Pellet assimilation and impurity transport in fusion plasma

Pellet injection is a standard technique for fueling and disruption mitigation in tokamak reactors. For thermal quench mitigation in a tokamak disruption, high-Z impurities replace plasma heat flux at the divertor/first wall with volumetric line radiations. This requires large quantity of high-Z impurities to be assimilated into the plasma on a time scale much shorter than the thermal quench duration. Spatial uniformity of the radiation load on the first wall also requires rapid spatial transport and homogenization of high-Z radiators over a flux surface despite the initially local deposition of pellet material. Here we employ the first-principles kinetic simulations to investigate the physics of pellet assimilation and the spatial transport and mix of high-Z impurities in a hydrogen plasma upon pellet assimilation. We find that the kinetic instabilities and collisions play important roles in pellet assimilation, where the tail high-energy electrons can be critical. Another finding is that the assimilated impurities will spread along the field line via propagating fronts, the fastest of which will be the fully ionized impurities that follow the cooling front in the thermal quench [1].

[1] Y. Zhang, J. Li and X-Z Tang, cooling flow regime of plasma thermal quench, submitted.