Calibration of Vacuum Ultraviolet spectroscopy diagnostic on J-TEXT tokamak

A space-resolved Vacuum Ultraviolet (VUV) spectroscopy system based on a 1-meter normal incidence spectrometer has been developed on J-TEXT tokamak. To obtain the impurity emission profile, the Optical path and absolute intensity calibration are implemented. An alignment plate customized with several different shaped apertures is mounted between the entrance of the spectrometer and the tokamak. The exact viewing angle is identified via the on-site test. The collisional-radiative line ratio method based on the impurity transport code STRAHL is applied for the absolute intensity calibration. Pairs of lines with different upper energy levels for the same ions, e.g. C III 465 nm/C III 97.7 nm, C IV 580 nm/C IV 154.8 nm are chosen to achieve the calibration. A calibrated visible spectrometer is used to provide the absolute intensity of the visible branch. The line emission profile in the VUV range could be evaluated from the visible line via STRAHL and the atomic data from ADAS to take into account the effect of temperature on the ratio of the selected line pair. Based on the calibrated coefficient, the radial emission profiles of C III 97.7 nm and C IV 154.8 nm are obtained.