Application of field-aligned coordinates to a spectral reduced MHD code for simulation of ballooning mode turbulence

To realize simulation of ballooning mode turbulence with high spatial resolution, the field-aligned coordinates are applied to a spectral reduced magnetohydrodynamics (rMHD) code which was originally developed by means of the cylindrical coordinates. A new computational scheme using a poloidal mode number shift is developed employing a spectral method in the field-aligned direction. The novel scheme enables a highly efficient nonlinear rMHD simulation that achieves not only a computational speedup in proportion to the maximum toroidal mode number but also suppresses numerical errors by applying the spectral method. Results of a benchmark test on the resistive ballooning mode instability and its turbulent transition agree well with those obtained by the original code. Improved computational efficiency of the newly developed code enables ones to employ high spatial resolution on the poloidal plane, leading to successful reproduction of the energy cascade process and generation of fine vortex structures.