The Hull Cutoff condition for magnetic insulation in crossed-field electron devices in the presence of a slow-wave structure

Abstract

Crossed-Field Vacuum Electron Devices are ubiquitous in the High-Power Microwave field in either an oscillator/source or amplifier variant. A typical configuration consists of a magnetically insulated laminar electron flow in an anode–cathode gap with crossed electric (∝V, voltage) and magnetic (B-) fields and a series of open resonant cavities/vanes located on the anode block that serve as a slow-wave structure (SWS). The SWS slows the phase velocity of the electromagnetic signal down so that the wave becomes synchronous with a layer of the electron flow but is often neglected when calculating the Hull magnetic field necessary to insulate the electrons. In particular, the guiding design equation for the critical cutoff B-field assumes a smooth anode wall. In this paper, we show that such an assumption severely narrows the operating regime in B–V space and that upon inclusion of a revised Hull Cutoff condition taking into account the SWS, operation at lower B and higher V is possible. This revised Hull Cutoff criterion for magnetic insulation in crossed-field devices is corroborated by Particle-in-Cell simulations using CST Particle Studio.