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Observation of visible M1 lines of tunsgten highly charged ions and its applications to tungsten measurements in LHD core plasmas

par Caroline CHAMPENOIS - publié le

séminaire du laboratoire
lundi 26 mars, à 14:30, service 361

Prof. D. Kato
National Institute for Fusion Science, Department of Fusion Science, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan
Department of Advanced Energy Engineering Science, Kyushu University, 6-1 Kasuga-koen Kasuga-city Fukuoka 816-8580, Japan

abstract :

Impact of tungsten contamination in ITER is a big concern because a large radiation power loss by the tungsten highly charged ions is predicted. Tungsten behaviors in core plasmas have intensively been studied using Extreme-Ultra-Violet (EUV) and soft X-ray emission lines of up to 48 times ionized tungsten ions with large devices, e.g. ASDEX-U, JET, JT-60U, LHD, EAST. It is noted that these charge states would be dominant in edge plasmas of ITER. More precice measurements are facilitated with UV-visible lines, however applications have exclusively been limited to neutral and very low charge states of tungsten in peripheral plasmas. In the present work, we investigate magnetic-dipole (M1) lines of the tungsten highly charged ions in the ground states predicted in the UV-visible range [1].
Using electron-beam-ion-trap (EBIT), quite a few visible M1 lines of W7+ - W28+ are reported [2-7]. Measurements of tungsten emission lines at Large Helical Device (LHD) are performed using solid pellet injection techniques [8,9]. We identified the tungsten M1 lines in the visible spectra of core plasmas (the central electron temperatures are about 1 keV) [10,11]. Photon emission coefficients of the M1 lines are calculated using a collisional-radiative (CR) model. The present CR model is constructed based on fully-relativistic ab-initio calculations of atomic data using HULLAC (Hebrew University Lawrence Livermore Atomic Code) [12]. Proton collision effects are evaluated using semi-classical perturbation theories and taken into account in the photon emission coefficients. Local ion densities of W26+ and W27+ in the core plasmas are then deduced from spacially resolved measurements of the M1 line intensities. Deduced ion density ratios W27+/W26+ obey ionization equilibrium values calculated with CADW ionization [13] and modified ADPAK recombination [14] rate coefficients in a temperature range of 0.5 – 1.5 keV. Only at the instance when a steep gradient of electron temperature profile at the edge is observed, a significant deviation from the ionization equilibrium is seen. Tungsten profiles in the core plasmas are obtained using a charge state distribution of the ionization equilibrium based on the present W26+ and W27+ densities. The tungsten profiles show an apparent correlation with corresponding electron temeprature profiles measured by Thomson scattering.
This work was partly supported by the JSPS-NRF-NSFC A3 Foresight Program in the field of Plasma Physics (NSFC : No.11261140328, NRF : No.2012K2A2A6000443). DK is grateful to financial support of KAKENHI (15H04235).
[1] D. Kato et al., 26th IAEA Fusion Energy Conference (17-22 October, 2016, Kyoto International Conference Centre, Kyoto, Japan) EX/P8-14.

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[14] T. Putterich et al., Plasma Phys. Control. Fusion 50 (2008) 085016.

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