Computational implementation of a coupled plasma-neutral fluid model

Abstract

This paper describes the computational transport of coupled plasma-neutral fluids in the edge region of a toroidally symmetric magnetic confinement device, with applications to the tokamak. The model couples neutral density in a diffusion approximation with a set of transport equations for the plasma including density, classical plasma parallel velocity, anomalous cross-field velocity, and ion and electron temperature equations. The plasma potential, gradient electric fields, drift velocity, and net poloidal velocity are computed as dependent quantities under the assumption of ambipolarity. The implementation is flexible to permit extension in the future to a fully coupled set of non-ambipolar momentum equations. The computational method incorporates sonic flow and particle recycling of ions and neutrals at the vessel boundary. A numerically generated orthogonal grid conforms to the poloidal magnetic flux surfaces. Power law differencing based on the SIMPLE relaxation method is modified to accomodate the compressible reactive plasma flow with a “semi-implicit” diffusion method. Residual corrections are applied to obtain a valid convergence to the steady state solution. Results are presented for a representative divertor tokamak in a high recycling regime, showing strongly peaked neutral and plasma densities near the divertor target. Solutions show large poloidal and radial gradients in the plasma density, potential, and temperatures. These findings may help to understand the strong turbulence experimentally observed in the plasma edge region of the tokamak.