Development of the cold BPM system for SHINE: Preliminary beam test results

The Shanghai High Repetition rate XFEL and Extreme light facility (SHINE) is a new-generation, high-repetition-rate free-electron laser (FEL) driven by a continuous-wave superconducting accelerator. It is designed to accelerate electron bunches to 8 GeV using cryomodules (CMs) integrated with superconducting radio-frequency (SRF) cavities. The SHINE accelerator mainly consists of a VHF electron gun, an injector, and four linear accelerator sections separated by three bunch compressors (BC1–BC3). The entire linac comprises 77 cryomodules, including 75 standard 1.3 GHz cryomodules and two 3.9 GHz cryomodules. Each cryomodule is equipped with a cold beam position monitor (cBPM), which is required to achieve a position resolution better than 200 μ m at 10 pC. Accurate beam position measurement is essential for cryomodule protection and beam orbit feedback. It effectively prevents electron beam drift and avoids severe consequences such as superconducting magnet quenches. Considering the extremely low thermal load requirement of the cryogenic environment, as well as structural simplicity and mechanical robustness, the non-magnetic button-type BPM was ultimately selected over two conventional schemes: cavity-type BPMs and stripline BPMs. The cBPM is mounted at the downstream end of each cryomodule, between the superconducting cavities and superconducting magnets. Surrounded by these two cryogenic components, the cBPM is cooled below 10 K via thermal conduction. Therefore, cryogenic resistance and thermal shock resistance are key design indicators for the cBPM. To date, the in-house development of the cold BPMs has been fully completed, covering design, fabrication, testing, and on-site installation. The first five cryomodules in the L1 and HL sections have been installed, and their cryogenic cooling commissioning has been finished. Preliminary beam-based experiments have been successfully conducted, achieving a system resolution better than 100 μ m at 50 pC. This paper presents the overall development of the cold beam position measurement system for the SHINE facility, with an emphasis on comprehensive experimental verification and performance testing.