Pulse shortening is an effect where the output power from a high power microwave tube terminates well before the end of the electron beam pulse. A less severe but related problem in lower power tubes is pulse distortion where the output power significantly degrades with time during the pulse. These effects often limit the energy per pulse and average power capability of many high power microwave sources. The cause of pulse shortening and distortion varies depending on the device and circumstances, and different causes can simultaneously contribute to the behavior which obscures the underlying mechanisms. In this paper, pulse shortening triggers are examined in tube-industry-standard, high-vacuum traveling wave tubes and backward-wave oscillators. While there are many different causes of pulse shortening such as arcing, mode competition, beam instability, etc.; the combination of sufficiently high power electron beams and poor vacuum conditions leading to excessive plasma generation in devices not designed to handle plasma often causes this problem. The presence of plasma significantly modifies the beam coupling to the circuit, which affects the microwave production efficiency. The situations leading to pulse shortening and distortion and possible solutions are discussed.
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