Superconducting Magnet Test

18 10 2010

All Photos.

I added another terminal to the superconducting coil.

It looks like this:

I operated the coil in the following configuration. About 12cm of SC cable was above the liquid nitrogen forming a resistor.

Putting 5A through the SC coil produced about 20 Gauss.

When I submerged the warm SC cable in liquid nitrogen (leaving the current source on), the field dropped off by half!

The strength of the magnetic field was sensitive to changes in the amount of YBCO at room temperature.

In summary:

We succeeded in directing the current into the main superconducting coil despite the short circuit. GOOD.

When we “turn off the heater” by dunking the warm SC cable into the liquid nitrogen, we lose much of the current going into the main coil.

Not exactly sure how to interpret the results.

My guesses for what’s needed:
1) Longer span of YBCO in the heater.

2) Insulate the heater.

3) Longer splice, better solder joint.

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3 responses

18 10 2010
Aaron

Are these measurements taking in account the 42g drop you mention here?
https://prometheusfusionperfection.com/2009/08/19/superconducting-magnet-test/
Also, how is exactly is your coil wound?, these numbers and your previous ones are far below what i believe what they should be
I also reccomend that instead of soldering your connections together, that you clean, polish, and then clamp/crimp them together, superconducting wire is a far better conductor than solder :)
You also didn’t detail what happened after your performed the full procedure from heat up to current shut down last time, i’m assuming that it didn’t work out.

19 10 2010
FAMULUS

I’m aware of he cryogenic drop in the magnetometer. I’m ignoring the absolute value and looking at change in value.

No matter how you splice the YBCO it will be a normal copper to copper joint.

19 10 2010
Jonathan

I like “anonymous”‘s test method of pulling the bypass out of the LN2, the simplest solutions are often the best :)

I would however have wanted the connections to the coil in LN2 also, so only the bypass would be non-SC, not a portion of the coil also; not sure this has any ill effects.

“””When I submerged the warm SC cable in liquid nitrogen (leaving the current source on), the field dropped off by half!”””

That is what I might expect from your circuit in a fully SC state, ignoring the whole superconducting magnetism stuff:
You have the coil with 0 resistance, in parallel with the bypass with 0 resistance, so half of the current goes through the coil, and half through the bypass, hence a drop of 50% in the field produced.
That said, this fails to take into account the resistance of the splice (or rather assumes the splice to be fully SC, which seems unlikely to me)

Also, I may be completely wrong because:
1) I don’t know exactly how currents are distributed in separate circuits with 0 resistance, it’s kind of a border case of Ohm’s law….
2) I may be misunderstanding more of the physics involved in persisting superconducting electromagnets….

I get that once the bypass is SC, the electromagnet should be one loop with current happily flowing through it.
But what about the Current source, should it not have any current flowing through it any more for the same coil current?

I can imagine several options :
1) Naive option: The Current source keeps pumping more and more current into the coil, which ends up going through the bypass, and the field grows linearly (strange and probably incorrect)
2) As you mentioned before: The Current source acts as a load and sucks energy out of the field (Until it gets to my above mentioned 50/50 coil/bypass currents)
3) Does the current source look like a 0-resistance wire, and nothing flows through the bypass?

Personally, I’d want to disconnect the current source ASAP after the bypass goes SC; based on the 30 seconds it takes for the field to drop, that should be more or less doable.
BUT if you disconnect before the bypass goes SC, you’ll have a DC spike caused by the energy left in the field. This will be limited by the resistive bypass, so it might not be a hazard, but I’d still be a little wary….

“””No matter how you splice the YBCO it will be a normal copper to copper joint.”””
Why? Is the YBCO wire coated in copper?
If so, it seems impossible to me that you could create a persistent field at all…

Regarding the magnetometer and the coil: how is it mounted, and what are the coil’s characteristics?
If your coil is a solenoid (as it seems to be), the magnetometer should (ideally) be mounted so it measures the field along the axis of the coil, in the center of the coil (at the end of the coil should have some effect, and off-center some more I believe)
Assuming the magnetometer measures perpendicularly to it’s leads’ plane, and the cable it’s connected to is in that plane, like this is pretty much OK: https://prometheusfusionperfection.com/2009/08/19/superconducting-magnet-test/
BUT, I would expect to see much less field (if any) with the magnetometer like this: https://prometheusfusionperfection.com/2010/10/16/superconducting-magnet-test-2/ because you would be measuring perpendicularly to the solenoid’s radius, where there should be hardly any field

How does your measured field compare to the theoretical field in a solenoid (could you post the coil geometry)?
ie: B(Tesla) = µ0*µr*(nb turns)*(current [A])/(length [m])
µ0 = 4π×10^−7 (Tesla*m/A)
µr ~= 1 for air
multiply by 10^4 for Gauss
(I expect you know the formula, but I had to look it up so I figured I’d post it for reference)

“””Are these measurements taking in account the 42g drop you mention here?”””
“””I’m aware of he cryogenic drop in the magnetometer. I’m ignoring the absolute value and looking at change in value.”””
What is the actual characteristic of this drop?
Is it really -42Gauss, reardless of the field, or actually a change in slope resulting in -42Gauss at (say) 200Gauss, or a bit of both?
Your graph seems to imply -37.5Gauss reading with no current in the coil, but is the slope still 1mV/Gauss ?
You can test this by running the coil at various currents in and out of LN2
The datasheet might tell you what to expect, also, but probably not down to 77K unless your magnetometer is specifically designed to work at such temperatures…

Soo many questions…. I think I’ll have to come and work for you :P

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