On the origin of the plasma current spike during a tokamak disruption and its relation with magnetic stochasticity

Abstract

A JOREK 3D non-linear MHD simulation of a disruption triggered by an argon massive gas injection in JET, which quantitatively reproduces the plasma current ( $I_\mathrm p$ ) spike (Nardon et al 2021 Plasma Phys. Control. Fusion 63 115006), is analyzed in order to investigate the origin of the $I_\mathrm p$ spike and its relation with magnetic stochasticity. The $I_\mathrm p$ spike is associated to a current density ( j _φ ) profile relaxation which appears to result from Shear Alfvén Wave (SAW) propagation along stochastic field lines, as proposed by Boozer (2019 Plasma Phys. Control. Fusion 61 024002; 2020 Phys. Plasmas 27 102305), possibly complemented by a macroscopic E×B flow structure. Using axisymmetric JOREK simulations involving a mean field Ohm’s law, we verify that the level of hyper-resistivity associated to SAWs is consistent with the prediction made in (Boozer 2019 Plasma Phys. Control. Fusion 61 024002; Boozer 2020 Phys. Plasmas 27 102305), which connects the $I_\mathrm p$ spike with the level of stochasticity. The relaxation comprises two main phases, the first one corresponding to a fast (0.1 ms) and almost complete j _φ flattening in the q < 2 region, while the second one is longer (0.5 ms) and corresponds to a more gradual, global and incomplete j _φ flattening. During the first phase, strong E×B flows develop that play a key role in mixing impurities into the core.

Keywords

• disruption
• 3D non-linear MHD simulations
• MHD relaxation