Effects of asymmetries in computations of forced vertical displacement events

C R Sovinec,K J Bunkers

doi:10.1088/1361-6587/aaf124

C R Sovinec, K J Bunkers

Open Access

https://doi.org/10.1088/1361-6587/aaf124

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Abstract

Visco-resistive magnetohydrodynamic (MHD) computations with the NIMROD code (Sovinec C R et al 2004 J. Comput. Phys. 195 355) are applied to a model tokamak configuration that is subjected to induced vertical displacement. The modeling includes anisotropic thermal conduction within an evolving magnetic topology, and parameters separate the Alfvénic, resistive-wall, and plasma-resistive timescales. Contact with the wall leads to increasingly pervasive kink and tearing dynamics. The computed 3D evolution reproduces distinct thermal-quench and current-quench timescales, a positive bump in plasma current, and net horizontal forcing on the resistive wall. The MHD dynamo effect electric field, is analyzed for understanding the nonlinear effects of the fluctuations on the spreading of parallel current density and the resulting bump in plasma current. Forces on the resistive wall are consistent with Pustovitov’s analysis (Pustovitov V D 2015 Nucl. Fusion 55 113032); the plasma remains in approximate force-balance with the wall, so net force is accurately computed from integrating stress over the wall’s outer surface. Improvements to the modeling that are needed for predictive simulation of asymmetric vertical displacement events are discussed.

Highlights

Discharge-terminating disruptive events in tokamaks take many forms and involve a variety of plasma dynamics [1, 2]
Asymmetries that develop during vertical displacement events (VDEs) lead to horizontal forces on electrically conducting structures [3,4,5,6], and the forcing may rotate at rates that are comparable to mechanical harmonics [7]
Our analysis examines the force on the resistive wall during the current quench (CQ) and the spreading of parallel current density, which transiently raises the plasma current starting at the thermal quench (TQ)

Summary

Introduction

Discharge-terminating disruptive events in tokamaks take many forms and involve a variety of plasma dynamics [1, 2]. Whether it is a root cause or a consequence of other disruptive activity, vertical displacement poses a significant risk, because it brings hot plasma in contact with surfaces that are not designed for extreme thermal loading. We report on nonlinear visco-resistive magnetohydrodynamic (MHD) computations of vertical displacement in a model configuration and on the consequences of significant asymmetry that develops through contact with the wall. Our analysis examines the force on the resistive wall during the current quench (CQ) and the spreading of parallel current density, which transiently raises the plasma current starting at the thermal quench (TQ)

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