FLUID SIMULATIONS OF TOKAMAK TURBULENCE IN QUASIBALLOONING COORDINATES

A set of coordinates for simulations in toroidal magnetic geometry, ca lled quasiballooning coordinates, is proposed and implemented. Quasiba llooning coordinates are straight-field-line coordinates in which one of the coordinate directions is as close as possible to that of the ma gnetic field consistent with the near-parallel grid lines meshing exac tly (without interpolation) as they cross the boundaries of the simula tion region. This allows the true periodicity conditions in the toroid al-poloidal plane to be satisfied in a straightforward and seamless wa y, even for sheared magnetic fields. Quasiballooning coordinates are u seful in the simulation of instabilities and turbulence of interest in fusion plasmas since the number of grid cells needed to represent str uctures that are elongated along the magnetic field with a given resol ution is greatly reduced compared with toroidal-poloidal or other nont wisting coordinates. For explicit codes, they allow shorter time steps , and it is anticipated that for particle codes, their use will natura lly minimize the numerical noise. The key details necessary for the im plementation of quasiballooning coordinates, both in finite-difference and pseudospectral fluid codes are presented, and a fluid code has be en written. The advantages of quasiballooning coordinates are demonstr ated by applying this code to turbulence driven by the v(parallel-to)' instability.