Efficiency enhancement in high power backward-wave oscillators

High power microwave (HPM) sources based on the backward-wave oscillator (BWO) have been investigated for the past two decades primarily because of their potential for very high efficiency (15 to 40%) operation. Several different effects have been proposed to explain this high efficiency compared to conventional BWO's, One of the major contributors to the high efficiency of the plasma-filled Pasotron HPM BWO source is the presence of optimally phased end reflections. The Pasotron uses a long-pulse (greater than or equal to 100 mu s) plasma-cathode electron-gun and plasma-filled slow-wave structure to produce microwave pulses in the range of 1 to 10 MVV without the use of externally produced magnetic fields, The efficiency of the Pasotron can be enhanced by up to a factor of two when the device is configured as a standing-wave oscillator in which properly phased reflections from the downstream collector end of the finite length SWS constructively interfere with the fundamental backward-wave modes and improve the coupling of the beam to the circuit. Operation in this configuration increases the efficiency up to 30% hut causes the frequency to vary in discrete steps and the output power to change strongly with beam parameters and oscillation frequency.