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Simulations of sawtooth instabilities in ASDEX Upgrade using the 3D nonlinear two-fluid MHD code M3D-C1

Author: Isabel Krebs
Requested Type: Poster Only
Submitted: 2015-01-19 14:31:03

Co-authors: S.C.Jardin, D.Meshcheriakov, S.Günter, V.Igochine, N.Ferraro, M.Hoelzl

Contact Info:
Max-Planck/Princeton Center for Plasma Physics
100 Stellarator Road
Princeton, NJ   08540
USA

Abstract Text:
We use the high-order finite element code M3D-C1 for 3D nonlinear MHD simulations of sawtooth instabilities in toroidal geometry. The resistive as well as two-fluid MHD simulations are based on equilibrium reconstructions and parameters of a typical sawtoothing ASDEX Upgrade tokamak discharge. Our studies are focused on the sawtooth reconnection process aiming at the comparison of the simulations results to features of the experimental observations, like the sawtooth crash time, the 3D temperature evolution and the evolution of the safety factor profile during the crash and the occurrence of (m=1,n=1) post-cursor modes.
Nonlinear simulation results of a sawtooth crash depend critically upon the initial conditions used in the simulation code, in particular the form of the pressure profile p(Psi) and the safety factor profile q(Psi). Our goal is to initialize the configuration with an equilibrium that is marginally stable. To this end, we have performed an extensive study of the linear stability of ASDEX-U-like equilibrium with a range of profiles. In resistive MHD, the (1,1) mode is always unstable when the central safety factor q(0) falls below 1. The configuration can also be unstable to an interchange-like mode when q(0) is just above 1 if the shear, dq/dPsi, is sufficiently low, even at low pressure. When q(0) is less than one and the shear is sufficiently large, at low enough pressure the unstable mode growth rate is small and depends strongly on the inverse of the value of the resistivity. This regime can be stabilized by sheared rotation and possibly certain two-fluid as well as kinetic effects. As the pressure increases, an ideal stability boundary is crossed at a critical pressure, dependent on the q profile, and the growth rate increases substantially to a value almost independent of the resistivity. We initialize the nonlinear calculation from experimentally measured values constrained to be near this critical point.

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