Study of neutral hydrogen transport in LHD core plasmas based on high dynamic-range Balmer-α spectroscopy

The radial distributions of the neutral hydrogen atom density and pressure in the large helical device (LHD) and their electron density (n_e) dependence were studied. The atom density distribution was determined from a detailed analysis of the intensity-calibrated Balmer-α line profile while the pressure distribution was obtained with a simple one-dimensional analytical model. We determined for the first time the atom density at the centre of a fusion-oriented plasma, which is more than three orders smaller than that at the edge. On the contrary, the atom pressure changes only by a factor of 10 from the edge to core regions. Both the atom density and pressure in the core region have negative n_e dependence. The smaller drop of the atom pressure profile and the n_e dependence of the atom density were discussed based on the diffusion theory.