Anode Sheath Contributions in Plasma Thrusters

Contributions of the anode to Magnetoplasmadynamic MPD thruster performance are considered. High energy losses at this electrode, surface erosion, and sheathionization effects must be controlled in designs of practical interest. Current constriction or spotting at the anode, evolving into localized surface damage and considerable throat erosion, is shown to be related to the electron temperatures T sub e rise above the gas temperature T sub o . An elementary one-dimensional description of a collisional sheath which highlights the role of T sub e is presented. Computations to model the one- dimensional sheath are attempted using a set of five coupled first-order, nonlinear differential equations describing the electric field, as well as the species current and number densities. For a large temperature nonequilibrium i. e., T sub e T sub o, the one-dimensional approach fails to give reasonable answers and a multidimensional description is deemed necessary. Thus, anode spotting may be precipitated by the elevation of T sub e among other factors. A review of transpiration cooling as a means of recouping some anode power is included. Active anode cooling via transpiration cooling would result in 1 quenching T sub e, 2 adding hot propellant to exhaust. and 3 reducing the local electron Hall parameter. However significant technical problems remain.