Multilayer mirror and foil filter AXUV diode arrays on CDX-U spherical torus

Recent upgrades to CDX-U spherical torus diagnostics include two 10-channel AXUV diode arrays. The multilayer mirror (MLM) array measures the $λ150$ O VI brightness profile in the poloidal plane using the $Mo/B4C$ synthetic multilayer structures as dispersive elements. The foil filter array has a tangential view and is equipped with interchangeable clear aperture, beryllium and titanium filters. This allows measurements of radiated power, O VI or C V radial distributions, respectively. The O VI and C V emissivity and the radiated power profiles are highly peaked. A Neoclassical impurity accumulation mechanism is considered as an explanation. For radiated power measurements in the $Te⩽100$ eV plasmas, photon energy dependent corrections must be used in order to account for nonlinear AXUV sensitivity in the range $Ephot⩽20$ eV. The arrays are also used for characterization of resistive MHD phenomena, such as the low $m$ modes, saw-tooth oscillations and internal reconnection events. Based on the successful operation of the diagnostics, a new ultra soft x-ray multilayer mirror diode AXUV diode array monitoring the 34 Å emissivity distribution of C VI will be built and installed on the National Spherical Torus Experiment.