Fast-convergence and asymptotic-preserving simulation of neutral particle flows in the plasma edge

Abstract

The neutral flows in the plasma edge play a pivotal role in the design of nuclear fusion devices such as divertors and pumps. These flows are generally multiscale, encompassing the continuum, slip, transition, and free-molecular flow regimes, thus necessitating the use of gas kinetic equations. Traditional numerical methods, such as the direct simulation Monte Carlo method and the discrete velocity method, are hindered by extensive computation resources when dealing with near-continuum flows. This paper presents a general synthetic iterative scheme to deterministically simulate the neutral flows in plasma edge accurately and efficiently. By alternately solving the kinetic equations and macroscopic synthetic equations, our method substantially decreases the number of iterations, while maintains asymptotic-preserving properties even when the spatial cell size is much larger than the mean free path. Consequently, a significant reduction in the computational cost, particularly in near-continuum flow regimes, is achieved. This advancement provides an efficient computational tool essential for the advancement of next-generation nuclear fusion reactors.