Initial studies of a long-pulse relativistic backward-wave oscillator utilizing a disk cathode

One of the major issues in high-power microwave device operation is pulse shortening, which often limits microwave pulses to less than 100 ns. This has been the focus of many studies on the long-pulse backward-wave oscillator (BWO) at the University of New Mexico. Previous diagnostics have indicated that significant plasma is produced by a graphite knife-edge or "cookie cutter" geometry cathode. This plasma caused the beam to expand radially to match the dimensions of the cutoff neck upon entrance into the slow-wave structure. This effect led to an impedance collapse at which point the microwave production ceased. In recent studies by Loza and colleagues, they have produced intense annular relativistic electron beams that maintain a stable cross section for 1-μs duration utilizing a disk cathode. Whereas the beams produced by Loza and colleagues were not used in microwave sources, we have incorporated such a disk cathode in a long-pulse relativistic BWO to study its effort on pulse shortening. This simple solution has led to an increase in pulselength and radiated microwave power up to a factor of two as compared to the "cookie cutter" cathode.