Multipactor with electric field retarding secondary emission

A systematic study of the effects of emission under opposing rf electric field in multipactors is performed including external magnetic field. It is shown that the small, 1 eV or less, kinetic energy of the emitted secondaries suffices to overcome opposing ac electric fields of the order kV/cm at the moment of emission, owing to the high frequency that the field changes sign. Thus, the static space charge limitation (no emission under the opposing field) does not apply, and ac driven avalanches occur for resonant impact phases with retarding electric field. Hence (a) the limit in the parameter space where multipactor occurs is pushed towards much higher ac fields, where the resonant impact phase is negative. (b) For mostly negative impact phases the space charge remains confined in a tight bunch during multipactor saturation. At lower ac fields, where the resonant impact phases are positive, the finite emission energy has a small overall effect.