A kinetic electron-neutral collision model for particle-in-cell plasma simulation

Details of a kinetic electron-neutral collision model for particle-in-cell plasma simulation codes are presented. The model uses an efficient scheme to randomly select collision events -- elastic, excitation and ionization -- with the appropriate probability [H. Sugawara, \textit{et al}., J. Comput. Phys. \textbf{223}, 298 (2007).] Ionization events create electron-ion pairs, and the secondary electrons can themselves ionize the gas. To maintain a manageable particle count, a particle merger algorithm can be used to periodically replace all particles of a given species in a cell with a new, smaller set that conserves charge, momentum, and energy [D. R. Welch, \textit{et al}., J. Comput. Phys. \textbf{227}, 143 (2007).] Small-scale tests show that results with the merger are in good agreement with non-merged runs. Large simulations can only be done with the merger on, and typically show excellent merger efficiency (\textgreater 90{\%}).