OPTICAL SPECTROSCOPY OF PLASMA IN HIGH-POWER MICROWAVE PULSE SHORTENING EXPERIMENTS DRIVEN BY A MU-S E-BEAM

Microwave pulse shortening experiments have been performed on a rectan gular-cross-section (RCS) gyrotron driven by the Michigan Electron Lon g Beam Accelerator (MELBA) at parameters V = -800 kV, I-tube = 0.3 kA, and pulselengths of 0.5-1 mu s. Pulse shortening typically limits the highest (10 MW level) microwave power pulselength to 100-200 ns, Pote ntial explanations of pulse shortening are being investigated, particu larly plasma production inside the cavity and at the e-beam-collector. We report the first optical spectroscopy diagnostic measurements insi de an operating gyrotron as a means of exploring plasma effects on pul se shortening. Plasma hydrogen H-alpha line radiation has been charact erized in both time-integrated and temporally-resolved measurements an d correlated with microwave power/cutoff. Hydrogen is believed to orig inate from water absorbed on internal tube surfaces in the gyrotron.