Experiment on a large-diameter plasma-filled backward-wave oscillator

A large-diameter plasma-filled backward-wave oscillator (BWO) is investigated experimentally. The parameters of slow wave structure are chosen so that the oscillation frequency is about 20 GHz at 60 keV beam energy. Plasma is produced by the beam and has favorable effects for beam propagation and Cerenkov oscillations. The output power of the BWO with plasma is observed to be three to six times that of vacuum BWO. The power level is several kilowatts and the efficiency is about 0.01%. For Cerenkov oscillations of a large-diameter BWO, the beam energy mainly determines the starting conditions for oscillation. The output power is strongly enhanced when the guiding magnetic field approaches the fundamental electron cyclotron resonance. This mechanism is closely related to the anomalous Doppler cyclotron resonance. The maximum power of 480 kW with an efficiency of 5% is achieved even for a relatively low beam energy of 60 keV.