Qualitative improvement of Z_eff diagnostic based on visible bremsstrahlung profile measurement by changing observation cross section in LHD

Full vertical profiles of visible bremsstrahlung continuum have been measured for diagnostics of effective ion charge, Z_eff, with parallel optical fiber arrays at horizontally elongated plasma cross section in the Large Helical Device (LHD). Measured bremsstrahlung profiles were asymmetric due to non-uniform bremsstrahlung emissions located in the stochastic magnetic field layer. Then, the Z_eff profile analysis has been done only for a magnetic configuration with relatively thin stochastic magnetic field layer, i.e. for plasmas at magnetic axis position of R_ax = 3.60 m. The effect of the non-uniform bremsstrahlung emission disappears in the lower half profile at R_ax = 3.60 m, while the Z_eff profile analysis is extremely difficult in other magnetic configurations due to the presence of the non-uniform bremsstrahlung emission over the entire vertical profile from top to bottom. To improve the difficult situation in the Z_eff diagnostic the fan array optical fiber system was newly installed at vertically elongated plasma cross section for full horizontal profile measurement. It is found that the non-uniform bremsstrahlung emissions disappear from the signal and the observed bremsstrahlung profile is almost symmetric between inboard and outboard radial profiles. The analysis on the Z_eff profile has been carried out based on the plasma equilibrium database in LHD, TSMAP. A preliminary result is obtained in high-density discharges, which shows a flat Z_eff profile and values of Z_eff ∼ 1. (author)