Space charge saturation in multipactor discharges with parallel magnetic field

Abstract

Numerical and theoretical models were developed to investigate space-charge effects in a multipactor discharge with an applied magnetic field. The magnetic field was assumed to be parallel to the RF electric field, and only the space-charge effects in the direction perpendicular to the fields were studied. Inclusion of space-charge effects allowed investigation of a saturation mechanism driven by electron diffusion. Numerical simulations showed that saturation of the multipactor discharge could be reached as diffusion balanced the electron production due to secondary emission. It was found that the steady state density reaches a limit as RF voltage was increased. Fluid equations for the electron motion were used to describe electron diffusion and derive an expression for the steady-state electron density in a typical multipactor discharge. It was found that in the presence of a magnetic field, electrons may be trapped by executing a slow rigid-rotor rotation, as described by the Brillouin condition. The electron behavior was found to be dynamic, satisfying the electron trapping condition while the electrons were moving between electrodes and violating the Brillouin condition near the electrodes leading to larger diffusion losses. Numerical simulations show that Brillouin trapping due to an applied parallel magnetic field can significantly increase saturation density by up to two orders of magnitude.