The pellet rocket effect in magnetic confinement fusion plasmas

Abstract

Pellets of frozen material travelling into a magnetically confined fusion plasma are accelerated by the so-called pellet rocket effect. The non-uniform plasma heats the pellet ablation cloud asymmetrically, producing pressure-driven, rocket-like propulsion of the pellet. We present a semi-analytical model of this process by perturbing a spherically symmetric ablation model. Predicted pellet accelerations match experimental estimates in current tokamaks ($\sim 10^5 \;\rm m/s^2$). Projections for ITER high-confinement scenarios ($\sim 10^6 \;\rm m/s^2$) indicate significantly shorter pellet penetration than expected without this effect, which could limit the effectiveness of disruption mitigation.