Effect of Halo Neutrals on Fast-Ion Charge Exchange Spectroscopy Measurements in LHD

Abstract

A fast-ion charge exchange spectroscopy (FICXS) diagnostic was recently installed on the Large Helical Device (LHD). The aim of this diagnostic is to evaluate fast-ion confinement property by measuring energy-resolved fast-ion spatial profiles in LHD. To accurately evaluate the FICXS profile, the effect of halo neutrals, which are created by the charge exchange between bulk ions and injected neutral beams (NBs), must be treated properly in the current analysis of the FICXS spectra in the LHD. We have estimated the effect of halo neutrals on the measurement from the shapes of Doppler-shifted H-alpha spectra. The ratio of halo neutrals to the injected NBs on the lines of sight (LOSs) is estimated by using the EIRENE-3D Monte Carlo code. The results of these two methods agreed well. This indicates that the effect of halo neutrals should be considered when interpreting the FICXS diagnostic data as well as that from the injected NB, when the probing NB is radially injected and the emitted photons are observed from perpendicular LOSs.