7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Thermal transport induced by stochastic magnetic fields during fast thermalquench in tokamak plasmas
Shengyang Xiao1 and Lu Wang1
1
Institute of Fusion and Plasma Research, Huazhong University of Science and Technology
Wuhan, Chinae-mail : m202172026@hust.edu.cn

The timescale of the thermal quench (TQ) stage ofthe tokamak disruption is always the focus of attention.
Generally the whole TQ stage can be further divided into
the initial 1-2 delay stage and the final fast quench stage,in which a large flattening of the temperature profile isfollowed by a decrease of the flattened profile[1]. It is
widely recognized that the TQ timescale is closely
related to the stochastic magnetic fields induced heatdiffusion, but there are still some open issues to beelucidated. On the one hand, from the statistical result of
the IDDB (ITER disruption database), the 1-2 delay time 12 and the fast quench time  2 are roughly scaling
with the minor radius a as  12  a1.5 and  2  a1 ,respectively[2]. The timescale of non-linear interactionsof 1/1, 3/2 and 2/1 magnetic islands agrees well with the1-2 delay time[3], while the scaling of the fast quench
time is not fully understood yet. On the other hand, therange of plasma temperature involving in the fast TQ
stage is quite large, mainly because: (a) The core electron
temperature can reach the order of several keV before theonset of fast TQ, while the temperature at the boundaryis only at hundreds of eV or less; (b) After thermal
collapse, the plasma temperature can drop by one or twoorders of magnitude. Therefore, the thermal transport
process involves multiple collisional regimes[4], which
requires a heat diffusivity expression valid in a wide
collisional parameter range for analyzing.
In this work, we focus on the timescale of fast TQ
in tokamak locked mode disruptions in order tounderstand the physical mechanism of fast thermaltransport during this stage. A general expression ofelectron thermal diffusivity has been obtained byconnection the thermal diffusivity induced by stochastic
magnetic fields in multiple collisional regimes, which
can be applicable to a wide range of collisionalparameters. The dependence of this general diffusivity onelectron temperature, density and plasmas size has been
analyzed, which provides a possible explanation for thelinear scaling of the fast TQ timescale in the plasma
minor radius. With different tokamak parameters, thecharacteristic timescales of fast TQ are quantitatively
calculated based on the general electron heat diffusivity.
It has been found that the electron temperature profile
can be rapidly collapsed in less than 1 ms in the initialstage of fast TQ. However, the fast thermal transportcannot be maintained due to flattening of the electron
temperature profile. This may indicate that other fasttransport mechanisms, such as heat convection, nonlocal
transport, and so on are necessary to maintain the fast TQ,
which are left for future work.
References[1] V. Riccardo, A. Loarte, the JET EFDA Contributors,
Nucl. Fusion 45, 1427 (2005)[2] ITER Physics Expert Group on Disruptions, PlasmaControl, and MHD, ITER Physics Basis Editors, Nucl.
Fusion 39, 2251 (1999)[3] F. C. Schuller, Plasma Phys. Control. Fusion 37,
A135 (1995)[4] J. A. Krommes, C. Oberman, R. G. Kleva, J. Plasma
Phys. 30, 11 (1983)