  The fragment ablation rate plays significant roles in the mitigation
efficiency of Shattered Pellet Injection (SPI) as a Disruption Mitigation
System (DMS). Current mainstream 3D MHD codes modelling SPIs mostly assume
instantaneous thermalization between the previously hot ambient electrons and
the newly released cold electrons, which results in underestimation of the
ablation rate if the hot electron thermalization time is comparable or even
longer than the fragment flying time. To resolve this doubt, we hereby
investigate the thermalization dynamics and the overall hot-electron impact.
The finite-time collisional thermalization of hot-tail electrons in a rapidly
cooling plasma, as well as the so-called ``self-limiting'' effect are
considered. The former effect tends to deplete the colder population within a
hot-tail species, while the latter is found to preferentially deplete the
higher energy population. The combined result is found to cause an almost
self-similar decay of the hot electron distribution function, while its shape
does not deviate much from that of Maxwellian distribution and the mean energy
does not change much during the thermalization process. Based on this
observation, axisymmetric JOREK D2 SPI simulations were carried out with
additional hot-tail contribution to evaluate their overall impact onto the
injection assimilation and penetration. It is found that the hot-tail effect
indeed causes enhanced assimilation and shallower penetration, although the
overall effect depends on the exact injection configuration, with the slow
injection showing negligible hot-tail effect while the fast single
non-shattered pellet case shows drastic hot-tail ablation enhancement. For
ITER-like SPI parameters, there is no significant deviation in the total
assimilation, but some deviation in the injection penetration is observed for
the fast injection velocity cases.
