re: it would be nice to see the technology develop without being 100% dependant of the Space program.
Electron beams are much better for welding in a vaccum. They heat uniformly throughout metal plates. The plasma would just heat the surface. Not generally good for welding.
You might be on to something with the chemical idea. It is generally a hydrogenic plasma so it would reduce what it comes into contact with when neutralizing. This idea is a commonly used plasma proscessing technique used in the IC industry I think.
Scailing down doesnt seem to help. Last I heard at 20kW of input RF, the efficiency was scailing linearly with power. This means the technology has room to grow.