Pulse-shortening observations in large-orbit, coaxial gyrotron high power microwave experiments

Abstract

Summary form only given, as follows. Mechanisms of microwave pulse shortening are being investigated with a multi-megawatt, large-orbit, coaxial gyrotron. Experiments are primarily concerned with plasma production inside the microwave cavity and electron-beam collector. This gyrotron operates in the S-band at /spl sim/10 MW and is driven by the Michigan Electron Long Beam Accelerator (MELBA) at parameters: V=-750 kV, I/sub beam/=6 kA, I/sub tube/=0.8 kA, and pulselengths of 0.5-2.0 /spl mu/s. Plasma H-/spl alpha/ line radiation is measured inside the microwave cavity and e-beam collector via fiber optic probes/monochromators and correlated with microwave power and microwave cutoff. The temporal correlation is being measured of microwave power to growing H-/spl alpha/ optical emission. A strong correlation would suggest that the plasma is cutting off the microwaves as the plasma reaches critical density (/spl sim/8/spl times/10/sup 10/ cm/sup -3/). Time frequency analysis is used to analyze heterodyne microwave data to investigate other pulse shortening mechanisms, including voltage fluctuations, mode hopping, and mode competition. RF plasma processing is being examined on the coaxial cavity and e-beam collector to determine its effect on the pulse shortening characteristics of this gyrotron device.