Simulation of direct current microdischarges for microthruster applications
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The structure of direct-current microdischarges (MDs) is investigated using a detailed two-dimensional multispecies continuum model. MDs are direct-current (DC) discharges that operate at pressures of 100-1000 Torr and geometric dimensions in the 10-100 micron range. The motivation for this work comes from our proposed application of a MD-based electrothermal thruster for small satellite propulsion. The physical and chemical character of MDs, including the sheath-bulk plasma structure, effect of high-pressure and high-temperature plasma chemistry, effect of geometric configuration, and effect of bulk gas flow through the active discharge region is studied. The two-dimensional model, developed as part of this study, consists of two modules, a plasma module and a flow module. The plasma module solves conservation equations for plasma species continuity, electron energy and neutral gas energy, vi and Poisson’s equation for the self-consistent electric field. The flow module solves bulk gas momentum and mass conservation equations. Two discharge geometries are considered, the microhollow cathode discharge (MHCD) geometry, and the flow-through hollow-electrode (HE) geometry. Results indicate that the MHCD operates in an abnormal glow discharge mode with charged and excited metastable species densities of order 1020 m−3 , electron temperatures of tens of eV, and gas temperatures of several hundreds of Kelvin above the ambient temperature. Most of the model predictions are in qualitative and quantitative agreement with experimental data for MHCD discharges under similar conditions. The HE geometry is used to study plasma-flow interactions. The effect of pressure (250-1000 Torr), power input (0.5-1 W), and flow rates (0-225 sccm) on helium discharge properties is studied. The results show that the HE discharge operates in an abnormal discharge mode that transitions toward an increasingly normal mode as either the pressure or the flow rate is increased. The discharge becomes more confined and collisional with increasing pressures or increasing flow rates. Intense gas heating results in high gas temperatures (300-500 K above the ambient), even in the presence of significant gas flow. Gas heating is found to have a strong influence on overall discharge behaviour. The study provides crucial understanding to aid the preliminary design of direct-current MD based thrusters.