Current capacity of copper wires in vacuum

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.)