Microwave power absorption coefficient of an ECR xenon ion thruster

A 20-cm diameter xenon ion thruster with electron cyclotron resonance (ECR) discharge generates 30 mN of thrust at a total electric power consumption of 1 kW for spacecraft propulsion by ejecting 1.1 keV ion beam. By optimizing the discharge chamber length, magnetic field and propellant flow injection, ion beam currents of 500 mA at a microwave power of 100 W had been obtained at a frequency of 4.25 GHz with SmCo magnets arranged on a flat discharge chamber. The performance was highly dependent on the propellant injection method that affects electron-heating process. Two-dimensional microwave E-field distributions inside the discharge chamber were experimentally investigated for the best and the worst injector layouts. Microwave power absorption coefficient was estimated using the E-field distributions with and without plasma discharge. The coefficient decreases as the microwave power decreases and electron density gets close to an ECR cutoff density in all cases. The worst injector layout showed larger reflection and smaller absorption coefficient even at small beam currents. In the best configuration, microwave reflection was sufficiently smaller than 10% and 70–90% of the microwave power launched into the discharge chamber was absorbed by plasma electrons.