Observation of W IV–W VII line emissions in wavelength range of 495–1475 Å in the large helical device

Vacuum ultraviolet spectra of line emissions from tungsten ions at lower ionization stages have been measured in the large helical device (LHD) using a high-resolution 3 m normal incidence spectrometer in the wavelength range of 495–1475 A. Tungsten was introduced in the LHD plasma by injecting a coaxial tungsten impurity pellet. Many tungsten lines of W IV–W VII were successfully observed in low-temperature plasmas just after the tungsten pellet injection. It is found that some W VI lines are emitted with extremely high intensity and entirely isolated from other intrinsic impurity lines, in particular, W VI at 605.926 A (5d–6p), 639.683 A (5d–6p), 677.722 A (5d–6p), 1168.151 A (6s–6p) and 1467.959 A (6s–6p). The result strongly suggests that those lines may be useful for the spectroscopic study in ITER and other magnetic fusion devices with tungsten materials as the plasma facing component. The ion temperature was also measured from Doppler broadening of W V and W VI lines. The result indicates that the measured ion temperature is clearly higher than the ionization energy of such ions. The reason is discussed with regarding to the pellet injection.