Enforcing conservation at Monte Carlo level in a coupled Finite Volume - Monte Carlo simulation

Plasma edge simulations generally require coupling a finite volume (FV) fluid plasma discretization with a kinetic neutral Monte Carlo (MC) model. From the MC neutral simulation, source terms are estimated and inserted into the FV plasma simulation, transferring MC stochasticity to the FV simulation. While the FV simulation is based on the physical principle of conservation, most common MC source term estimators do not strictly conserve mass, momentum, and energy. Via the coupling, this also breaks the conservation in the FV simulation. Losing conservation can lead to numerical instability on the one hand and the inability to fulfill global mass, momentum, and energy balances on the other. Presently, the SOLPS-ITER code rescales the neutral flux launched from the target to mitigate the instability. In this paper, we propose to instead independently rescale each of the source term estimations. We show that rescaling introduces a bias that depends on the number of MC particles and is negligible compared to the statistical error in the simulation. The resulting method restores all conservation properties in the FV/MC code, which will allow monitoring the convergence through stabilization of the global quantities.status: submitte