Commissioning and first operation of the pulse-height analysis diagnostic on Wendelstein 7-X stellarator

The Wendelstein 7-X (W7-X) stellarator started its operation at the end of 2015. The first operational phase (0P1.1) has been conducted with helium and hydrogen as working gas. The initial experimental programs have been devoted to commissioning, tests and optimization of diagnostic systems. This paper reports on the commissioning of the 3-channel Pulse Height Analysis (PHA) diagnostic, which has been installed at W7-X in the middle of 2015. The system measures X-ray radiation from the plasma core in the energy range of 0.25-20 keV in order to estimate the electron temperature of the plasma (from the Bremsstrahlung radiation) and impurity content. The PHA diagnostic is designed for making measurements on long-lasting plasmas (up to 30 min.) in future operation phases of W7-X. During the first experimental campaign, the PHA system was successfully put into operation and tested. The 1st PHA channel has been optimised, however the 2nd and 3rd still need more time for proper adjustment. Registered spectra were characterized by satisfactory counting rates, which allowed to observe well separated spectral lines, thereby allowing impurity identification. More additional tests before OP1.2 and during the restart of W7-X should help to find ideal combination of diagnostic settings. (C) 2017 Elsevier B.V. All rights reserved.