In situ wavelength calibration system for the X-ray Imaging Crystal Spectrometer (XICS) on W7-X

An in situ wavelength calibration system for the X-ray Imaging Crystal Spectrometer (XICS) on W7-X has been developed to provide routine calibration between plasma shots. XICS is able to determine plasma flow profiles by measuring the Doppler shift of x-ray line emission from highly charged impurity species. A novel design is described that uses an x-ray tube with a cadmium anode placed in front of the diffracting spherically bent crystal. This arrangement provides calibration lines over the full detector extent for both the Ar¹⁶⁺ and Ar¹⁷⁺/Fe²⁴⁺ spectrometer channels. This calibration system can provide a relative wavelength accuracy of 3 × 10^-7 Å across the full spatial extent of the detector, which corresponds to 50 m/s in the W7-X system. An absolute wavelength calibration of 1 × 10^-5 Å is expected, corresponding to 1 km/s, based on the current known accuracy of the calibration wavelength and the achievable measurement of the absolute positioning of the hardware. This calibration system can be used to independently calibrate XICS systems on both stellarators and tokamaks, without the need for special plasma conditions often used for calibration, such as locked modes on tokamaks. Experimental and simulated results are shown along with expected results, and the complete design of the calibration hardware that is to be installed in the W7-X XICS system.