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I’d like to know the current carrying capacity of copper wires in vacuum.
The current plan (NPI) calls for sending ~3 amps through the vacuum space of a cryostat.
Has anyone done this? Have any references, links, or sage words of advice.
I found this,

http://snebulos.mit.edu/projects/reference/International-Space-Station/TM102179.pdf

and a few other references. (Kurt J. Lesker gives single numbers.) The slope of those curves in vacuum is about 2. Which at least makes some physics sense. Heat generated goes as I^2 and heat dissipated goes as T^4 (Assuming all the heat dissipation is by radiation… Stefan- Boltzmann law.)

Oh one kinda crazy idea would be to put a thick layer of heat shrink tubing over the wire. Better emmisivity and a larger area. (Perhaps something other than heat shrink.)

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  • \$\begingroup\$ We do it for short bursts. I think much heat loss is via conduction. We don't let the wires get hot enough to damage the insulation. Are the temperatures sensible to start with? Things get strange as you approach 0K. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Jul 21, 2014 at 14:25
  • \$\begingroup\$ Oh yeah, Temperature range from ~80 to 400K. This is for some B-field coils. Originally they were going to be outside the vacuum chamber, now they've moved inside. I think it's a bad idea, but I'll design what's put before me. \$\endgroup\$
    – George Herold
    Commented Jul 21, 2014 at 14:30
  • \$\begingroup\$ Can you handle copper atoms subliming into the vacuum? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Jul 21, 2014 at 14:52
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    \$\begingroup\$ Do you know how hard the vacuum will be? Heat shrink tubing is a bad idea because of the plastizers in it will out gas in the millitorr range (but do check that out - don't take my word for it as it is brand dependant). You can probably get some PTFE tubing (Teflon) to cover your copper. \$\endgroup\$
    – placeholder
    Commented Jul 21, 2014 at 16:05
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    \$\begingroup\$ @IgnacioVazquez-Abrams - You don't get sublimation until you're well into the red-hot regime. \$\endgroup\$
    – Connor Wolf
    Commented Sep 2, 2014 at 3:52

2 Answers 2

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For the steady state current limit, you need resistive power loss in the wire, to be matched by the radiated power at your chosen working temperature (and environmental temperature). Depending on the application, and the end termination, you may well be able to operate at much higher temperatures than would be normal for plastic insulated wires.

As you say, Stefan- Boltzmann is your friend here to calculate the radiated power for a given temperature difference.

My own experience with high current wires in cryogenic vacuum, is that the key design balance is often between heat leakage through the wires relative to the heat generation from the resistive losses, with the goal being to minimize the thermal load on the system. This is particularly the case when you have a liquid cryogen bath system rather than an actively cooled system. This balance may well push you toward various bronze and SS alloys which have a better thermal/electrical resistance ratio than copper.

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Since the current capacity of a wire increases in vacuum (about 20%, per your citation), you can use the design parameters allowed at 14.7 psi, and know that it will work safely in vacuum.

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    \$\begingroup\$ The citation shows the current capability decreases with vacuum. It kind of makes sense since the current capability is related to temperature and with a decreasing vacuum the ability to remove heat via convection decreases (while the radiated capability should remain constant) \$\endgroup\$
    – user16222
    Commented Dec 1, 2014 at 10:58
  • \$\begingroup\$ As noted by user16222, the current carrying capacity of wires in a vacuum is lower not higher, as there is minimal convective/conductive heat dissipation. \$\endgroup\$
    – colintd
    Commented Aug 28, 2023 at 9:13

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