Optical spectroscopy of plasma in high power microwave pulse shortening experiments driven by a microsecond electron beam

Summary form only given, as follows. Spectroscopic measurements have been performed to characterize the undesired plasma in a multi-megawatt coaxial gyrotron and a rectangular-cross-section (RCS) gyrotron. These gyrotrons are driven by the Michigan Electron Long Beam Accelerator (MELBA) at parameters: v=-800 kV, I/sub tube/=0.3 kA, and pulselengths of 0.5-1 /spl mu/s. Pulse shortening typically limits the highest (/spl sim/10 MW) microwave power pulselength to 100-200 ns. Potential explanations of pulse shortening are being investigated, particularly plasma production inside the cavity and at the e-beam collector. The source of this plasma is believed to be due to water vapor absorbed on surfaces which is ejected, dissociated, and ionized by electron beam impact. Plasma H-/spl alpha/ line radiation has been characterized in both time-integrated and temporally-resolved measurements and correlated with microwave power and microwave cutoff. Measurements from a residual gas analyzer (RGA) will be used to support this interpretation. Experiments involving RF plasma cleaning of the coaxial cavity are planned.