Implementation of field-aligned coordinates in a semi-Lagrangian gyrokinetic code for tokamak turbulence simulation

Field-aligned coordinates have been implemented in the gyrokinetic semi-Lagrangian code NLT, Ye et al (2016 J. Comput. Phys. 316 180), to improve the computational efficiency for the numerical simulations of tokamak turbulence and transport. 4D B-spline interpolation in field-aligned coordinates is applied to solve the gyrokinetic Vlasov equation. A fast iterative algorithm is proposed for efficiently solving the quasi-neutrality equation. A pseudo transform method is used for the numerical integration of the gyro-average operator for perturbations with a high toroidal mode number. The new method is shown to result in an improved code performance for reaching a given accuracy. Some numerical tests are presented to illustrate the new methods.