Hydrogen ice pellets have been injected using two different location configurations, namely outer port injection and coil side injection, in the large helical device. The behaviour of the pellet ablation has been observed using a fast camera, which possesses high spatial and time resolution. Striking toroidal deflection of the pellet trajectory is observed. The deflection is in the direction of tangential neutral beam injection. The toroidal velocity ultimately reaches 1000 m s−1 or more. The possibility of a rocket effect due to a unilateral ablation by the fast ions is discussed. The effective penetration depth of the pellet, which is measured by images, agrees with the prediction from the neutral-gas-shielding model. The penetration depth is compared with the measured deposition profile of the pellet.
Pellet ablation theory and experiments