Spectroscopic investigations of tungsten in the EUV region and the determination of its concentration in tokamaks

Tungsten was injected by means of laser ablation in both ohmic and additionally heated plasmas of the ASDEX Upgrade tokamak experiment. Spectroscopic investigations were performed in the extreme ultraviolet (EUV) wavelength region, 4 to 140 nm. Besides the quasi-continuum structure at ≈ 5 nm emitted by tungsten ions around W27+, isolated lines of tungsten were observed in the same spectral region and also in the range from 12 to 14 nm. By comparison with calculations from the HULLAC and RELAC codes, these lines could be identified as transitions of bromine to nickel-like tungsten ions. The concentration cW of tungsten after laser ablation was determined from comparisons between the total tungsten radiation PW and the calculated radiation losses. Calibration of the quasi-continuum intensity with the help of the PW measurements allows the tungsten concentration to be determined from spectroscopic observations, which are more sensitive. Both concentration measurements (quasi-continuum, PW) agree well in discharges with laser ablation. From a comparison of the intensity of the isolated lines with code results, cW could be estimated in the central region of hot, additionally heated plasmas. The lower detection limit of the spectroscopic method allows the extraction of cW during the tungsten divertor experiment of ASDEX Upgrade.