The microwave arcjet thruster uses microwave energy to create a free-floating plasma discharge within a microwave resonant cavity. This discharge absorbs the input microwave power and converts it to thermal energy which is then transferred to the flowing propellant gas. The thruster can be operated in a fixed configuration where neither the cavity geometry nor the tuning mechanisms are adjusted; optimization of this geometry for a given propellant results in power coupling in excess of 97%. The prototype has demonstrate its ability to operate in this fixed configuration within a vacuum tank facility where it has been extensively tested using both N2 and NH3. The current design is capable of efficient operation over a wide range of power levels and mass flow rates resulting in representative specific powers of 6.5 MJ/kg for N2 and 77.2 MJ/kg for NH3. Current work is focused on development of a compact power supply, redesign of the current prototype to allow for high temperature operation, and design of a low-mass prototype design which mates the magnetron supply directly to its power coupling probe.