Optimizing the performance of plasma based microthrusters

Summary form only given. Microdischarges (MDs) having diameters of 100s of mum and operating with back pressures of 10 to 50 torr are being investigated for use as sources of thrust for small satellites. A typical design includes a pair of ring electrodes separated by 100s to 1000s mum embedded into high temperature compatible ceramic. A DC discharge is sustained between the electrodes producing a glow and, in some cases, an arc. By virtue of being nozzles across a pressure gradient, these MDs produce thrust by the natural expansion of the gas. The purpose of the discharge is to add enthalpy to the gas flow to both increase the exit speed (and hence increase the thrust) and to enable a higher degree of control of the thrust. The major development issues include designing the electrodes and power format to optimize thrust while being consistent with system requirements. In this talk, results from a computational investigation of this class of MD will be discussed. The model used in this investigation is a 2-dimensional plasma hydrodynamics simulation having an unstructured mesh to resolve non-equilibrium electron, ion and neutral transport using fluid equations. Sheath accelerated, beam-like electrons are resolved using a Monte Carlo simulation. A compressible Navier-Stokes module provides the bulk fluid velocities and temperatures. A circuit model enables the discharge to be operated in a constant current, constant power or constant voltage mode, as well as using pulsed or AC power formats. Results from a parametric investigation of pressure (a few 10s of torr), power (a few Watts), and flow rates (10s sccm) will be discussed for rare gas mixtures with the goal of maximizing the thrust and the thrust per unit power. Exit speeds of up to 100s m/s, a factor of two larger than without the plasma, have been predicted in the throats of the devices with electron densities of 10¹³-10¹⁴ cm ^-3. Up to 50% of the ionization is produced by the beam electrons. The plasma produced addition of enthalpy is to some degree limited by heat transfer to the supporting structure, and so designs which utilize the enthalpy as thrust before it is conducted to the walls is important