In-situ wavelength calibration and temperature control for the C-Mod high-resolution x-ray crystal imaging spectrometer

An x-ray crystal imaging spectrometer with high spectral and spatial resolution is currently being used on Alcator C-Mod to infer time histories of temperature and velocity profiles. An in-situ wavelength calibration using a 1 $\mu$m palladium filter in between the crystal and the detectors of choice is being proposed as a natural wavelength-marker using the transmission changes across the L-II and L-III edges at 3722.9 mA and 3907.1 mA, respectively. Recent results also indicate that the crystal temperature should be kept constant within a fraction of a degree since the thermal expansion of the quartz crystal will change the interplanar ($2d$) spacing and introduce fictitious velocity measurements of several km/s. A detailed temperature scan indicates a thermal expansion coefficient ($\alpha_{\perp}$) of $13.5\times10^{-6}$ $/^{\circ}$C and thus a false Doppler shift of $4.05\cdot\Delta T[^{\circ}$C$]$ km/s.