Neoclassical transport simulations with an improved model collision operator

Abstract

The improved model collision operator proposed by Sugama et al. [Phys. Plasmas 26, 102108 (2019)], in which the original Sugama collision operator [Phys. Plasmas 16, 112503 (2009)] is extended to recover the friction–flow relation of the linearized Landau (exact) collision operator, is newly implemented in a global full-f gyrokinetic simulation code, GT5D. Neoclassical transport simulations of a single ion species plasma in a circular concentric tokamak are performed over the wide collisionality regime. The improved operator is verified to reproduce the neoclassical thermal diffusivity of Braginskii precisely in the Pfirsch–Schlüter regime, where a friction–flow relation of higher accuracy is required than in the lower collisional regime. In addition, it is found in all collisionality regimes that the overestimation of the neoclassical thermal diffusivity and the magnitude of the parallel flow coefficient observed for the original Sugama operator can be eliminated by using the improved operator, demonstrating that collisional processes described by the exact operator are correctly retained in the improved operator.