Liquid Nitrogen

28 09 2010

All photos.

Gearing up for another test of the superconducting magnet. So liquid nitrogen.

What is this… Halloween?

Heat Shield

1 12 2009

The red hot fusor grid reminds me  - I must address thermal issues from plasma, xrays and neutrons for polywell fusion without boiling the superconductor’s liquid nitrogen.

I asked for help with thermal modeling on the polywell talk forum. Good feedback.

Here is a rough draft of the superconducting magrid with a vacuum separated heat shield:

The trick is, the shield must have a gap so that you can weld the lid to the chassis. The welded magrid would have a gap in the shield along the midplane of the torus.

This gap would bring the vacuum between the heat shield and the inner superconductor holder. Well actually it would be  ~10 mToor of ionized deuterium.

This design does not include liquid water cooling. Although it’s easy to add cooling channels with the Arcam process, the real challenge is connecting fluid channels when you weld the lids onto the chassis.

Standoff for Superconducting Magrid

29 08 2009

I’ve been doing some brainstorming on the standoff for the superconducting magrid. This is a messy problem! You need a cryogenic feedthrough that is also a high voltage standoff. Then you need to pass in the YBCO superconducting cables, and wiring for the persistent switch.

Yesterday I realized that we can get most of the way there by welding together two off the shelf components:

standoff_feedthroughThis diagram refrences parts 9812107 and 9611005 from insulatorseal which is a subsidiary of MDC Vacuum. I’ve sent this drawing to insulatorseal for a quote. One problem is that part 9611005 is only rated up to 6kV so we will need a custom variant to get to the 10kV to 40kV range.

The idea with this setup is that the high voltage could come through a standard HV feedthrough and connect to the insulated tip of this feedthrough via a connecting wire:


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.

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.


10 08 2009

So we’ve been really struggling with the Swagelok interconnects. Amazingly we were able to get a representative from Swagelok to come out to our lab! He’ll be here on Wednesday the 12th at 10am. Hopefully we can fully spec and purchase all the remaining parts for the deuterium handling system, the RGA unit, and possibly cryogenic interconnects. Shout out to Deez for making this meeting happen.

Fun with Liquid Nitrogen

16 04 2009

So our dewar checks out. Here is some fun with liquid nitrogen:

Also check out safe handling of liquid nitrogen.

Liquid Nitrogen

15 04 2009

I’m getting quotes for liquid nitrogen (available at welding supply shops). These guys can fill my 30 L dewar on site for:

$70 nitrogen, $30 delivery fee, plus tax. I hope to place an order today.


These guys can do it for $78 including shipping. I have 30 L of liquid nitrogen coming tomorrow afternoon.

New Arrivals

24 03 2009

30 Liter Dewar flask for liquid nitrogen for the superconducting coils, and the rough pump arrived.




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