Spatial Structure of Volume Recombination in JT-60U Detached Divertor Plasmas

Two-dimensional distributions of Dα, Dβ, Dγ and Dδ are reconstructed from line-integral measurements. The two-dimensional emission ratio of Dδ to Dγ indicates that the volume recombination occurs throughout the inner divertor plasma. In particular, at an emission peak above the inner strike point, it is shown that the population density of high n levels (n = 4, 5 and 6) is determined by the electron-ion volume recombination with an electron temperature and density of 0.2-0.5 eV and 1 × 1020 m-3 , respectively, and that the population density of the n=3 level is not explained by the recombining plasma component with or without the ionizing plasma component.