Cathode multipactor induced by traveling waves

While ac multipactors between gaps have been extensively studied, traveling wave multipactors, widely thought responsible for hub formation in cold-ca thode crossed-field devices, have received much less attention. It is shown that a single surface multipactor is induced by a slow wave with two-dimen sional space variation. Phase locking with the rf occurs when the distance traveled, and flight time between successive impacts correspond to a collis ional drift velocity upsilon(c) = Delta y/Delta t equal to the phase veloci ty upsilon(p). The flight time is not a multiple of the rf period, as in ac multipactor, but closer to the cyclotron period. A stable resonant orbit i s an attractor in space, pulling nearby orbits; in a rf-synchronous moving window, particles cluster into a Larmor radius size ring, as the rest of th e wavelength is swept clean. The bunched charge is later dispersed due to t he mutual repulsion, creating a nonuniform hub. The parameter space where a valanching occurs for secondary yield above unity is found much more extend ed than for an ac multipactor. A crossed-field device based on secondary em itting ("cold'') cathode must operate in the same parameter range. (C) 1999 American Institute of Physics. [S1070-664X(99)02901-8].