Numerical electromagnetic modeling of a low-power microwave electrothermal thruster

Microwave electrothermal thrusters-which promise moderate specific impulses, high efficiency, and long lifetimes-are characterized by a free-floating arc plasma discharge. The location of the discharge, which forms at the region of maximum microwave power density, is determined by the pattern of electric power density within a microwave resonant cavity absorption chamber. Proper cavity design produces patterns that result in an axially located plasma that is positioned directly upstream from a nozzle incorporated into one end of the cavity. The electromagnetic design of the cavity is of extreme importance to the operation of the thruster. To this end, numerical electromagnetic modeling has been used to evaluate the field structure in existing prototype lab thruster designs. This modeling goes beyond the qualitative mode modeling of past efforts and allows for an investigation of additional components and features within the cavity. As such, its use has allowed further refinement of the electric field profile and provides guidance with respect to antenna placement, cavity height and diameter, material selection, etc. This paper reports on the modeling efforts of a 70-W, 7.5-GHz thruster for use on small spacecraft and satellites. Although specific to this thruster, the work should provide design guidance to other MET designs, including the kW-class, 2.45-GHz MET.