The timescale of thermal quench during disruptions in EAST

Since 2015, the timescale of thermal quench (TQ) during disruptions on the EAST tokamak has been examined using electron cyclotron emission and soft x-ray diagnostic systems. The database includes both major disruptions (MDs) and hot vertical displacement events (VDEs), where the TQ duration of the former is within 56-788 & mu;s, and the latter is approximately within 100-3000 & mu;s. In particular, for MDs, the lower bound of TQ duration, indicating the minimum durations at different current plateaus, decreases as the plasma current increases. This decrease is due to the connection length shortening and the plasma temperature increasing. For MDs, two typical TQ processes, single-stage TQ and double-stage TQ, are characterized by different magnetic perturbations. In single-stage TQ, a fast-loss stage is triggered by magnetic perturbation exceeding 4.3 x 10(-3) T with a fast growth rate of 1.5 x 10(-2) & mu;s(-1). In contrast, fast quench is triggered by a slightly smaller magnetic perturbation of 3.6 x 10(-3) T in double-stage TQ, and the growth rate 5.3 x 10(-3) & mu;s(-1) is an order of magnitude smaller than single-stage TQ. For hot VDEs, the plasma temperature collapses step by step from the edge to the core, and every progressive collapse corresponds to a magnetic perturbation, whose growth rate is approximately equal to or less than double-stage TQ. The whole process of TQ energy release can be divided into the transport in a stochastic field within the separatrix and in the scrape-off layer and, according to the typical parameters of EAST, TQ duration in MDs is roughly estimated to be 245 & mu;s by an approximate formula, which is consistent with the experimental results.