  A muon collider or Higgs factory requires significant reduction of the six
dimensional emittance of the beam prior to acceleration. One method to
accomplish this involves building a cooling channel using high pressure gas
filled radio frequency cavities. The performance of such a cavity when
subjected to an intense particle beam must be investigated before this
technology can be validated. To this end, a high pressure gas filled radio
frequency (rf) test cell was built and placed in a 400 MeV beam line from the
Fermilab linac to study the plasma evolution and its effect on the cavity.
Hydrogen, deuterium, helium and nitrogen gases were studied. Additionally,
sulfur hexafluoride and dry air were used as dopants to aid in the removal of
plasma electrons. Measurements were made using a variety of beam intensities,
gas pressures, dopant concentrations, and cavity rf electric fields, both with
and without a 3 T external solenoidal magnetic field. Energy dissipation per
electron-ion pair, electron-ion recombination rates, ion-ion recombination
rates, and electron attachment times to $SF_6$ and $O_2$ were measured.
