Collisionless transport mechanisms for thermal quench in stochastic magnetic fields open at the wall boundary *

The break of closed magnetic surfaces due to locked modes can result in a very broad stochastic layer and lead to a rapid loss of energy confinement in tokamaks [1]. Here we report on a first-principles-based simulation study of plasma transport in stochastic magnetic fields based on a global gyrokinetic model to address some specific challenges of thermal quench in tokamak disruption [2]. Even though magnetic field lines become stochastic and open at the wall, the majority of electrons were found to be confined in the system due to trapping by the magnetic mirror force and positive electrostatic potential developed in the stochastic layer. In this study, we present a comprehensive understanding of the dynamics of passing and trapped electrons and the 3-D topology of the stochastic layer, taking into account the consistent coupling of electron and ion dynamics through the ambipolar electric field. The 3-D ambipolar potential builds up in the stochastic layer to maintain quasi-neutrality in the plasma during the thermal quench. The associated ExB vortices mix particles across the stochastic open field lines, providing a collisionless detrapping of electrons that plays a major role in the loss of high-energy electrons along favorable open field lines. In addition, the 3-D electric field also drives a significant perpendicular transport, directly contributing to the thermal quench. As a result, the electron temperature decreases steadily in the typical thermal quench time scale of milliseconds.