Simulation of high-Z impurity assimilation along the magnetic field line using a novel coupling of PIC and collision-radiative model

High-Z pellet injection is a standard technique for disruption mitigation in fusion reactors. 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 plasma at time scale much shorter than 1 millisecond; and to (2) spread the radiation uniformly on the first wall with uniform high-Z radiators over a flux surface despite the initially localized ablation of the pellets. Therefore, the emphasis on post-ablation spatial transport of the pellet materials is a critical aspect for successful thermal quench mitigation in a tokamak reactor, which is largely determined by the power flux from the surrounding hot plasmas. Through 1D3V PIC simulations, we have shown that the plasma kinetics plays an essential role in controlling such power flux and hence the impurity assimilation physics. Particularly, the impurities are limited by an expansion front, scaling with a local impurity ion sound speed [endif]-->, into the hot plasma, where [endif]--> is mainly governed by the ambient electron temperature via a collisionless mixing of the hot (from the ambient) and cold (from pellet) electrons. Thus, the radiative power loss could act as a major factor affecting the impurity transport by modifying both [endif]--> and [endif]-->. By integrating a collisional-radiative model into the PIC code, the impact of radiative cooling by high-Z impurities on its assimilation is revealed.