> [xenon is] great for in-space propulsion because it’s fairly heavy (so you get more ooomph per atom)
More specifically, for a given exhaust velocity and grid spacing, the space charge limited thrust density (thrust/area) of an ion engine scales as the square of the mass/charge ratio of the ions. So you really want heavy singly charged ions. This is completely unlike thermal rockets, where you want low molecular weight exhaust gases.
Plasma engines that accelerate a quasi-neutral plasma aren't subject to space charge limits, but even there heavy ions help because they reduce the energy used in ionizing the propellant per unit propellant mass.
BepiColombo [0] uses 581 kg of Xe gas for its electric propulsion. I remember reading at the time this was being built that it consumed a measurable portion of the global xenon production for that year. This post reminded me to look that up, and it seems to be only ~1% of the ~50 tons, which is quite a bit less than I remember but still quite significant for a single application to use a non-trivial amount of the supply.
[0]https://sci.esa.int/web/bepicolombo/-/60642-bepicolombo-mtm-...
Given that ~100 million tons of oxygen are produced annually, extracting all the xenon from that air would yield about 170 tons/year. So there's a bit of room for growth.
The BepiColombo number is similar, I think, to the amount of xenon made annually in nuclear reactors (where it occurs in spent fuel as the result of fission.)
I think it might have taken a larger percentage of high-grade ultrapure xenon, a narrower market than the global bulk supply. A 1% impurity is fine if you are using xenon for welding, not so much if you are firing zenon plasma at a grid carrying a few hundred volts. A little bit of o2 in there and your grid would be rust very quickly.
Does anyone use xenon for welding? Argon, yes, but xenon is five orders of magnitude less common in air.
You could. It is heavier and so can carry more heat. There may be some specific metal/tool combo where that might be needed.
Inert gas in welding isn't used to carry heat, it's used as shielding to prevent oxidation, nitridation, and ingress of hydrogen. In any case, the heat capacity of the noble gases are almost identical. What xenon might do is reduce diffusion of heat away from the weld, as its thermal conductivity is just 1/3rd that of argon.
In practice I think a combination of argon and CO2 is typically used for inert gas welding of steel.
It depends on the process. Argon/CO2 is used for MIG welding, while TIG generally uses pure argon. In some situations that justify the expense, helium is used instead as it allows deeper weld penetration.