Observation of the Effect of Energetic Ions on Pellet Ablation in the Large Helical Device

Ablation of a solid hydrogen pellet in hot plasmas was investigated in the Large Helical Device (LHD). The penetration depth of the injected pellets in the experiment was compared to a theoretical model employing ablation due to the heat flux of fast ions as well as thermal electrons. Shallower penetration than that predicted based on the model considering only electrons was observed in LHD plasmas in the presence of highly energetic (up to 180 keV) fast ions due to neutral beam injection (NBI). This discrepancy can be quantified by the contribution of fast ions in terms of the stored energy of fast ions. The ablation model calculation taking into consideration the effect of thermal electrons and fast ions agrees with the experimental results obtained by LHD. Scaling which includes the fast-ion effect on penetration depth in LHD was compared with the wider multi-experiment database (IPADBASE) [L.R. Baylor et al., Nucl. Fusion 37, 445 (1997)].