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 (AF-MPDTs) is carried out in order to provide guidelines for scaling and controlling AF-MDPT performance. Thruster voltage measurements were made at different current, applied magnetic field and mass flow rate levels in a 30 kW lithium-fed AF-MPDT. The efficiency was then calculated using the voltage data along with a semi-empirical thrust formula derived and verified previously for the same thruster. The non-useful voltage component (the voltage associated with the thruster’s power losses) was found to scale linearly with current and applied magnetic field and inversely with mass flow rate. This behavior was attributed to electrode sheath effects and decreased conductivity with increasing applied magnetic field. The efficiency was found to increase with 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 is reduced. The observed minimum in the efficiency vs current curve was related to interplay between the components of the thrust and was shown experimentally and analytically to increase with increasing applied field and decreasing mass flow rate.