Collisionless cooling of $T_{eperp}$ in a tokamak thermal quench

Thermal quench (TQ) marks the point of no return in a tokamak disruption. It not only brings a thermal load management issue at the divertor plates and first wall, but also determines the runaway seeding for the subsequent current quench. There are two ways to trigger a TQ, one is the globally stochastic magnetic field lines that connect the hot core plasma to the cold boundary, while the other is high-Z impurity pellet injection. In both situations, a nearly collisionless magnetized plasma is made to intercept a radiative cooling mass (RCM), being that an ablated pellet or a vapor-shielded wall. JET, DIII-D and EAST data have shown a wide range of TQ time with and without high-Z pellet injection. Our previous results have shown that the TQ due to a localized RCM is dominated by convective energy transport as opposed to conductive energy transport, and as a result, the $T_{eparallel}$ cooling comes in the form of four propagating fronts with distinct characteristic speeds, turning the core $T_{eparallel}$ cooling into four different stages, with major cooling of $T_{eparallel}$ in the collisionless stage. However, due to the lack of collisions in the collisionless stage, the fast cooling of the perpendicular electron temperature $T_{eperp}$, is yet a mystery. Here, we will briefly discuss the underlying physics of these propagating fronts and the staged cooling of electrons, with a focus on the collisionless $T_{eperp}$ cooling. We will show that the self-excited whistler modes play a crucial role in collisionlessly cooling $T_{eperp}$.