Particle conservation in numerical models of the tokamak plasma edge

Abstract

The test particle Monte-Carlo models for neutral particles are often used in the tokamak edge modelling codes. The drawback of these models is that the self-consistent solution suffers from random error introduced by the statistical method. A particular case where the onset of nonphysical solutions can be clearly identified is the violation of the global particle balance due to non-converged residuals. There are techniques that can reduce the residuals—such as internal iterations in the code B2-EIRENE—but they may pose severe restrictions on the time-step and slow down the computations. Numerical diagnostics described in this paper can be used to unambiguously identify when the too large error in the global particle balance is due to finite-volume residuals, and their reduction is absolutely necessary. Algorithms that reduce the error while allowing large time-step are also discussed.