Adding a fluid neutrals model to the gyrokinetic code GENE-X for edge/SOL turbulence simulation in divertor geometries

Developing first-principle codes to accurately simulate the plasma edge and SOL is crucial for predicting heat and particle exhaust in future fusion devices. Significant quantities of neutrals in the SOL interact with the plasma through complex collisions, necessitating their inclusion in plasma turbulence codes.

In this work a novel coupling of a continuum gyrokinetic model with a fluid neutrals model using the GENE-X code[1] is detailed. GENE-X is a global, full-f, electromagnetic, collisional gyrokinetic turbulence code for studying the edge and SOL in realistic geometries, including magnetic X-points. Neutrals are treated as a fluid to minimize computational cost, with density evolution (1-moment model) modeled by a pressure-diffusion equation, where diffusion is caused by charge exchange collisions. Two centered finite difference schemes, a 4th order and a 2nd order direct discretization of the divergence operator, are implemented to study trade-offs between accuracy and stability. Plasma-neutrals coupling is performed using conservative Krook operators to model ionization and recombination sources and sinks. A three-operator, 2nd-order Strang splitting algorithm allows explicit integration, treating the Vlasov plasma, Coulomb collisions, and neutrals independently. To lift the time-step constraints imposed by the diffusive nature of the latter two, a 2nd order RK-Chebyshev (RKC) scheme has been implemented, also allowing the number of steps to be chosen to maximize the possible time-step in the simulations. The implementation has been verified using the method of exact solutions, obtaining the expected order of accuracy in slab, circular and toroidal geometries, and first blob simulations are performed.

References

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