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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 μ m⁠) 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 …