Improved Efficiency and Throttling Range of the VX-200 Magnetoplasma Thruster

Testing of the Variable Specific Impulse Magnetoplasma Rocket VX-200 engine was performed over a wide throttle range in a 150 m3 vacuum chamber with sufficient pumping to permit exhaust plume measurements at argon background pressures less than 1×10−3 Pa (1×10−5 torr) during firings, ensuring charge-exchange mean free paths longer than the vacuum chamber. Measurements of plasma flux, radio frequency power, propellant flow rate, and ion kinetic energy were used to determine the ionization cost of argon and krypton in the helicon discharge. Experimental data on ionization cost, ion fraction, exhaust plume expansion angle, thruster efficiency, and thrust are presented that characterize the VX-200 engine performance over a throttling range from 15 to 200 kW radio frequency power. A semiempirical model of the thruster efficiency as a function of specific impulse indicates an ion cyclotron heating efficiency of 85±7%. Operation at a total radio frequency coupled power level of 200 kW yields a thruster efficiency of 72±6% at a specific impulse of 4900±300 s with argon propellant. A high thrust-to-power operating mode was characterized over a wide parameter space with a maximum thrust-to-power ratio of 51±5 mN/kW at a specific impulse of 1660±100 s for a ratio of ion cyclotron heating radio frequency power to helicon radio frequency power of 0.7∶1.