Traveling wave induced multipactor and hub formation in crossed-field devices

Summary form only given, as follows. The onset of multipactor discharge by traveling waves (RF), of wavelength comparable to the cascading particles orbit size, is examined. Two dimensional effects are important, as opposed to one dimensional approach in AC driven multipactors. The process is relevant to creating and sustaining the cathode layer (hub) in crossed-field devices with secondary emitting cathodes. The phase locking condition, where the bounce-drifting velocity for cascading electrons matches the RF phase velocity, is obtained as a function of the operation parameters. When such a phase exists the particles bunch in space within a wavelength, until mutual repulsion due to space charge build-up disperses them along the hub. We also find that secondary avalanches can exist without the existence of phase locked orbits if the from impact to impact phase slippage relative to the wave is small enough to allow many cathode hits before the phase turns unfavorable (RF field opposing emission).