Study of plasma shaping effects on ITG instability using global gyrokinetic code GKNET with analytical magnetic equilibrium

We extended the global gyrokinetic code GKNET (GyroKinetic Numerical Experiment of Tokamak) to non-circular shaped plasmas with analytical magnetic equilibria, which satisfy the Grad-Shafranov equation up to the second order with respect to aspect ratio. The extended version allows us to set the equilibria with non-unity elongation and non-zero triangularity, where the finite Shafranov shift is consistently determined. The allocated mesh follows the magnetic field line with periodic boundary conditions along the poloidal and toroidal directions so that the calculation cost for solving the gyrokinetic quasi-neutrality condition can be reduced by utilizing 1D FFT and MPI_–ALLtoALL transpose technique. Based on the developed code, we studied the effect of elongation and triangularity on linear Ion Temperature Gradient (ITG) instability with adiabatic electrons in a non-circular shaped Tokamak by paying attention to the symmetry breaking of mode structure due to global profile effects. It is found that elongation reduces ITG instability owing to the effective reduction of flux-surface averaged ion temperature gradient and increases the asymmetry characterized by the Bloch angle θ_b. On the other hand, when elongation is approximately unity, triangularity weakly affects the growth rate, while negative triangularity stabilizes ITG modes and increases the asymmetry in a large elongation regime. (author)