shielding simply take care of that all?

I'm not sure what you are asking. The outer walls of a support standoff might be metal or ceramic. No surface charge is needed like the magrid casings themselves. Other concerns may dictate some charge for other reasons. The superconductor itself, immersed in liquid nitrogen or liquid helium would be within a magnetically permeable structure which might be made up of any number of materials that provide the thermal insulation, electrical insulation, etc and live comfortably at cryogenic temperatures. This might be metal coated with a ceramic or other insulator, pure ceramic, etc. Electrical insulation between windings does not need to be much as the drive voltage for the electromagnets will be low. Electrical insulation between these low voltage magnet windings and the high voltage shell can be handled with insulating wrappings, and ceramic standoffs that will be separating the coolant and insulating layers within the magrid. If you did desire some magnetic shielding of the standoff, I suspect a single strand of superconducting wire would probably do. I suspect this is not the case for the magrid proper. A single superconducting wire might hold enough energy, but the magnetic field distortion where the wire enters and leaves the magrid (even if through the same support/ standoff) would probably play havoc with containment. Having multiple windings of the superconductor would minimize this. What would be the minimum, 5, 20, or more windings?

Dan Tibbets

[sd_matt]

Would the coolant routes through the supports have to be metal? If so then would the single wire and it's field be enough to keep the field lines away?

Or is there a non-conducting material that can handle cryogenic temps?

Or does the shielding simply take care of that all?

I'm not sure what you are asking. The outer walls of a support standoff might be metal or ceramic. No surface charge is needed like the magrid casings themselves. Other concerns may dictate some charge for other reasons. The superconductor itself, immersed in liquid nitrogen or liquid helium would be within a magnetically permeable structure which might be made up of any number of materials that provide the thermal insulation, electrical insulation, etc and live comfortably at cryogenic temperatures. This might be metal coated with a ceramic or other insulator, pure ceramic, etc. Electrical insulation between windings does not need to be much as the drive voltage for the electromagnets will be low. Electrical insulation between these low voltage magnet windings and the high voltage shell can be handled with insulating wrappings, and ceramic standoffs that will be separating the coolant and insulating layers within the magrid. If you did desire some magnetic shielding of the standoff, I suspect a single strand of superconducting wire would probably do. I suspect this is not the case for the magrid proper. A single superconducting wire might hold enough energy, but the magnetic field distortion where the wire enters and leaves the magrid (even if through the same support/ standoff) would probably play havoc with containment. Having multiple windings of the superconductor would minimize this. What would be the minimum, 5, 20, or more windings?

Dan Tibbets

Can you clarify what the difference is, if any, between the electromagnets and the magrid proper? Is the magrid the virtual cathode?

As for the rest I guess it answers the question. Someone earlier mentioned the supports/standoffs being in the shadow of the magrid or something like that. Does this mean that there is no line of sight from the center of the well to the standoffs (assuming the standoffs are connected to the flanges that allow access to the interior of the chamber)

[sd_matt]

Ok went back and did some reading.....big difference between the magrid and the virtual cathode.

[KitemanSA]

Can you clarify what the difference is, if any, between the electromagnets and the magrid proper? Is the magrid the virtual cathode?

The MaGrid is the set of electromagnets plus the positively charged sheath. The positively charged sheath acts like the positive grid in an Elmore-Tuck-Watson fusor. The magnets protect the grid against electron impact/losses. The virtual cathod is created by the electrons that are accelerated by the grid and trapped by both the grid and magnets.

This link provides a good overview of Polywell theory, design and history.
http://www.askmar.com/ConferenceNotes/2 ... 0Paper.pdf

[DeltaV]

Someone earlier mentioned the supports/standoffs being in the shadow of the magrid or something like that. Does this mean that there is no line of sight from the center of the well to the standoffs (assuming the standoffs are connected to the flanges that allow access to the interior of the chamber)

Correct. They are hidden by the magrid from the core, which is emitting fusion products (energetic alpha particles for p-B11), bremstrahllung X-rays, gamma rays and some neutrons. Shielding effect due to the magrid varies depending on the type/intensity of radiation, magrid potential, coil current, etc. Neutrons and photons plowing through the magrid and charged particles following curved paths due to E and B fields can still hit the supports, but losses are not expected to be great there. The alphas from p-B11 (the major energy carriers) are bunched together by the coil fields into crude beams along each coil axis and are not expected to hit the magrid anyway.

[hanelyp]

There was some talk about shielding the nubs. I wonder if they tried that in 7.1?

Like DeltaV, I suspect it doesn't matter with mounts because of the shape of the field lines.

On one of the predecessors to WB6 they tried negatively charged plates behind the cusps to reduce electron losses. Ion losses soared. I expect similar results if nubs are used and electrically insulated. Electrons hitting the insulated nubs would stick and build up a negative charge. Insulated standoffs outside the magrid shouldn't have that problem being far enough outside the magrid for the internal plasma to not see them.

[KitemanSA]

Move the "nubs" out of the line-like cusps and the losses will go down too.

[MSimon]

There was some talk about shielding the nubs. I wonder if they tried that in 7.1?

Like DeltaV, I suspect it doesn't matter with mounts because of the shape of the field lines.

On one of the predecessors to WB6 they tried negatively charged plates behind the cusps to reduce electron losses. Ion losses soared. I expect similar results if nubs are used and electrically insulated. Electrons hitting the insulated nubs would stick and build up a negative charge. Insulated standoffs outside the magrid shouldn't have that problem being far enough outside the magrid for the internal plasma to not see them.

There was also the thought that coating the standoffs with a slightly conductive material so that the potential gradient on the standoffs matched the field gradient might help.

[D Tibbets]

There was some talk about shielding the nubs. I wonder if they tried that in 7.1?

Like DeltaV, I suspect it doesn't matter with mounts because of the shape of the field lines.

On one of the predecessors to WB6 they tried negatively charged plates behind the cusps to reduce electron losses. Ion losses soared. I expect similar results if nubs are used and electrically insulated. Electrons hitting the insulated nubs would stick and build up a negative charge. Insulated standoffs outside the magrid shouldn't have that problem being far enough outside the magrid for the internal plasma to not see them.

There was also the thought that coating the standoffs with a slightly conductive material so that the potential gradient on the standoffs matched the field gradient might help.

Cusp plugging with charged plates were apparently tried on several machines. In a sense the electron guns (or ion guns) have a similar effect. According to the recent patent application, attention to the placement of the guns were critical to prevent undesired effects. I believe the recipe was to have the electron guns some fraction of the magrid diameter outside the cusp.

Concerning what M. Simon said, I assume he meant having conductive coatings on the nubs that were insulated from the rest of the magrid casing, so that this area would float and accumulate a negative charge as it was bombarded by escaping electrons, and thus repell further electrons. This sounds like another variation on cusp plugging (or at least diversion). I would think it would have similar problems as other cusp plugging attempts (?).

The recirculation must be dominated by the electrons traveling only a short distance past the grid before being reversed by the strong positive charge on the magrid and pulled back in. Apparently, this distance was too great for the interconnecting nubs that were placed outside the mid line of the magrids, especially as they had a positive charge. Presumably the separate supports/ standoffs on each magnet is far enough outside the magrid ( as measured by an electron flowing along a curving field line) to avoid much of this problem. There may even be plans to add a negative surface charge to the standoffs, similar to what M. simon said. This might work better in this case due to the position and distance of these structures compared to interconnecting nubs.

Dan Tibbets