Yesterday I tried the the superconducting magnet‘s persistent switch again.
I failed to make a persistent superconductor, but all the circuits and LabView worked properly. More WIN than FAIL.
Superconducting magnet submerged in liquid nitrogen.
Conceptually this is the circuit we are testing. The heater functions as a variable resistor. The IGBT functions as the switch. Both are computer controlled.
This is the procedure:
I built a LabView VI to trigger the SC coil a variable number of millisecond after the heater:
We can measure the magnetic field produced with the DC magnetometer:
When I ran the experiment with 5A through the SC coil, I only saw a tiny magnetic field:
6 Gausse from the SC magnet
Furthermore, use of the heater seemed to make no difference at all.
As a control I ran the magnet in this configuration to see what magnetic field strength we should expect:
The produced a much stronger field:
15 Gauss When connected directly.
So the full current is not going through the main coil, but through the heater. I suspect either the heater resistor is not working (I can hear and see it boil the liquid nitrogen) OR the splice in the coil has more resistance than the coil heater:
The last time I ran this experiment the YBCO corroded from condensation:
This time I ran 2A of current through the coil for several hours to warm and evaporate any moisture.
Prometheus Fusion Perfection 
Why is the heater in the same circuit (connected) as the Superconducting coil?
Why not two separate circuits; and controls?
Ray
They are not connected. This schematic clarifies:
http://picasaweb.google.com/102602129941374127392/20101016#5528716002757212834
Okay, It still looks like you are shorting out the SC. I presume this is some kind of thermal conduction scheme. But if your heating a wire to transfer heat to the SC I would just heat the lead to the SC.
I have no experience with SC.
Yes, we are shorting the SC coil. That is a requirement for persistent mode.
http://en.wikipedia.org/wiki/Superconducting_magnet#Persistent_mode
Is it possible that the heater is incapable of heating the SC because of the LN2?
Unless the heated portion is somewhat insulated, would not be surprised if the temperature is not rising enough.
LN2 will boil at 77K, and extra LN2 will move in, trying to keep the temperature down at 77K, but you need to get to your Tc (93K for YBCO AFAIK) to go resistive. Thermal conduction in the YBCO wire will also try to keep the heated portion cold.
Have you tried measuring the resistance of a portion of SC wire with and without heater? (while in LN2 of course)
Also, how about putting a thermocouple in close thermal contact with the heated portion of SC wire under the heating coil?
I would suspect your splice might be problematic: The way I understand, once cold, the coil should be one continuous SC loop, but AFAICT your splice is a soldered connection; I don’t imagine solder is SC!
I would just clamp the bare SC wires on to each other so you have a true SC loop, or maybe crimp them?
You would probably have to clean the exposed surfaces to avoid oxidated bits, then maybe lacquer the connection to be safe.
Cheers,
Jo
Great point about the insulation. What would be a good around the shop material to insulate with?
Not sure, I’d try anything that will be at least midly insulating, but most importantly limit the influx of LN2 to replace the boiled-off N2
Also, that will not disintegrate due to heat stress too quickly, and not be melted by the heater (if it gets hot)
If the max temperature is sane (~100C ?), how about just a piece of heat-shrink, or even duck tape, possibly with some padding (think pair of socks, or some fiberglass wool) for a super-ghetto solution.
Is it me, or is the magnetic field produced by the magnet kinda really weak? 15 gauss is kinda underwhelming when considering the entire setup.
Agreed, although I only put 5A thought it so far.
why dont you lift the part of the sc wire that is suposed to become resistive out of the LN2? kinda getho but should be easier to do and avoids the probelm of LN2 creeping under the heater coil. also jonathan is probably right about the solder conection.
Moin
While reading your post I had two questions :
1. What is the resistance of your ‘switch’ with and without the heater turned on.
2. What is the resistance of your sc coil.
Because the switch shortens the coil (which is of course required for persistent current), therefore coil and switch are two resistors in parallel to your power supply.
If the switch doesn’t have a reallly high resistance, when heated, you’ll always have a small current flowing through the switch.
(Ohm’s Law R1/R2 = I1/I2)
Applying the formula for the field strength of a soleniod (B = (N*I)/L) to your results, I guess that without the switch the current through your coil is about 3 times higher.
Therefore I assume that the heated the SC material has not enough resistance.
(Given your heater works.)
I would connect the switch without the SC coil to a power supply, put it into LN2 an measure the current going through it for a fixed voltage.
Then take it out give it time to warm up and measure again.
(This way you don’t need to worry about your heater working.)
That’s better then measuring the resistance directly, because a normal multimeter is not really accurate for resistances below 10Ohm.
Sebastian