Over-ablation and deflection of hydrogen pellets injected into neutral beam injection heated plasmas in the Large Helical Device

摘要

During hydrogen pellet injection experiments in the Large Helical Device (LHD), over-ablation caused by fast ions (due to tangential neutral beam injection (NBI) heating at 150180keV beam energy) is shown to dominate the measured penetration depths. The neutral gas and plasma shielding model, including the interaction of the pellet with fast ions, is applied, and its predictions are shown to agree well with the measured H _α emission profiles. The toroidal deflection of the pellet trajectories observed in the direction of beam injection is reproduced by the model when unbalanced ablation on the two sides of the pellet is included. The attenuation of the fast-ion and electron heat fluxes entering the ablation cloud is examined, showing that the high-energy part of the fast-ion distribution function is responsible for the high ablation rates, and that the thermal electron population is not the dominant ablating species. An analytical scaling for the balanced NBI condition is derived, which reasonably reproduces the measured penetration depths included in the LHD database. In the LHD, for given pellet parameters and beam energy, the fast-ion density is shown to be the major parameter affecting the experimental penetration.