Coupling plasma and neutral kinetic models: Considerations and solutions

In the edge and divertor regions of magnetic confinement devices, the plasma and neutral species are both highly non-Maxwellian. First principles simulation of this environment therefore requires self-consistent kinetic theory. To this end, the gyrokinetics edge simulation suite XGC [1] is coupled to DEGAS2 [2], a Monte Carlo transport solver for the neutral species. Earlier iterations of this coupling identified a lack of energy and momentum conservation. This unphysical behavior was due to the simplified treatment of the charge-exchange interaction: effective Maxwellians were used to represent the field particle distributions in each of the respective collision operators that couple the plasma to the neutral gas [3]. A rigorous treatment recognizes charge-exchange as a nonlinear interaction between (unknown a priori) non-Maxwellian populations.