Z(eff) profile measurement system with an optimized Czerny-Turner visible spectrometer in large helical device

Z(eff) measurement system using a visible spectrometer has been newly designed and constructed instead of an old interference filter system to eliminate line emissions from the signal and to measure the Z(eff) value in low-density plasmas. The system consists of the Czerny-Turner-type spectrometer with a charge-coupled device camera and 44 optical fibers vertical array. The spectrometer is equipped with an additional toroidal mirror for further reduction in the astigmatism in addition to a flat and two spherical mirrors and three gratings (110, 120, and 1200 grooves/mm) with 30 cm focal length. The images from 44 optical fibers can be detected without astigmatism in a wavelength range of 200-900 nm. Combination of the optical fiber (core diameter: 100 microm) with the lens (focal length: 30 mm) provides spatial resolution of 30 mm at the plasma center. Results clearly indicate a very good focus image of the fiber and suggest the absence of the cross-talk between adjacent fiber images. Absolute intensity calibration has been done using a standard tungsten lamp to analyze the Z(eff) value. The bremsstrahlung profile and resultant Z(eff) profile have been obtained after Abel inversion of the signals observed in large helical device plasmas with elliptical poloidal cross section.