Simulation of rapid startup in microwave magnetrons with azimuthally-varying axial magnetic fields

Recent experiments have shown that the sidebands and the close-in noise in a microwave oven magnetron may be virtually eliminated by the addition of an azimuthally varying axial magnetic field. Not only the low noise behavior was observed, but also the startup was found to be substantially hastened when the number of the perturbing magnets equals the number of electron spokes in the operating mode (usually the α-mode) of the magnetron. The starting current was also observed to be one order of magnitude lower. In this paper, we show that this rapid startup is a result of "magnetic priming", which is achieved by imposing n=N/2 azimuthal variations in the axial magnetic field in an N-cavity magnetron that operates in the α mode.