Radial displacement of pellet ablation material in tokamaks due to the grad-B effect

During pellet injection in tokamaks, a rapid movement of pellet ablation substance towards the low-field or outward major radius R direction is observed, favoring pellet injection from the high-field side in order to promote deeper fuel penetration. The motion has been attributed to a vertical curvature and ∇B drift current induced inside the ionized ablated material by the 1/R toroidal field variation. The uncompensated vertical drift current inside the weakly diamagnetic (β<0.1) ablation cloud will cause charge separation at the boundary. The resulting electrostatic field induces the E×B drift to the large-R side of the torus. The calculated fuel penetration depth is consistent with inside launched pellet experiments on the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)]. The dependence of the penetration depth with plasma parameters suggests that low velocity inside launched pellets may provide a unique solution to the refueling problem in larger and hotter machines of the future.