Characterization of charge-exchanged energetic particle flux observed with an imaging neutral particle analyzer in large helical device

Electron density dependence on the energetic neutrals escaping from the plasma, due to charge-exchange reaction between beam ions and neutrals in the plasma peripheral region, was investigated using an imaging neutral particle analyzer (INPA) on the large helical device. The INPA provides simultaneous spatially and energy-resolved measurements of charge-exchanged energetic neutral flux with temporal resolution and is designed to focus on the behavior of helically trapped beam ions. Neutral beam heated hydrogen plasma experiments were performed by scanning the line-averaged electron density from 1.3×1019 to 3.7×1019 m−3. We observed that the total INPA signal decreases with increasing electron density. This is consistent with the slowing-down behavior of energetic ions as described by the Spitzer slowing-down time. To further understand the phenomenon, a numerical simulation of the INPA signal, based on the orbit-following models combined with line-integrated calculation, was conducted using the experimental data. We qualitatively reproduced the electron density dependence of the INPA signal as observed in the experiments. This work provides the first characterization of the electron-density dependence of charge-exchanged energetic neutral flux measured by an INPA in a stellarator.