VASIMR VX-200 thruster throttling optimization from 30 to 200 kW

The VASIMR$^{\mbox{{\textregistered}}}$ VX-200 experimental plasma thruster incorporates a 40 kW helicon plasma source with a 180 kW Ion Cyclotron Heating (ICH) acceleration stage integrated in a superconducting magnet. Argon propellant mass flow is injected up to 140 mg/s. Rapid plasma start up ($<$ 100 ms) and high pumping speed ($>$ 10$^{5}$ liters/s) in a 150 m$^{3}$ vacuum chamber achieve performance measurements with the charge exchange mean-free-path greater than 1 m in the background neutral gas (pressure $<$ 10$^{-5}$ Torr). The thruster efficiency at 200 kW total power is 72 $\pm $ 9{\%}, the ratio of effective jet power to input RF power, with an Isp = 4900 $\pm $ 300 seconds (flow velocity of 49 km/s), and an ion flux of 1.7 $\pm $ 0.1 $\times $ 10$^{21}$/s. The thrust increases steadily with power to 5.8 $\pm $ 0.4 N until the power is maximized and there is no indication of saturation. The plasma density near the device exit exceeds 10$^{18}$ m$^{-3}$ with a power density over 5 MW/m$^{2}$. An extensive study of thruster performance, efficiency and thrust-to-power ratio, as a function of Ar propellant flow rate and ICH-to-helicon RF power ratio has been carried out over a total power range of 30 to 200 kW. Optimized throttling set points are determined. The experimental configuration and results of this study are presented.