Numerical simulations of the plumes of electric propulsion thrusters

The plumes of two electric propulsion thrusters are simulated using a computer code which combines the direct simulation Monte Carlo (DSMC) and the Particlein-Cell (PIC) techniques. The DSMC method used for rarefied gas flow problems is described. The PIC method used to simulate the plasma behavior in the plume is also described. A discussion of the assumptions and the physical models used for the plasma dynamics is included. Details of the collision models are also discussed. Computational techniques permit the physical models and collision models in the simulations to be examined. These techniques include a particle weighting scheme and a routine for simulating expansion into a finite background pressure. Both techniques address resolution problems which occur when species have large differences in density.; The simulations capture the plume behavior qualitatively. The sensitivity of the plume profile to various physical assumptions and flow conditions is examined. Comparisons with available experimental data reveal which assumptions are valid and which quantities are insensitive to the uncertainties. The electron temperature and its variation in the plume are important features which affect the ion profile. Many of the simulations show good agreement with much of the data. Simulations which include the full chamber of the experimental facility in which the data were obtained lead to good agreement with the data from the centerline to large angles in the thruster back flow region. This level of agreement is not found if the background pressure of the vacuum chamber is not included in the simulations. Discrepancies between the simulations and the experimental data indicate areas where the modeling needs improvement.