Numerical accuracy and convergence with EMC3-EIRENE

The iterative Monte Carlo (MC) code EMC3-EIRENE is frequently used for plasma edge simulations in 3D applications. So far, a quantitative evaluation of the numerical quality of the code results remains an open issue. In this paper, we demonstrate a framework for the practical assessment of accuracy and convergence with EMC3-EIRENE. Moreover, we provide a first accuracy analysis with EMC3-EIRENE for a DIII-D divertor edge plasma case. First, we introduce post-processing averaging to efficiently reduce the variance of the statistical error. Then, we estimate the deterministic error contributions based on their theoretical reduction rates by comparing solutions with a different grid resolution, time step, or number of MC particles per iteration. Finally, using parameterized expressions for the error and the computational time, suitable numerical parameters are determined to achieve faster and/or more accurate results. We found that simulations can be more than twice as fast without losing accuracy by making use of post-processing averaging and choosing optimal parameters. In addition, we conclude that the discretization error is the dominant error contribution for the case selected in this paper, which demonstrates the importance of constructing an adequate mesh.