First demonstration of disruption mitigation using shell pellets for core impurity deposition on DIII-D

Experiments injecting boron-filled diamond shell pellets into the DIII-D tokamak provide the first demonstration of disruption mitigation through core impurity deposition. Core impurity injection shows promise to provide “inside-out” radiative cooling of the plasma, as well as high impurity assimilation and global stochastization of the plasma field lines to suppress runaway electron seed formation [1]. The shell pellet technique utilizes a thin, minimally perturbative shell to transport the enclosed radiating impurity (boron dust) to the plasma core before dispersal, delaying the onset of global MHD that is typically initiated by conventional edge-cooling techniques (e.g. massive gas injection). Visible imaging shows the shell ablating gradually until the boron is released near the magnetic axis. 0-D mitigation metrics generally improve with injection velocity, indicative of the importance of deep deposition. Density measurements account for a large fraction of electrons provided by the pellets, indicating high impurity assimilation fraction. Future work and the design of ITER-relevant shells are discussed.

[1] V. A. Izzo and P. B. Parks, Phys. Plasmas 24, 060705 (2017)