Shell Pellet Experiments on DIII-D

Injection of hollow shell pellets filled with a dispersive payload is a possible method for rapid shutdown of future large tokamaks to avoid wall damage during disruptions. Preliminary shell pellet experiments have been performed in the DIII-D tokamak by firing small (OD $\sim 2$ mm) polystyrene shells filled with either pressurized (10 atm) argon gas or with boron powder into quiescent discharges. Pellet slowing from 350 m/s down to 100 m/s was observed, which is not well-understood at present. Using the measured pellet velocity, the observed pellet burn up at $r/a \sim 0.5$, appears consistent with ablation rate calculations. Successful delivery and rapid ($<15~{\rm ms}$) dispersal of the pellet payloads into the plasma core was observed. Negligible plasma current contraction or MHD onset were seen as a result of the shell burn up in the plasma edge, consistent with calculations. Planned experiments with large (OD $\sim 1$ cm) shell pellets will also be discussed.