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Comment by analog31 | Hacker News Reader

Does the refrigerator still need helium? Getting rid of helium might be a bigger deal than just raising the temperature.

Edit: Thanks for the comments. Adding a note: As I understand it, in some of these materials, the transition temperature decreases when the material is in a strong magnetic field, so there's a limit to how strong a magnet can be made, and further cooling may be needed to deliver sufficient field for an application. Still, I'm excited, and hoping that the "grail" of helium free refrigeration is achieved soon, due to the tricky issue of world helium supply.

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Generally "high-temperature" means liquid-nitrogen temperatures - in general, 77K is a common lab benchmark for these materials.

EDIT: See, for example, this paper on the superconductor in question, which gives properties at 77K and zero magnetic field: https://www.sciencedirect.com/science/article/abs/pii/S09214...

They're cooling it to about 30K, at which point nitrogen is frozen solid.

Interesting. That's around the liquid hydrogen range - still cheaper than helium, but probably not as easy to work with as nitrogen. I guess their requirements for critical field were high enough that the 77K GdBaCuO performance wasn't good enough?

Going by the link stephen_g found, it has around 1.8x the needed capacity at 30K, 1x at 50K, and 0.2x at 77K.

Given the spectacular ways wind turbines can fail, I'm not sure if I'd like liquid hydrogen to be involved there.

It'd probably evaporate before it reaches the ground

Which is the scary part. A mix with air us explosive.

It should also dissipate very quickly. There would be a brief window when it'd be dangerous, but you'd need as little wind as possible. The volumes used in these chillers are relatively small.

The (closed cycle) cooling compressor uses helium as a working fluid, but Tue quantities are usually very low. Hight temperature superconductors are usually cooled with just liquid nitrogen

Per this: https://aip.scitation.org/doi/abs/10.1063/1.99713

Superconducting transition occurs between 86K to 94K, so liquid nitrogen would be more than sufficient for the task.

I found this other document that has some information I think about this project at the end [1], which says it's conduction cooled by the cold heads. So perhaps it doesn't use either LN or liquid helium.

1. https://indico.cern.ch/event/588810/contributions/2477331/at...

What does "conduction cooled with cold heads" mean?

"Cold head" means the cold end of a cryocooler. "Conduction cooled with cold heads" means that the superconductor is in direct thermal contact with the cold end, rather than through some more complicated arrangement.

Still need a refrigerant in your cryocooler to have a cold head. See my comment above - at those temps, probably liquid hydrogen.

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