Commissioning and first test results of the beam emission spectroscopy diagnostic on Wendelstein 7-X*

A novel beam emission spectroscopy (BES) diagnostic is being installed and commissioned on Wendelstein 7-X (W7-X) to study 2D structure of ion-scale turbulence dynamics. An avalanche photodiode (APD) detector module has been assembled to receive light from a common semi-telecentric optical system, where the light is collimated and bandpass selected at the desired wavelength, allowing a uniform channel-to-channel performance with a reduced manufacturing cost. Two of the 4-by-8 element avalanche photodiode (APD) arrays are embedded at the image plane of the optical system. Signal from each pixel of APD array is then distributed to individual channel on the customized preamplifier circuit for further signal processing. The detector module can be actively cooled down to -20^oC using a built-in thermoelectric cooling (TEC) unit, providing a significantly improved signal-to-noise ratio. 

The installation of the BES front-end optics is scheduled on the W7-X maintenance phase 2.4 in 2025, whereas the detector module is ready for commissioning beforehand. Here we present the details of the detector assembly including the designs of main sub-modules, and the noise characteristics. By replacing the BES interference filter with a passband at 656 nm with a filter that has a passband close to 528 nm, and using a fiber that is available from the charge-exchange recombination spectroscopy (CXRS), it enables us to test the detector module before the final installation of the BES front-end optics. The performance of the detector module using the CXRS fiber is initially evaluated by measuring the charge-exchanged emission from carbon at 528 nm in the upcoming W7-X campaign.