Assessment of advanced fluid neutral models for the neutral atoms in the plasma edge and application in ITER geometry

Abstract

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.

Fluid, kinetic and hybrid approaches for neutral and trace ion edge transport modelling in fusion devices

“…Moreover, the accurate numerical discretization of the (isotropic) neutral transport fluxes on grids misaligned with respect to the magnetic field was proven essential is this picture [29]. Through the use of boundary conditions based on TRIM data, the impact of wall materials on the reflection properties is now captured by the fluid models [16,30]-an effect that was inaccessible with earlier fluid neutral approaches.…”

Advanced spatially hybrid fluid‐kinetic modelling of plasma‐edge neutrals and application to ITER case using SOLPS‐ITER

“…These fluid neutral boundary conditions have been proven to be accurate in HCRs. [1,2,7,14] A separate surface source of thermally released kinetic molecules (Γ mol,k ) now has to be added, with a strength of…”

“…For simplicity, we take a spatially constant L = 0.1 m, representing a rough average of the gradient length scales encountered in the problem, as explained in Ref. [14]. It would be physically more appropriate to introduce a spatially dependent L(r), based on local gradient length scales of the plasma, as was done in Ref.…”

“…Speed-up of the EIRENE part of the hybrid simulations compared to the fully kinetic approach, including also the atom-only Note: Left half: speed-up for the particle distributions from Table 3. Right half: speed-up, including the correction for the statistical noise, determined by Equation (14).…”

Hilbert expansion based fluid models for kinetic equations describing neutral particles in the plasma edge of a fusion device

“…The literature on fluid models describing neutrals in the plasma edge mainly focuses on solving steady-state problems using models based on a variant of the Method of Moments. [6][7][8][9][10][11][12][13][14][15][16][17][18][19] These fluid models typically introduce approximations and simplifications (e.g., discarding terms or adding additional constraints) in the fluid description based on an intuitive, phenomenological basis. Incorporating knowledge of domain experts, these models are accurate given the right physical conditions, but have a rather unclear range of validity and offer no consistent approximation to the particle velocity distribution.…”