Modeling of Radiation Transport in Fluctuating Edge Plasmas: A Statistical Approach

The radiation due to atomic lines in the tokamak edge and divertor can be strongly coupled to the plasma dynamics. Self-consistent (charged particle - atom - photon) simulations carried out for ITER conditions with the transport code B2-EIRENE have indicated modifications of the effective ionization rate induced by photon absorption effects. Up to now, all of the investigations done so far were made assuming a plasma background whose typical spatial variation scales are much larger than the radiation transport scale. In this paper, we examine the validity of this approximation within an effective transport model, relying on a statistical parameterization of the turbulent fluctuations. The model is inspired from the quasilinear theory used for plasma turbulence and assumes a small ratio lturb/lmfp between the turbulence correlation length and the photon mean free path. We apply the model to calculations of the penetration of a radiation pencil in an optically thick turbulent plasma. The validity of this model for intermediate lturb/lmfp is next discussed through comparison to a model-coefficient approach, widely used in problems involving linear stochastic differential equations and suitable for arbitrary turbulence correlation length ((c) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)