Effects in an HPM BWO induced by the spent electron beam

As a result of the interaction with an operating wave in a relativistic backward wave oscillator (BWO), electrons end up with a large energy spread regardless of the efficiency of operation of the oscillator. Calculations show that the distribution of electron density as a function of energy is almost symmetric in relation to the average electron energy after transfer of part of electron´s energy to the electromagnetic field. One portion of the distribution has electron energy that is significantly greater than the initial one, while the other portion has smaller electron energy that leads to a decrease of the transported current, limited by the electrons´ own space charge. Local virtual cathodes can appear in the output channel. A portion of the reflected electrons reaches the cathode, decreasing its emission and leading to a modulation of the output radiation. The amplitude and spectrum of the modulation depend on the circumstances of the beam dump for spent electrons. In experiments with an X-band relativistic BWO, an increase of the microwave radiation pulse duration was observed when the electron beam dump was placed close to the interaction space. Pulse shortening could be caused by a decrease of electron emission due to reflected electrons decreasing the beam current below the start current. Therefore, conditions in the beam dump can interrupt the regime of stationary generation.