Direct Numerical Simulation of Rocket Acceleration of Pellets and SPI Fragments in Tokamaks

Direct numerical simulations of the rocket acceleration of pellets have been performed using PELOTON, a 3D Lagrangian particle pellet code [R. Samulyak et al, Nuclear Fusion 61 (4), 046007 (2021)]. The pellet rocket acceleration is driven by grad-B drift of the ablation cloud that creates asymmetry and non-uniform heating of the cloud. As a result, the increased pressure on the HFS compared to the LFS leads to pellet (fragment) acceleration. To enable the resolution of rocket acceleration, an improved pellet cloud charging model that computes a spatially varying electrostatic sheath potential of the cloud has been implemented in PELOTON. Numerical simulations of the pellet rocket acceleration have been validated using experimental data from ASDEX upgrade and JET tokamaks and applied to the study of pellets injected into ITER from HFS. Simulations predict a significant rocket acceleration of hydrogen pellets in the ITER pedestal, affecting their trajectories. We show that asymmetric properties of hydrogen and neon clouds are similar, leading to similar rocket acceleration forces despite much slower grad-B drift in neon clouds. The reduced rocket acceleration of neon pellet is caused mainly by their larger mass and not by smaller grad-B drift velocities.