Extension of Global Gyrokinetic Code for Tokamak Edge Turbulence Simulation

A field-aligned coordinate system using a shifted metric technique has been implemented to a global gyrokinetic code GKNET. This coordinate system allows for simulations with lower resolution in the direction along the magnetic field line, which is especially effective in the outer core regions with higher q values. Realistic tokamak geometries, including up-down asymmetric equilibria, have also been implemented by using a newly developed interface code that connects GKNET and a free-boundary 2D Grad-Shafranov equation solver. This is essential for tokamak edge simulations, which will be developed in the future. As an application, the nonlinear simulation of the ion temperature gradient (ITG) mode with the JT-60SA ITER-like plasma [M. Nakata et al., Plasma Fusion Res. 9, 1403029 (2014)] has been performed. The result shows that the linear ITG instability with high poloidal modes and resultant zonal flow generation are properly traced. In this case, it is estimated that the number of computational grids can be reduced to 1/94 compared to that of the flux surface coordinate system.