Mechanism of microwave pulse shortening in the relativistic backward wave oscillator

Spontaneous pulse shortening occurring in the relativistic backward wave oscillator (BWO) at gigawatt power levels is studied experimentally and theoretically. It is experimentally demonstrated that this phenomenon is accompanied by formation of explosive-emission plasma at the surface of a corrugated slow-wave structure. Termination of microwave emission is explained in terms of the increase of the BWO starting current due to the absorption of the operating electromagnetic wave by electrons emitted from the plasma while the intensity of absorption radically increases owing to the presence of positive ions emitted from the plasma. Application of oil-free vacuum and electrochemical polishing of the SWS surface in an X-band BWO allowed generation of 3-GW, 26-ns microwave pulses with an energy of similar to 80 J, thereby demonstrating pulse lengthening with a factor of four.