Magnetohydrodynamic features and particle transport during thermal quench in HL-2A with density source

Abstract

Diverse disruption mitigation strategies based on massive material injection have been developed in recent decades, aiming to uniformly deplete the thermal energy stored within plasmas during the thermal quench (TQ) while simultaneously elevating electron density to facilitate runaway electron suppression. Irrespective of the detailed dynamics of the material delivery scheme, deposition location and subsequent density mixing are pivotal in achieving highly efficient mitigation, however, which are markedly influenced by magnetohydrodynamic (MHD) activities. In order to assess the influence of MHD-induced transport on disruption mitigation, a simulation of TQ triggered by pure deuterium (D) deposition is conducted using a three-dimensional (3D) nonlinear reduced MHD code, JOREK. Steady density sources (the deposition rate of 1024 D atoms per second is greater than in real experiments) are introduced at various locations to explore the dynamics. The findings distinctly reveal two types of TQ processes, contingent on locations of deposition (LoD) of the neutral D source. Evidently, the results underscore the effectiveness of proper density mixing and moderated MHD in disruption mitigation. Nonlinear mode coupling emerges as a significant factor in shaping the final outcomes of TQ. Specifically, the 5/2 mode contributes to edge collapse, whereas the 3/2 mode is instrumental in core collapse. Additionally, the investigation indicates that the rise in core density is contingent on LoD, exhibiting threshold behavior. This threshold is observed within the q = 2/1 surface of equilibria, and a rapid increase in core density is witnessed when the density source crosses this threshold. The outcomes point toward the important role of E × B convection due to the 1/1 mode evolution in the process.