Investigation of Efficiency in Applied Field MagnetoPlasmaDynamic Thrusters
An experimental and theoretical investigation of the scaling of thrust efficiency with the operational parameters (J,B, mdot) of applied-field magnetoplasmadynamic thrusters (AF-MPDTs) is carried out to provide guidelines for scaling and controlling AF-MDPT performance. This investigation is based on characterization of the various power dissipation mechanisms in AF-MPDTs with a focus on the acceleration and anode sheath power components.
A semi-empirical model is derived for the anode sheath voltage fall in AF-MPDTs and verified by comparison to experimental data on a 30 kW lithium-fed steady-state AF-MPDT obtained using a hot langmuir probe. It is found that the anode sheath voltage fall increases approximately linearly with current and applied magnetic field and is inversely proportional to mass flow rate. It is shown that, although the electrons in the anode sheath are unmagnetized the voltage fall is attributed to plasma density reduction at the sheath edge, which is a result of increased plasma pinching at higher applied magnetic field values. It is also concluded that increased thermionic emission from the anode surface leads to an increase in the anode sheath voltage fall; therefore anode material with a high work function is preferred.
A thrust efficiency model is formulated by employing a thrust formula previously derived and verified for the same thruster, and composing expressions for the different voltage components in AF-MPDTs. It is demonstrated that the efficiency increases with applied magnetic field for all current and mass flow rate values, and the en- hancement of the efficiency by the applied magnetic field is greater when the mass flow rate is reduced. It is shown that the efficiency-current curves have a decreasing-increasing behavior due to an interchange between the different thrust components, each of which dominates in a different current regime and thus affects the scalability of the acceleration power component with current.
It is demonstrated that electrodes power losses, primarily anode sheath power losses, are the dominant power dissipation mechanism in AF-MPDTs. It is also demonstrated that resistive power deposition, which is responsible for plasma heating and ionization, has little effect on the overall efficiency, except in the low current regime in which resistive power losses can account for more than a third of the total thruster power.
The physical insights obtained from this study can aid in forming design criteria and general guidelines for AF-MPDT design and control.