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Cold bubble formation from 2/1 tearing mode during massive gas injection in a tokamak

By Shiyong Zeng, Ping Zhu, Haolong Li and Zhonghe Jiang

Abstract

Massive gas injection (MGI) experiments have been carried out in many tokamaks to study disruption dynamics and mitigation schemes. Two events often observed in those experiments are the excitation of the m = 2,n = 1 magnetohydrodynamic (MHD) mode before the thermal quench (TQ), and the formation of cold bubble structure in temperature distribution during the TQ. Here m is the poloidal mode number, n the toroidal mode number. The physics mechanisms underlying those phenomena, however, have not been entirely clear. Recent NIMROD simulationsofMGIprocessinatokamakhavereproducedmainfeaturesofbothevents, which has allowed us to examine and establish the causal relation between them. In particular, the 3/1 and 2/1 islands are found to form successively after the arrival of impurity cold front at the corresponding q = 3 and q = 2 rational surfaces. At the interface between impurity and plasma, peaked poloidal magnetic perturbation along with a thin current sheet moves inward following the gas cold front. This eventually leads to the formation of an inner 2/1 mode structure in the region between q = 2 and q = 1 surfaces, which has an opposite phase to the dominant 2/1 mode on the q = 2 surface. ItisthroughtheO-pointoftheinner 2/1 modethattheimpurityfrontfurther penetrates inside the q = 1 surface, and enables the formation of a cold bubble at the beginning of TQ. In addition, a 1/1 mode appears inside the q = 1 surface after the impurity penetration, which dominates the subsequent start of current quench (CQ)

Topics: Physics - Plasma Physics
Year: 2020
OAI identifier: oai:arXiv.org:2008.08420

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