[Light Speed]

Cooo-ool!
I used to operate a Plasma machine ,large scale...underwater cut..for an American firm out of Houston.
My Plasma used "Nitrogen, and Chryogenic Nitrogen" for cut process.
A poster here at FR works aero space and pointed me to the Boeing "Plasma thruster" engine web pages...
Boeing Ion mass drivers use Xenon for propulsion...
My Plasma table had computers to millisecond mix via solenoids and such..sometimes things would not work right..and "Big detonations" occured..
The machine really didn't freak me too much..one learned to maintain it..but the 12 bottles of Liquid Nitrogen just a few feet away..were allways unnerving.

[RadioAstronomer]

Boeing Ion mass drivers use Xenon for propulsion...

I built a small ion engine as a project once! :-)