Accelerator-enabled non-uniform mesh procedures for pic simulations targeting plasma surface interactions

Abstract

In fusion reactors, when a high-velocity plasma species strikes a material wall it sputters the wall impurities into the domain which can adversely affect the sustainability of the fusion reactions. High-fidelity plasma-surface interaction models are needed to understand the behavior of impurities and their impact on fusion reactions. The overall impurity transport problem requires a multi-scale model. In particular, it requires a small-scale model to perform high-fidelity simulations of the surface yield as well as the sheath region (a tiny portion closest to the wall, with a thickness of few millimeters) and a device-scale model for high-fidelity impurity transport simulation in the fusion domain (which involves dimensions in the order of meters or tens of meters). In this work, the small-scale model involving the surface and sheath region is based on kinetic descriptions of particles to simulate the sputtering of impurities from the material wall into the plasma domain as well as the sheath electric field. Subsequently, the impurities are tracked in the device-scale transport model to simulate the migration and interaction kinetics of the impurities and any further surface events/interactions. Problems of interest include conditions that often produce high gradients in the specific portions of the domain which calls for highly anisotropic and non-uniform/graded meshes. The primary focus of this work is on the design and development of accelerator-enabled (e.g., GPU-enabled) capabilities (including data structures and algorithms) for particle-in-cell (PIC) simulations on highly non-uniform/graded meshes. Specifically, we target two aspects based on novel procedures designed for: (i) block-structured, non-uniform meshes to simulate sheath physics, and (ii) 3D complex geometries and unstructured anisotropic meshes to simulate device-scale impurity transport. In particular, this research has enabled two new types of PIC simulations on non-uniform meshes targeting: (i) sheath electric field, and (ii) integrated impurity transport. In both cases, highly performant and scalable procedures as well as software on GPUs are developed. The utility of the current computational tools are demonstrated on multiple problems of interest including tokamak devices such as ITER and DIII-D. Specifically, a one-of-its-kind integrated impurity simulation, including high-fidelity 2D sheath simulation, is performed.