The neutral atoms in the plasma edge of nuclear fusion devices are typically modeled using either a fluid or kinetic approach. The kinetic approach is most accurate, but it has two main disadvantages. First, the usual solution of the high-dimensional kinetic equation using Monte Carlo techniques introduces statistical noise, which hampers the convergence of the coupled plasma-neutral model. Second, the computational time strongly increases for highly collisional regimes. For these reasons, deterministic fluid neutral models remain an attractive alternative, in particular for the highly collisional conditions where their accuracy is expected to be high. In recent years, efforts have been undertaken to improve the agreement between the fluid and kinetic approach by introducing consistent transport coefficients and consistent boundary conditions in the fluid models. In this work, these so-called advanced fluid neutral models are further enhanced by introducing different strategies to cope with the high heterogeneity of the ion-neutral collisionality encountered in realistic plasma-edge geometries, namely isotropic neutral flux limiters and an automated selection criterion for the optimal neutral boundary conditions. The validity of the resulting fluid neutral models is thoroughly assessed for various representative simulation cases with different geometries, divertor collisionalities, and wall materials, including, for the first time, simulations in a realistic ITER plasma edge geometry. Strong quantitative agreement between the fluid and kinetic models is achieved for cases with highest divertor collisionality.
Hilbert expansion based fluid models for kinetic equations describing neutral particles in the plasma edge of a fusion device