WB6 Coil question

[MSimon]

Tom says:

There are further tradeoffs regarding magnet electrical/cooling arrangements. I favor making each magnet independent rather than connecting all in series. I have three reasons for this. The first is, you want to make the cooling path as short as possible. The second is, it would be nice to eliminate those short interconnecting stubs on WB6, right in the spot where they wanted to eliminate the "funny cusp". The third reason is it would be convenient to make all the magnets identical to avoid the hassle of the interconnection bits, and to allow very easy replacement of individual magnets.

The downside is, each magnet must be on a big feedthru plate, on, I think, four insulated standoffs of substantial size. Two carry magnet current and water, two are just support. These will interfere with outside electron circulation, but I'm betting that is less trouble than the interconnects(relatively low electron density outside the coils), an using four insulators allows them to be place wherever the models suggest the density is lowest.

That is the same conclusion I came to with the LN2 cooled Bitter magnets. Add one more. Standard Cu tube is limited to about 500 psi. If the magnets are in series you will go well above that. You have to balance electrical resistance against pressure drop. With any cooling method your outlet pressure must be well above the boiling point at the outlet temp to allow for local hot spots. That means a flow restrictor on the outlet to keep the pressure up.

I like LN2 because the Cu resistance goes down allowing much smaller magnet supplies. I like the Bitter design (even though it is not volumetrically efficient) because of its mechanical strength and because it is just a series of punchings (perhaps gold or silver plated at the overlaps) and mylar insulators .0005" thick maybe less. I figured a volume fill (square form factor) of 90% with .001" mylar. I could probably go to 99% with careful handling of the mylar (.0001 thick).

Cu has a pretty high temp. coeff. of resistance making water cooling problematic. Of course it becomes self limiting in terms of current so that helps. Unless you have constant current supplies. Which is what you want for experimental purposes since every thing must balance - well depth, drive voltage, B field.

My plan was to use the "dead space" in the coil forms as flow headers and for running D-D gas injection tubes.

Some design studies (better than my +/- 20% BOE calculations) would need to be done to choose between the alternatives.

Of course LN2 limits you to 30 minutes to an hour per day of run time. Good enough in the beginning. In fact at the beginning a few seconds of run time would be more than enough. Given the usual problems.

Another thing LN2 gets you is high insulation resistance without having to worry about contaminates.

[Solo]

Thanks for sharing that, Tom!

I've got a question: is there a need to minimize the maximum distance between the surface and the core of a conductor element, so that heat doesn't, say, build up in the center of the copper wire? I'm guessing that the heat conductivity of copper makes that a moot point, but if we add the insulation/epoxy that might cause problems. Tom, is that why you suggested the pie-shaped wedges, so that the copper parts act kinda like radiator fins to bring the heat to the outside of the magnet? I imagine a solution like that would be better than trying to make small channels thru something like a epoxy-potted wire-wound magnet and trying to force the coolant through that. Small channels in series means high pressure drop, but in parallel means uneven flow, and either of those can lead to coolant vaporization.

[Tom Ligon]

Solo,

The pie-shaped conductor idea stems mainly from the need to achieve a uniform field around the conductor using what must, due to the cooling needs, be fairly large conductors. Using tubing or square channels to build up a round cross section magnet won't fill the case smoothly.

But my first idea was to snip off the tips of the wedges to make a channel down the center, which could be cooled with LN2. The thought was just what you suggest ... the copper is VERY thermally conductive, and you could chill the entire magnet very effectively between runs. This system would still only allow short run times, but you could re-cool it fairly quickly, and the heat dissipation would be far lower than a room temperature magnet so the heating would be reduced.

You don't want a huge distance between the conductors of a copper magnet and the skin of the magrid, but you should be able to have a reasonable layer of insulation. If you wanted to go for net power, you would probably want a cooling layer on the inner faces. I have always felt the best way to introduce ionized fuel was via waveguides in the inner surfaces ... use the magnetic field and microwaves tuned to that particular strength to cause ECR in the waveguide, so any fuel introduced by that path would be totally ionized. The waveguides would require thickening the magnet shells.

Superconductor magnets will definitely be buried under 2 or 3 cooling shells, with vacuum jackets between them. Liquid helium superconductors would need a vacuum jacket around the inner magnet, then LN2 intermediate cooling to conserve the liquid helium, another vacuum jacket, and then a water jacket (or some exotic fluid) around that to soak up the heat from the plasma.

[tonybarry]

Tom, I gather from your writing that you see the major heat input to the MaGrid being from coil I^2R heating rather than fusion product heating. Is this correct? (At least for the time being, say WB-7, WB-8, maybe WB-9.)

Regards,
Tony Barry

[Tom Ligon]

Tony,

For little machines on the pattern of WB7, using copper coils, I think I^2R in the copper is going to be the overwhelming source of heat in the coils. That's based on my experience with WB2 and WB3 running on a pickup truck worth of lead acid batteries to power the magnets, and some preliminary work on the WB4 battery bank and cooling system. Battery power during the course of a test absolutely dwarfed the available power to the building. Fusion power at peak might possibly have hit a milliwatt for a fraction of a millisecond in WB6. If WB7 can manage a reasonable fraction of a watt of fusion, that would be a coup. I don't think they expect to. I've seen their address on a MapQuest aerial photo, and I would not want to be in an adjoining building in that little industrial area! Fusion power in the little machines would not touch I^2R magnet heating, by many orders of magnitude.

If we were to build net power deuterium-burning machines using copper magnets, magnet power would still be a major consideration. You might still be able to reach net power by some profitable margin, but ohmic magnet heating will never be trivial in copper. Dropping resistance dramatically using LN2 would be a huge benefit if it can be done practically, in which case a cooling jacket with something like borated (B10) water would probably be essential to minimize heat loading the copper with fusion energy. Silver conductors might marginally improve the situation.

Once we're working with net power machines and superconducting magnets (let's hope we have to deal with this problem) the concern shifts decisively to fusion energy heating.

[hanelyp]

While we're discussing coil construction, I've been musing on something similar to a Bitter magnet, but wound from a ribbon conductor rather than stacked from stampings. A corrugated or porous insulation layer would pass the coolant running between the ribbon layers, rather than through holes in the conductor.

On a related matter, anyone know how well a ribbon of high temperature superconductor ceramic would take to this kind of winding?

[Tom Ligon]

hanelyp,

I dreamed up the same thing while at EMC2. A "watch spring" magnet winding would eliminate the ungraceful steps in winding at the end of each layer, unavoidable when using wire or tubing. It should cool very effectively.

The problem with this configuraton is making it conform to a circular cross-section case. The straight-forward way of making these would fit in a square cross-section case. I suppose you could play with a roller pressure, and leave the ends thick and narrow, the middle flat and wide.

The samples of early high-temperature superconductor winding we played with at EMC2 were powder sandwiched between two silver foil layers. The ribbon was about 3-4 mm wide, thin, and fairly delicate. It would maglev easily in LN2, but we never tried making any magnets with it. Dr. Bussard was intrigued by the possibilities for the future, but nobody had much experience with it at that time, especially us, and we had doubts we could make a magnet physically robust enough, considering the high mutual repulsion of the magrid magnets.

[tombo]

The pie shaped conductors are standard technology in high current long distance power cables.
Easing the pieces together is not much of a problem, I think.
There are companies that specialize in custom cable manufacture.
Once the $$$ are available, the skills are available.
Just like almost everything else in this project.

What do you think of making the hollow wedges by rolling copper tube to the right profile?

Is there really enough temperature drop across the profile of the copper to justify this much effort?
As opposed to, say, using very thick wall copper tube.

Another random thought.
Maybe use nested concentric copper tubing with standard tube sizes.
Using longitudinal Cu wires in the gaps to hold the spaces open during tube bending and operation.
They would probably have to be soldered to the inner one before sliding it into the bigger one.

[Tom Ligon]

I looked into rolling copper tubing to an oval to control the relative cross sections of copper and channel.

The goal of making the conductors wedge-shaped is to make the resulting field as smoothly circular as possible around the magrid skin. Any kind of coarse winding is likely to make a lumpy field that directs the closer electrons in for a hit. I suppose you can overcome this with more field strength, but the less field we can get by with, the less power we waste and the less demanding the cooling.

[MSimon]

Tom,

Assuming we can get enough current through the coils the cross section (square vs round) makes hardly any difference in the external field.

It is probably not worth optimizing in a test reactor.

The Bitter design has a lot to recommend it in terms of mechanical ruggedness. The windings are held in place with bolts. The dead space is also useful as a header for coolant and for gas feed piping.

If we used LN2 for short one shot tests (1 to 10 seconds - once an hour) and LHe at 50K for longer continuous runs that should work fine. Cu resistance drops very quickly below 77K. I need to rerun the numbers but the difference between 80K and 50K is significant.

Of course LHe would require a reliquefying apparatus to keep costs down.

[tombo]

Yes,Smooth is essential.
The concentric tubes concept was intended to have the largest tube be the same od as your wedges.

The oval can be rolled further to make the pie sections.

The pie sections could also be made from copper strip (say 25mm wide by 2mm thick in 1000' rolls)
Rolled into a U shape then close the top of the U to make the pie shape as long as you want to make it.
Look at the website for Hyper Tech Research's CTFF process to see the kind of tooling I'm talking about.
The edges can be welded together if you need to keep the coolant confined that well or not if there is an outer jacket.
I would design it with the edges toward the cable axis and nest them around a round central tube.
(I have done jobs where we did this kind of thing to copper strip.)
The machine is called a "Tube Mill".
Usually they are set up to make round tube but could be set up to make any shape within the elastic limits of the copper.


hmmmmm.......
That center tube might be carrying the LN2 or LH2
Or....
The center tube could be another set of pie shaped coolant channels for LN2 around yet another of LH2 and then the superconductor in the very center.
That might be a viable way to build up the multilayer conductor.
I was wondering how to keep all those layers concentric and aligned and stable while forming them to shape and in use.
I think you are onto something here.

[tombo]

The Bitter design has a lot to recommend it in terms of mechanical ruggedness. The windings are held in place with bolts. The dead space is also useful as a header for coolant and for gas feed piping.

I wonder.
Would a Bitter coil work in a cone shape?
Say with the cone's vertex at the reactor center.

[MSimon]

The Bitter design has a lot to recommend it in terms of mechanical ruggedness. The windings are held in place with bolts. The dead space is also useful as a header for coolant and for gas feed piping.

I wonder.
Would a Bitter coil work in a cone shape?
Say with the cone's vertex at the reactor center.

Things get tricky when you have to keep the loss per unit length constant to prevent uneven heat loads. Thickness goes up. Bolting problems increase. And the gain is not too great because you are not adding a lot of amp-turns due to increasing thickness. And then there are the increased mfg. problems.

An alternative might be a tape wound coil. If you could figure a way to drill cooling holes in the tape without shorting out adjacent turns. Laser drilling might work.

Preventing undesigned hot spots is essential due to the fact that LN goes gaseous (at any pressure) around 120K. Once that happens it ceases to be a liquid coolant and the failure is regenerative.

[hanelyp]

An alternative might be a tape wound coil. If you could figure a way to drill cooling holes in the tape without shorting out adjacent turns. Laser drilling might work.

See my previous post this thread. run coolant not through holes in the tape but between layers of tape.

[MSimon]

An alternative might be a tape wound coil. If you could figure a way to drill cooling holes in the tape without shorting out adjacent turns. Laser drilling might work.

See my previous post this thread. run coolant not through holes in the tape but between layers of tape.

It would be interesting to see which gives a higher volume of copper for equivalent cooling. I'm betting holes in the coils. Plus there are mechanical issues. I'm betting the forces involved are better constrained with successive turns in close contact vs spacing between turns.