Am. Dimits, FLUID SIMULATIONS OF TOKAMAK TURBULENCE IN QUASIBALLOONING COORDINATES, Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 48(5), 1993, pp. 4070-4079
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.