Effective ion charge (Z_eff) measurements and impurity behavior in KSTAR

A visible bremsstrahlung detector array diagnostic system has been developed on the Korea Superconducting Tokamak Advanced Research (KSTAR) to view the whole minor radius in a narrow region of the continuum free of spectral lines. The interference filters coupled with photomultiplier tubes have been employed to determine the effective charge Z_eff by using visible bremsstrahlung data during neutral beam injection in the KSTAR plasma. The Z_eff profiles are typically flat for L-mode plasmas and evolve to hollow profiles during the H mode in the KSTAR. A comparison of the visible bremsstrahlung emission based on the calculated Z_eff profiles is consistent with measured values of Z_eff from a visible spectrometer in the core plasma. The electron temperature is measured by X-ray imaging crystal spectrometry, and electron density needed for the analysis is taken by the assumption of parabolic profiles of these parameters. The line of sight averaged local bremsstrahlung emissivity is determined with low uncertainty, and the radial emissivity is obtained by using the Abel inversion technique. In addition, a dependence of effective charge Z_eff on the line-averaged electron density is evaluated, and Z_eff is also determined to observe the effect of boronization.