Improved collision operator for plasma kinetic simulations with multi-species ions and electrons

A linearized collision operator, which preserves the conservation properties of particles, momentum, and energy, and the self-adjointness relation (H-theorem) for arbitrary particle species, is applied to Eulerian kinetic simulations of multi-species ions and electrons. Velocity-space grid number scans clarify that numerical errors in like-particle and unlike-ion collisions are sufficiently small under turbulence-simulation relevant resolutions. For electron-ion/ion-electron collisions, it is identified that highly steep and localized nature in the velocity-space leads to relatively larger conservation errors in the standard implementation without error corrections. In order to resolve such numerical difficulties, a modified field-particle operator based on the original one is newly constructed, so that the conservation properties are improved to achieve the round-off error levels. It is confirmed that the modified collision operator successfully reproduces the collisional relaxation of multi-species ions and electrons towards their equilibrium distributions satisfying the H-theorem.