May 8-10

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Author: Haotian Mao
Requested Type: Consider for Invited
Submitted: 2023-03-24 15:45:17

Co-authors: Y.Zhang, X.Tang

Contact Info:
Los Alamos National Laboratory
3790 Gold Street Apt 5
Los Alamos, New Mexico   87544
United States of Ame

Abstract Text:
Pellet injection is a standard technique for fueling and disruption mitigation in fusion reactors like ITER. 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 disruption mitigation of tokamak thermal quench, 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 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 local assimilation of the pellets. Therefore, the emphasis on post-assimilation spatial transport and mixing is a critical aspect for successful thermal quench mitigation in a tokamak reactor. Here we employ the first-principles kinetic simulations and analysis to investigate the physics underlying the high-Z impurities assimilation along the magnetic field line from an ablated pellet. We find that, the high-Z impurities’ transport is limited by the cooling front, which, by definition is where the ambient hot ions meet the cold recycled ions, is formed when a fusion-grade plasma intercepts with a cooling spot (cold pellet in our case). Such cooling front among other propagating fronts significantly modify the heat flux to the pellet and hence its ablation. In contrast with a pure hydrogen pellet, it is shown that the high-Z impurities in pellet can diffusively reflect the ambient hot plasmas, which, by replenishing the distribution void, can substantially reduce the heat fluxes and hence the plasma cooling. The high-Z impurities’ assimilated front in the plasma is found to be governed by the averaged charge number.

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