Rapid assimilation of high-Z impurities along the magnetic field line from an ablated pellet

Pellet injection is a standard technique for fueling and disruption mitigation in fusion reactors. Although the nominal goals of these two applications are similar, namely to deliver materials into the fusion core, there are important distinctions on the specifics. Particularly, for thermal quench mitigation of a tokamak disruption, the primary aim is to (1) replace plasma power exhaust at the first wall with line radiation by high-Z impurities, which requires substantial amount of high-Z impurities to be assimilated into the core plasma on time scale much shorter than 1 millisecond; and to (2) spread the radiation as uniformly as possible on the first wall, which requires rapid spatial transport and homogenization of high-Z radiators over a flux surface despite the initially localized deposition of the ablated pellet cloud. The emphasis on post-ablation spatial transport of the pellet materials is a critical aspect for successful thermal quench mitigation in a tokamak reactor. Here we employ the first-principles kinetic simulations to study the impurity assimilation into the ambient plasma along the magnetic field line. We find that impurity assimilation comes in the form of propagating fronts at the impurity ion sound speed, which is related to the ratio of the averaged charge of impurity ions and impurity ion mass. Furthermore, the impurity front speed is mostly set by the ambient plasma temperature, and has very weak dependence on the temperature of the ablated pellet cloud. As a result, a lower ambient plasma temperature implies poorer impurity assimilation and stronger radiation asymmetry.