Pulsewidth Limitation in the Relativistic Backward

Spontaneous pulse shortening occurring in a rel- ativistic backward wave oscillator (BWO) at gigawatt power levels is studied in experiment and theory. It is experimentally demonstrated that this phenomenon is accompanied by formation of an explosive-emission plasma at the surface of the corrugated slow-wave structure (SWS). Termination of microwave emission is explained by the increase of the BWO starting current from the absorption of the operating electromagnetic wave by electrons emitted from the plasma, whereas the intensity of the absorption radically increases owing to the presence of positive ions emitted from the plasma. Application of oil-free vacuum and electrochem- ical polishing of the SWS surface in an X-band BWO allowed generation of 3-GW, 26-ns microwave pulses with an energy of J, thereby demonstrating pulse lengthening by a factor of four. Index Terms—Explosive emission, gigawatt relativistic back- ward wave oscillator (BWO), pulse lengthening, slow-wave structure, space charge neutralization, spontaneous shortening of microwave pulse, starting current, surface conditioning, wave absorption.