An anisotropic hybrid structured-unstructured mesh approach enabling PIC simulations of the Scrape-Off-Layer

In this research, we introduce an anisotropic hybrid structured-unstructured mesh approach for Particle-In-Cell (PIC) scheme based kinetic simulations of the Scrape-Off-Layer (SOL) for cases with non-trivial wall shapes. The steep gradient of the electrostatic potential results in an anisotropic solution variation, e.g., in the plasma-sheath region, for which using a uniform mesh, or even a nonuniform/graded isotropic mesh, leads to an excessively large mesh. To address this challenge, we propose an anisotropic hybrid mesh approach. This approach employs a structured or layered anisotropically refined mesh, including near the non-trivial wall shapes, and smoothly transitions into a relatively coarse unstructured triangular mesh with rapid gradation for regions with a marginal solution variation. The anisotropic layered mesh uses a much smaller mesh size in the direction of the solution variation, e.g., mesh size of the Debye length in the wall-normal direction in the sheath region, while using a larger mesh size in the other directions. The effectiveness of the proposed approach is demonstrated through kinetic simulations of large-scale SOL using the hPIC2 code on multi-GPU computers.