A hybrid fluid-kinetic model for hydrogenic atoms in the plasma edge of tokamaks based on a micro-macro decomposition of the kinetic equation

Abstract

Monte Carlo (MC) simulations of the full kinetic equation for the neutral particles in the plasma edge become computationally costly for reactor-relevant regimes. To accelerate the simulations, we propose a hybrid fluid-kinetic approach that is based on a micro-macro decomposition of the kinetic equation. This leads to a macro/fluid model with kinetic corrections that follow from an MC simulation of the micro/kinetic part. We distinguish three hybrid models with different underlying fluid equations: (i) a pure pressure-diffusion equation with equal neutral and ion temperatures (Tn=Ti); (ii) a continuity and parallel momentum equation with pressure-diffusion transport retained in the directions perpendicular to the magnetic field lines, with Tn=Ti; and (iii) the same model as (ii), but with a separate neutral energy equation (Tn≠Ti). To facilitate the future integration in more complete plasma edge codes, we neglect some kinetic correction terms. Hence, the hybrid model is not exactly equivalent to the full kinetic equation. We assess the hybrid performance on the basis of the reduction of the CPU time compared to an MC simulation of the full kinetic equation for the same statistical error on a certain plasma source. This is done for a high recycling slab case. Only the models with parallel momentum equation ((ii)-(iii)) are able to significantly reduce the CPU time. However, due to the incomplete kinetic corrections there is a remaining hybrid-kinetic discrepancy that mainly pops up in the ion energy source from the model with energy equation (iii).