Global gyrokinetic simulation code utilizing field-aligned coordinates for studying electromagnetic turbulence in tokamak plasmas

The global electromagnetic gyrokinetic simulation code, GKNET, is extended by incorporating magnetic-field-aligned coordinates. This development significantly reduces computational costs, thereby enabling the investigation of microturbulence in parameter regimes that include the actual ion-to-electron mass ratio and larger machine sizes. The updated code facilitates the study of microturbulence in numerically computed MHD equilibria of tokamak plasmas. To validate the GKNET code, we perform a comparative analysis of the linear growth of microturbulence against the results from other global gyrokinetic simulation codes, specifically for the cyclone-base-case tokamak parameters. The results from these codes on the normalized pressure beta dependence of the growth rate and real frequency of the ion-temperature-gradient modes and kinetic ballooning modes are in good agreement. Nonlinear simulations show that the ion heat diffusivity, attributed to ion-temperature-gradient turbulence, decreases as beta increases, owing to the electromagnetic stabilizing effects on the ion temperature gradient mode. While the reduction rate of the ion heat diffusivity agrees with the prediction by the quasilinear mixing-length theory, its magnitude is less than half of the prediction. On the other hand, the electron heat diffusivity shows little change with increasing beta.