Multiple reflections of electrons and the possibility of intense positive-ion flow in high ν/γ diodes

Variations of the nature and pressure of the background gas used for neutralizing the charge of a relativistic electron beam have been observed to alter the current waveform of the propagating beam, the time delay before propagation begins, and the voltage and current waveforms of the diode which accelerates the beam. This interaction between the beam region and the diode occurs during the delay phase before beam propagation begins and can be explained by (1) the reflection of electrons back into the diode from a virtual cathode which exists in the beam region during the delay phase and (2) the flow of a significant positive-ion current in the diode during this same period. A theory describing positive-ion flow in the presence of multiply reflected electrons is developed and compared to experimental observations.