Force-free plasmas are a good approximation where the plasma pressure is tiny
compared with the magnetic pressure, which is the case during the cold vertical
displacement event (VDE) of a major disruption in a tokamak. On time scales
long compared with the transit time of Alfven waves, the evolution of a
force-free plasma is most efficiently described by the quasi-static
magnetohydrodynamic (MHD) model, which ignores the plasma inertia. Here we
consider a regularized quasi-static MHD model for force-free plasmas in tokamak
disruptions and propose a mimetic finite difference (MFD) algorithm. The full
geometry of an ITER-like tokamak reactor is treated, with a blanket module
region, a vacuum vessel region, and the plasma region. Specifically, we develop
a parallel, fully implicit, and scalable MFD solver based on PETSc and its
DMStag data structure for the discretization of the five-field quasi-static
perpendicular plasma dynamics model on a 3D structured mesh. The MFD spatial
discretization is coupled with a fully implicit DIRK scheme. The algorithm
exactly preserves the divergence-free condition of the magnetic field under the
resistive Ohm's law. The preconditioner employed is a four-level fieldsplit
preconditioner, which is created by combining separate preconditioners for
individual fields, that calls multigrid or direct solvers for sub-blocks or
exact factorization on the separate fields. The numerical results confirm the
divergence-free constraint is strongly satisfied and demonstrate the
performance of the fieldsplit preconditioner and overall algorithm. The
simulation of ITER VDE cases over the actual plasma current diffusion time is
also presented.Comment: 43 page