The behaviour of neutral particles around the ergodic layer has been investigated by H α spectral profile measurements by means of a polarization-separation technique. Emission locations, atomic temperatures and velocity components along the lines of sight at the inner and outer peripheral regions are determined for a large helical device plasma with a magnetic field axis of R ax = 3.80 m. A high intensity of H α emissions is localized around the ergodic layer.

Superdense core plasmas with a highly peaked electron density profile have been obtained in reduced recycling discharges in the Large Helical Device ͓O. Motojima et al., Phys. Plasmas 6, 1843 ͑1999͔͒. The polarization separation spectroscopy technique has been applied to characterize H␣ emissions in the peripheral region of superdense core plasmas. Fitting positions of the high intensity H␣ line are located just outside of the last closed flux surface in the local island divertor configuration. In the helical divertor configuration, high intensity emissions are observed around the inner and outer X points and along the divertor legs. In both configurations the hydrogen atoms have an inward velocity along the lines of sight.

The behavior of hydrogen neutral particles in and around the ergodic layer of Large Helical Device plasmas has been investigated through Hα emission spectral line profiles using plasma polarization spectroscopy (PPS). The PPS technique enables us to quantitatively evaluate emission locations, atomic temperatures and velocity components along the line-of-sight (LOS) for both inner and outer peripheral regions. The emission locations and the LOS components of atomic velocities are determined by varying the magnetic field axis R ax from 3.60 m to 4.00 m, shot by shot. The high intensity region of Hα emissions is localized in the inner ergodic layer for the inward configuration. With an increase in R ax , the high intensity region of Hα emission moves outward.