Superconducting Magnet

15 10 2009

Gearing up for a second superconducting magnet test. This time computer controlled. Here is the new bobbin with 133 turns:


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Superconducting Magnet Test

19 08 2009

Today we ran the first test putting current through the superconducting magnet. First step is wiring up an AC connection for the DC power supply (0-100VDC, 0-120 Amps):

IMG_3928I learned how to crimp. It’s easy, but you need the right crimping tool. This power supply uses 240V single phase on the AC input side. I include these rather mundane steps because they matter. They need to be done for the project to move forward; they need to be done well; they often have their own challenges, “right ways”, tricks and considerations. I learn more than I expect from the simple stuff like connecting a power supply.

So here is the superconducting magnet soldered to a 50 amp twisted copper cable for main power, and the persistent switch with it’s nicrome heater wires:


On the other side we have the probe for the magnetometer secured with Kapton tape:


Here is the whole setup with a high wattage power supply for the main superconducting current and a low wattage power supply to power the heater coil. Dewar flask below.


Magnetometer check:

IMG_3939OH. Speaking of this magnetometer: I found out today that it drops -42 gauss when you dip it in liquid nitrogen. At first I though my gauss meter was broken from falling off the table!

So here is how it went:

First we lowered the experimental apparatus into the dewar of liquid nitrogen. It took several minutes for the nitrogen to stop boiling.

Next we turned on the heater up to 1 amp, based on previous experimentation. From there we incrementally raised the amperage, waiting about 1 minute between steps. The good news is the superconducting magnet seems to work. Here are the magnetometer readings for each amperage (adjusted for the -42 G drop from cryogenic):

5 A: 58 G

10 A: 126 G

15 A: 183 G

20 A: 232 G

So far so good!

Next we tried putting the magnet into persistent mode by turning off the heater allowing the persistent switch to become superconducting again.

Upon turning off the heater, the magnetometer jumped from 220 G to 115 G with no change in amperage to the magnet.

From here I lowered the amperage to zero over 10 seconds. The magnetic field returned to baseline. In other words FAILURE – the current did not persist.

Stuart suggest the problem may be that we are leaving the power supply connected, and it’s acting as a resistive load. We can certainly test this by adding a circuit breaker at the power supply.

So… not a complete success, but we have moved the game forward. I learned a lot today. I plan to retool this experiment, get some fresh LN2 and try it again.

Superconducting Magnet

13 08 2009

I spent some time yesterday with my friend jewelry design Max Steiner. He helped me design and fabricate a small acrylic bobbin for testing the YBCO as a superconducting magnet. This will fit in the dewars flask:


The silvery metal is the Hastelloy substrate the YBCO is deposited on.

I’ll test this out when we receive the DC magnetometer.

Progress Update

12 08 2009

Todd T. From Swagelok stopped by the shop and we quickly built a parts list for the deuterium system and the RGA. Turns out we are using a VCR style Swageloks, and they require a single use steel gasket much like the copper gasket in a conflat.

Also made a first pass at building the persistent switch. Here you see 50cm of insulated nicrome wire wrapped around 5cm of YBCO:


Cover in Kapton tape:


Connect a multi meter across the YBCO, and wire up the nicrome wire to a variable power supply:IMG_3920

And into the cold:


Unfortunately I was not able to get any reliable readings from the ohm meter. Before I turned the heater on, the meter registered between -0.5 ohms to  0.4 ohms. Any change in resistance from turning on the heater was lost in the meters margin of error. However the heater does turn on and work. You can hear the LN2 boiling off when you set the heater current to about 1.5 amps. So not exactly sure how to meter and test the persistent switch… for all we know this one works. And really this makes perfect sense: the resistance of the multimeter probes should be greater than the strip of superconductor even in it’s resistive state.

Coming out of the LN2:


Persistent Switch

10 08 2009

I’ve decided to run the superconducting magnet tests inside the dewar of liquid nitrogen. This will give us maximum cooling with minimal loss of LN2. The dewar is non-magnetic. The experimental apparatus must pass through the 50mm opening of the dewar.

I have 180cm of insulated nicrome wire which will serve as a heater for the persistent switch. This length of wire clocks in at 40 ohms, so 0.222 ohm/cm. I’ve ordered Kapton Tape W/silicone Adhesive:

We’ll use this Kapton Tape to secure the nicrome wire to the YBCO cable, and to provide some insulation for the nicrome heater.

Liquid nitrogen boils at 77 K (−196 °C). The critical temperature for YBCO is ~ 92K (-181 °C ). So we need to raise the temperature of the YBCO by 15 °C.

Unfortunately we really don’t know how much of the heat from the nicrome wire is going to warm up the YBCO, and how much is just boiling off LN2. Should be pretty easy to find how much current we will need  with a little trial and error.

DC Magnetometer

9 08 2009

Just bought this DC Magnetometer from AlphaLab to measure the strength of the superconducting magnets we are about to build:

Persistent Switch

2 08 2009

I’m starting to fabricate the persistent switch for the superconducting cable. Cryogenic Control Systems sells these heater cartridges for the purpose. Another option is to fabricate our own heater with the nichrome wire used for the reprap’s heater barrel. With a persistent switch we can test out the superconducting cable with a current.

Finicky Superconductor

5 06 2009

This thread on the Polywell talk forum covers some of the challenges working with superconducting magnets. They are finicky and can quench easily. In summary: SC magnets must be brought up to current slowly; the cleanliness of the power supply is important; field overlaps in SC magnets increases the chance of quench, the Polywell shape will be especially challenging; movement between two charged magnets can cause quench.

Loss of Electrical Resistance

22 04 2009

These photos show the drop in electrical resistance when you pour liquid nitrogen on the YBCO. Here is the before at 1.3 ohm:


After liquid nitrogen:

ohmsAs a point of reference, the lowest the meter goes when the clips are connected together is 0.5 Ohm.

Frost forms from the extreme cold:


Superconducting Levitation

22 04 2009

Finally got a good photo of superconducting levitation produced by the Meissner effect (the white disk is a tiny magnet):


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