JOREK simulation of heat load asymmetry during unmitigated and mitigated thermal quenches in EXL-50U

In this study, the 3D nonlinear JOREK code is utilized to conduct a pair of thermal quench (TQ) simulations for the Spherical Tokamak (ST) EXL-50U. As a comparison, the divertor and the first wall heat flux and the temperature rise during the TQ with and without Massive Gas Injection (MGI) are investigated. The main focus lies on the following aspects: the heat load and its asymmetry on the Plasma Facing Components (PFCs), the seed runaway electron (RE) transport via the stochastic field lines during the TQ, and the radiated energy fraction for the MGI case. The heat flux deposition shows notable inner-outer asymmetry as well as toroidal asymmetry during the TQ. It is found that the heat load on the outer (low field side) divertor is significantly stronger than that on the inner (high field side) one, which distinguishes its transient disruption behavior from conventional tokamaks but is consistent with earlier MAST device experimental observations of thermal quenches. Poincar'e plot of stochastic magnetic field lines and Particle Tracing Codes (PTC) on hot electrons further confirms this geometrically driven trend, showing that field line ``fingers'' in the open field line region are far more prone to terminate on the outboard side. Concurrently, the heat flux toroidal asymmetry on the divertors corresponds to the toroidal mode number of the core-dominant modes, as is anticipated. As a supplement, the test particle simulation of the seed RE and the hot electron also exhibits a helical asymmetry in their deposition. As for the PFCs temperature rise, although its maximum value may approach the energy impact threshold of stainless steel and CFC in the unmitigated case, the heat load could be remarkably alleviated by MGI with over 95\% radiated energy fraction. It is also found that such temperature rise is ``smoothed out'' by movement of the strike points over time in both cases. The temperature rise stays well below the tungsten melting limit across all PFCs for both cases.