Most plasma spheres operate in the range of 5,500 to 7000 microns. Air
being 7600 microns (no vacuum at all).
I use xenon, argon, helium, krpton in my mixtures.
The container cleaning is the problem issue. You have to bake it at high
temp for 24 hrs before filling. Also, use a 60 kV discharge in Argon gas to
microscopically "clean" all the residuals off the interior walls. Failing
to take this step can result in a fuzzy ball after a few months as the
plasma dislodges molecules adhering to the glass internal walls.
Dr. Resonance