Neutral gas model for gyrokinetic edge and SOL turbulence simulations with GENE-X

Understanding turbulent transport in the edge and scrape-off layer (SOL) is key to managing heat and particle exhaust while ensuring optimal core confinement. Neutrals, especially in the SOL, are present in significant quantities and interact with the plasma through complex collision processes. This impacts the radial plasma profiles and the resulting gradient-driven instabilities[1], the particle transport across the separatrix[2], and the blob dynamics in the SOL[3].

This work presents the first-of-its-kind coupling of a continuum full-f gyrokinetic model with a fluid model for neutrals using the GENE-X code[4]. GENE-X is dedicated to studying the edge and SOL in realistic magnetic geometries, including X-points, via a flux-coordinate independent (FCI) approach. The evolution of the neutral density is modeled by a pressure-diffusion equation, with diffusion caused by charge exchange collisions. Plasma-neutral interactions, including ionization, recombination, and associated radiation, are modeled using specially designed Krook operators. These operators ensure the conservation of total momentum and the combined kinetic and radiated energy.

After the verification of the implementation in standard geometries, relaxation studies of a three-species system -- consisting of electrons, protons, and hydrogen -- are performed to observe the impact of plasma-neutrals inelastic reactions on plasma distribution functions and equilibriation dynamics. Further, gyrokinetic simulations of seeded blobs in divertor geometries are carried out to investigate the impact of plasma-neutrals interactions on radial transport across the separatrix and in the SOL. The interplay with magnetic geometries and the relevance to turbulence simulation are discussed.

References

[1] W. Zholobenko et al., Nuclear Fusion 61, 116015 (2021).

[2] A. S. Thrysøe et al., Physics of Plasmas 25, 032307 (2018).

[3] T. N. Bernard et al., Physics of Plasmas 30, 112501 (2023).

[4] D. Michels et al., Physics of Plasmas 29, 032307 (2022)