Interspecies fractional collisions

Abstract

Relaxing the direct simulation Monte Carlo (DSMC) method to be compatible with variable particle weights has significant advantages when applied to simulations with a large density variation, as the statistical noise in the low density region can be controlled. The stochastic weighted particle method [S. Rjasanow and W. Wagner, J. Comput. Phys. 124, 243–253 (1996)] enables the control of particle weight but has never been applied to a multicomponent gas mixture or plasma. This paper focuses on charge-exchange collisions, but the method is applicable to any interspecies collision reactions including ionization and excitation collisions, providing the correct collision rate between different species with different weights in the context of no-time-counter scheme, which can easily be extended to other DSMC schemes. The method is first verified through two test cases observing the depletion of high-energy ions as impacting background neutral particles and looking at the spatial variation and temporal evolution of different species. Then, the method is applied to a simple electric propulsion plume simulation, demonstrating the controllability of species ratio and capability in reducing statistical noise for the charge-exchange ions.