Upgrades in the KSTAR soft X-ray arrays in preparation for multichannel measurements

To upgrade the soft X-ray (SXR) arrays in KSTAR, we developed a new readout system using silicon photomultipliers (SiPMs) and optimized the optics of the previous array system. The upgraded array system employs a YAG:Ce scintillator to convert incident SXR radiation into visible light, which is transmitted via optical fibers to remote SiPM detectors. We conducted single-channel SiPM tests in the laboratory to evaluate light collection efficiency and spectral response. RGB-type SiPMs, selected for their spectral compatibility with the scintillator, demonstrated higher sensitivity than near ultraviolet–type devices. Optical coupling was also improved through lens optimization and the introduction of tapered light guides. Based on these results, the 16-channel SiPM array hardware has been installed in KSTAR, with full-scale commissioning and data acquisition planned for the 2025 campaign. This work establishes a foundation for multichannel SXR measurements in KSTAR.