Plasma impurities observed by a pulse height analysis diagnostic during the divertor campaign of the Wendelstein 7-X stellarator

Published Online: 27 September 2018 | Accepted: July 2018

Review of Scientific Instruments 89, 10F111 (2018);
M. Kubkowska, A. Czarnecka, T. Fornal, M. Gruca, S. Jabłoński, N. Krawczyk, L. Ryć, R. Burhenn, B. Buttenschön, B. Geiger, O. Grulke, A. Langenberg, O. Marchuk, K. J. McCarthy, U. Neuner, D. Nicolai, N. Pablant, B. Schweer, H. Thomsen, Th. Wegner, P. Drews, K.-P. Hollfeld, C. Killer, Th. Krings, G. Offermanns, G. Satheeswaran, F. Kunkel, and W7-X team

The paper reports on the optimization process of the soft X-ray pulse height analyzer installed on the Wendelstein 7-X (W7-X) stellarator. It is a 3-channel system that records X-ray spectra in the range from 0.6 to 19.6 keV. X-ray spectra, with a temporal and spatial resolution of 100 ms and 2.5 cm (depending on selected slit sizes), respectively, are line integrated along a line-of-sight that crosses near to the plasma center. In the second W7-X operation phase with a carbon test divertor unit, light impurities, e.g., carbon and oxygen, were observed as well as mid- to high-Z elements, e.g., sulfur, chlorine, chromium, manganese, iron, and nickel. In addition, X-ray lines from several tracer elements have been observed after the laser blow-off injection of different impurities, e.g., silicon, titanium, and iron, and during discharges with prefill or a gas puff of neon or argon. These measurements were achieved by optimizing light absorber-foil selection, which defines the detected energy range, and remotely controlled pinhole size, which defines photon flux. The identification of X-ray lines was confirmed by other spectroscopic diagnostics, e.g., by the High-Efficiency XUV Overview Spectrometer, HEXOS, and high-resolution X-ray imaging spectrometer, HR-XIS.

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