Importance of Gyrokinetic Exact Landau Collisions in Fusion Plasma Turbulence (PhD Oral-24)

Gyrokinetic simulations are routinely performed to understand and predict magnetic confinement. Previous works have used model collision operators (e.g., Lorentz, Abel, Sugama models) with approximate field-particle terms of unknown accuracy and/or have neglected collisional finite Larmor radius (FLR) effects. This work moves beyond models to implement a gyrokinetic exact linearized Fokker--Planck collision operator for the first time in a gyrokinetic code (the GENE code)\footnote{Q. Pan, D. R. Ernst, P. Crandall, Phys. Plasmas \textbf{27} (2020).}. The conservative and symmetric Landau form\footnote{Q. Pan, D. R. Ernst, Phy. Rev. E \textbf{99} (2019).} preserves the conservation laws and H-theorem. The new exact operator allows the accuracy of collision models to be assessed. Comparison with the recent Sugama model implemented in the same code\footnote{P. Crandall et al., Comput. Phys. Commun. \textbf{255} (2020).} shows significant differences for temperature-gradient-driven trapped electron mode (TEM) turbulence (up to 68\% in fluxes) and zonal flow damping, also for microtearing modes in a JET-ILW pedestal. The difference is parameter-dependent; the two operators closely agree for density-gradient-driven TEM turbulence and some drift-type modes in the JET pedestal.