Decawell Moon Shot

3 02 2009

I’m starting to think it makes sense to take a moon shot, and build a superconducting core from go. This would allow for continuos operation, smaller chassis, higher field strengths per turn, and is generally Badass.     

I have 13 meters of superconducting YBCO tape on the way (was more expensive with copper matrix and insulation). That’s enough to build a one turn dodecahedral polywell, with one meter of superconducting cable for each of 12 coils.  I’m not exactly sure how to apply the manufacturers specs for the cable. This is my guess:

Superconducting properties (@ 77K) 

Critical current IC*: 200 – 250 A/cm 

Engineering crit. current*: 200 – 250 A/mm2 Density:   9 g/cm3 

Icvalues range from 80 –110 Amps at 77 K in 4 mm width 

Engineering Current Density (Je) = 21 –29 kA/cm2 

It looks like I take the Critical current IC* 200 – 250 80 –110 A/cm and multiply it by the length of cable 1300cm, which gives us 26,000 104,000 Amps. I’m working out the B field calculations using Ampère’s force law

Now regarding the chassis.


A few weeks ago I contacted POM Group, requesting a quote for fabricating the chassis and lids in stainless steel using direct metal deposition, based on my EGIS files of the parts. POM wrote back: “unfortunately it would not be feasible to manufacture the chassis with our process. It would be feasible to DMD the lids”. I’m eagerly awaiting more information on what makes it infeasible. My guess would be too great an angle of overhang. Although I’m not writing off DMD just yet, I want to explore other options such as casting.

I came across an interesting technology called  ZCast Metal. This uses the Z Corporation rapid prototyping technology to directly produce a ceramic negative for metal casting. Currently this process only works for aluminum, brass, zinc and magnesium which have lower melting points than steel. It cost far less than traditional rapid prototype casting, where the negative is made on a CNC machine, followed by a wax injection mold, followed by a ceramic coating, followed by the actual casting. Another limitation is the size of the Zcorp printer, the maximum build envelope being 254 x 381 x 203 mm.

This raises the question: Does the chassis _have_ to be made of stainless steel? Bussard indicated that stainless steel was the ideal material, but other metal may also be suitable (keep in mind we are being a prototype). The chassis must do a few things. It must be strong enough to resist the mechanical forces exerted by the electromagnetic coils. It must be electrically conductive so it can be set to a high positive electric potential. An in the case of superconducting coils, it’s must be sealed well enough to prevent the liquid nitrogen boil off from poisoning the vacuum. I’m not sure if the magnetic properties of the metal are important. Depending on the alloy, stainless steel can be either magnetic or non magnetic. 

Here is useful tensile strength comparison chart. This shows Aluminum with a tensile strength of 145 Mpa. Stainless Steel’s tensile strength can range from 200 to 700 Mpa depending on alloy and annealing. So it may be possible to use aluminum for the chassis, though with a reduced operating envelope.

In terms of outgassing, aluminum varies widely depending on treatment, and seems comparable to stainless steel. 

I’m considering designing the chassis so that the coils are serviceable, ie have the lids bolted on not welded together. This way you can upgrade the coils with more turns later on.  I really like this idea. We could get it working with a single turn of superconducting cable, then when more money comes in, upgrade the unit to 10 or 20 turns. On the down side this would introduce issues with bolts breaking the smooth profile of the chassis, ie not being conformal to the coils. Also, it would be more challenging to seal in the liquid nitrogen with a non welded design. 

Today I’m getting quotes to have the chassis and lids cast in stainless steel using rapid prototype casting as well.




2 responses

3 02 2009
M. Simon

I believe critical current Jc is given in Amps/sq cm of conductor area.

You might want to contact the mfg. and get the specs for your ribbon. Also minimum bending radius.

You might want to devote all the SC tape to one coil to get the Amp turns high enough. BTW critical current is lower in a magnetic field. You might want to get that spec too.

How to charge it up:

Make the coil into a continuous loop. Make the device with a heater across the SC. Cool the SC. turn on the heater – when you have made a small section non-superconducting apply your power supply across that section and slowly ramp up the current to your final value. (20 minutes) Turn off the heater. You should now have a circulating current.

You might want to contact some one who uses your SC wire and get a definitive report on how they do it. The above is what I have read.

I suggest using a stand pipe to fill the coils with LN2. Two stand pipes actually. One to vent atmosphere from the coils and the other to feed in LN2. Make sure the stand pipes are big enough so that in a boil off event (loss of superconductivity) you do not get LN2 explosively hurled out of the system.

And don’t forget to design to apply HV to the coil housings. No aluminum housings. Stainless or copper. That means HV breaks in the standpipe.

3 02 2009
M. Simon

BTW all the vacuum guys I have heard from say: no aluminum. Ever. Too much grief. The housings must be non-magnetic to avoid enclosing or distorting the field.

Talk to people who make vacuum chambers to get something more definitive or a suggestion for which aluminum alloy. It may need to be forged to eliminate voids.

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