Scaling of efficiency with applied magnetic field in magnetoplasmadynamic thrusters

An investigation of the scaling of thrust efficiency with the applied magnetic field in applied-field magnetoplasmadynamic thrusters is carried out in order to provide guidelines for scaling and controlling appliedfield magnetoplasmadynamic thruster performance. Thruster voltage measurements were made at different current, applied-magnetic-field, and mass-flow-rate levels in a 30 kW lithium-fed applied-field magnetoplasmadynamic thruster. The efficiency was then calculated using the voltage data along with a semiempirical thrust formula derived and verified previously for the same thruster. The nonuseful voltage component (the voltage associated with the thruster's power losses) was found to scale linearly with the current and applied magnetic field and inversely with the mass flow rate. This behavior was attributed to electrode sheath effects and decreased conductivity with an increasing applied magnetic field. The efficiency was found to increase with the applied magnetic field for all current and mass-flow-rate values, and the enhancement of the efficiency by the applied magnetic field was found to be greater when the mass flow rate was reduced. The observed minimum in the efficiency versus the current curve was related to the interplay between the components of the thrust and was shown experimentally and analytically to increase with increasing the applied field and decreasing the mass flow rate.