Pulse shortening causes in high power BWO and TWT microwave sources

Pulse shortening, an effect where the microwave output power from a high-power tube terminates or significantly degrades well before the end of the electron beam pulse, severely limits the energy per pulse and average power capability of many high power microwave (HPM) sources. The cause of pulse shortening varies from device to device, and different causes can simultaneously contribute to the observed power reduction behavior which tends to obscure the underlying mechanisms and possible solutions. In this paper, we show a variety of experimental situations that lead to pulse shortening in HPM sources. The mechanisms of the different pulse shortening triggers are examined in detail in high-vacuum traveling wave tubes (TWT) and plasma-filled backward-wave oscillators (BWO). We find that there are many different causes of pulse shortening such as arcing, mode competition, beam instability, etc. However, the most commonly observed situation that leads to pulse shortening is the combination of sufficiently high power electron beams and poor vacuum conditions that lead to plasma generation. The presence of plasma significantly modifies the beam coupling to the circuit, which can affect the microwave production efficiency on very short time scales. The situations lending to pulse shortening and possible solutions are presented.