Model for computing volume-averaged plasma properties in electron-bombardment ion thrusters

Abstract

This paper describes a model for computing volume-averaged values of the plasma properties inside electron-bombardment ion thrusters, using spatially varying Langmuir-probe measurements. The proposed model is intended to identify the correct plasma property averaging process that transforms the measured, spatially varying plasma parameters within a given thruster into their equivalent, spatially uniform values. This transformation technique allows thruster designs to be evaluated and compared on the basis of a single average value for each plasma parameter. In addition, it allows valid quantitative comparison with modern theoretical models of ion thruster discharge plasmas that are based on the characteristics of a spatially uniform plasma. Average values of the Maxwellian electron temperature and density, as well as the primary electron energy and density, are computed, compared, and interpreted for several different operational set points of two 30-cm diam mercury ion thrusters; a ring-cusp thruster and a J-series thruster, J-2. For low values of beam-ion-production cost, the superior performance of the ring-cusp thruster is correlated with a higher volume-averaged Maxwellian electron temperature and primary electron density compared to the J-series thruster. The volume-averaged plasma properties for both thrusters are compared with the analytical predictions of a detailed ion thruster performance model that is based on the characteristics of a spatially uniform plasma. Good quantitative and qualitative agreement is obtained between the volume-averaged and the ion thruster performance model calculations.