Plasma sources for micro-thrusters

Summary form only given. Micro-thrusters are used in spacecraft to generate thrusts ranging from sub-/spl mu/N to mN for station keeping and attitude control. Currently, pulsed plasma thrusters (PPT) and Hall effect thrusters have been used in larger spacecraft but scaling them to small thrusts presents challenges. Micro-ion engines have higher specific impulse and potentially higher thrust to power ratios and operate with noncontaminating propellants when compared with PPT systems. Inductively Coupled (ICP) sources for micro-thrusters are attractive due to their simpler designs, potentially longer service lives, and ability to achieve high ion densities. Scaling ICP sources to small dimensions is challenging due to the need to increase frequency and plasma density to obtain reasonably small skin depths. To maximize the conversion of power to thrust (through acceleration of ions using grids), the fractional ionization should be as large as possible. In this regard, plasma densities of 10/sup 10/-10/sup 11/ /cm/sup 3/ at 2 W and 2-4 Torr pressure have been achieved in ICPs sustained in argon having dimensions of a few mm. In this talk, we report on results from a 2-dimensional (2d) computational investigation of plasma sources for micro-thrusters similar to those described by Minayeva and Hopwood. The 2d model uses 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. Power deposition is resolved by solving Maxwell's equations coupled with the electron and ion transport. Results from a parametric study of pressure ( a few to 10 Torr), power (a few Watts) and a geometry for sources with dimensions of a few mm discussed for rate gas mixtures with the goal of optimizing the ionization fraction.