Computational acceleration of orbital neutral sensor ionizer simulation through phenomena separation

Simulation of orbital phenomena is often difficult because of the non-continuum nature of the flow, which forces the use of particle methods, and the disparate time scales, which make long run times necessary. In this work, the computational work load has been reduced by taking advantage of the low number of collisions between different species. This allows each population of particles to be brought into convergence separately using a time step size optimized for its particular motion. The converged populations are then brought together to simulate low probability phenomena, such as ionization or excitation, on much longer time scales. The result of this technique has the effect of reducing run times by a factor of 103-104. The technique was applied to the simulation of a low earth orbit neutral species sensor with an ionizing element. Comparison with laboratory experiments of ion impacts generated by electron flux shows very good agreement.