Destabilization of diocotron modes inside structured anode cavities

Previous stability analysis of low space charge omega(e)(2)/Omega(2) much l ess than 1, E X B drifting flows inside smooth anode cavities has shown tha t they are practically stable against slow wave omega/ck much less than 1 d iocotron instabilities when the flow touches the cathode (cathode layer). I t is shown here that the presence of periodically structured anode destabil izes cathode layer modes when their phase velocity matches that of an empty cavity eigenmode, as structured cavities support slow waves in vacuum. An analytic dispersion relation that self- consistently includes the structure d anode effects is obtained in the guiding center approximation. The growth rate, related to the onset of magnetron oscillations, scales as gamma/omeg a similar to omega(D)/omega similar or equal to 1/kd; it is independent of, and remains finite as, omega(e)(2)/Omega(2) --> 0. The most unstable frequ ency corresponds to a resonant layer below the flow surface (slightly below the Buneman-Hartree frequency) and the growth rate is found symmetric rela tive to the detuning from resonance. (C) 1999 American Institute of Physics . [S1070-664X(99)04501-2].