Observation of three-dimensional motion of the pellet ablatant in the Large Helical Device

Application of two-point stereoscopic diagnostics using a fast camera and bundled fibre has enabled the observation of the three-dimensional nature of the pellet trajectory in the Large Helical Device. It has been observed that the pellet trajectory deviates from its injection direction, toroidally and vertically depending on the direction of the tangentially applied neutral beams. The magnitude of the toroidal deviation is similar in the clockwise as well as counter-clockwise neutral beam directions and is of 15–20 cm with a deflection speed of up to 400 m s−1. In contrast, an asymmetry in trajectory deflection has been observed in the vertical direction. Experimental results also indicate that the starting radius of the pellet trajectory bending in the counter-clockwise neutral beam case is more inward to that of the plasma. Collectively this leads to less penetration of the pellet inside the plasma and is prominent in the case of the clockwise neutral beam. Additionally, this fact supports evidence that the fast ion plays an important role in the pellet ablation process in the Large Helical Device. The pellet deflection is explained by the rocket effect due to unilateral ablation by the fast ions. The possible cause of the difference in the vertical deflection is explained by considering the geometrical aspects of the magnetic field structure.