Author: Kyle J Bunkers
Requested Type: Poster
Submitted: 2016-02-14 17:23:27
Co-authors: C.R. Sovinec
University of Wisconsin-Madison
1500 Engineering Dr
Madison, WI 53706
Disruptions associated with vertical displacement events (VDEs) have potential for causing considerable physical damage to ITER and other tokamak experiments. For example, VDEs can create large forces and heat loads on the confinement vessel, in addition to creating runaway electrons. VDEs in disruptions often include non-axisymmetric activity. This activity includes external kink modes, which are driven unstable as contact with the wall eats into the q-profile. In order to understand the effects of disruption, external kinks must be accurately modeled and understood. The NIMROD code [Sovinec, et al., JCP 195, 335] is being applied to study external-kink-unstable tokamak profiles in toroidal and cylindrical geometries. Simulations with external kinks show the plasma swallowing a vacuum bubble, similar to Ref. [Rosenbluth, Monticello, Strauss and White, Phys. Fluids 19, 1987]. NIMROD reproduces external kinks in both geometries, but it runs into an expected challenge as the 3D plasma-vacuum boundary distortion grows. A large number of Fourier components is needed to avoid Gibbs-like phenomena. In under-resolved cases, the time step also becomes restrictively short. When Spitzer resistivity is used, the resistivity changes rapidly as a function of toroidal angle, due to plasma-vacuum boundaries. A Fourier representation of resistivity can lead to negative values, which makes the system ill-posed. Here, we report on progress and tests with various methods to improve the representation in order to allow longer time steps, enabling the simulation of the dynamics of external kink well into the nonlinear phase.
*This effort is supported by the U.S. Dept. of Energy, award numbers DE-FG02-06ER54850 and DE-FC02-08ER54975.