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Numerical Cherenkov Instabilities in Kinetic Plasma Particle-in-Cell Simulations based on Structured and Unstructured Meshes

Particle-in-cell simulations are widely utilized for the analysis and design of state-of-the-art plasma-based devices. However, detrimental artifacts present in relativistic PIC simulations exist, called numerical Cherenkov instabilities. These instabilities are caused by grid dispersion and beam aliasing effects. Here, we analyze Numerical Cherenkov instabilities in finite-element-based PIC algorithms on unstructured grids and compare against results from conventional finite-difference-based PIC algorithms on structured grids. We assess the conditions for the onset of numerical Cherenkov instabilities by performing a complete grid dispersion analysis of finite-element field-solvers for various mesh layouts. The analytic predictions are validated against PIC simulations of relativistic plasma beams. We observe that highly irregular meshes alter the beam physics the least among different mesh layouts, preventing coherent spurious wave-particle interactions.