Design, development, and operation of a fiber-based Cherenkov beam loss monitor at the SPring-8 Angstrom Compact Free Electron Laser

A fiber-based Cherenkov beam loss monitor (CBLM) consisting of large core (400 μm), long (≥150 m) multimode fibers, has been developed as an online long-range detection tool with high sensitivity and good position resolution for the 8 GeV SPring-8 Angstrom Compact Free Electron Laser: primarily designed for radiation safety in order to limit the dose outside the shielding of the machine, this monitor also serves as an early warning tool to avoid radiation damages done by lost electrons to the undulator magnets. This paper presents the approach chosen to insure that the required sensitivity (≤1 pC) could be obtained over more than 100 m. A beam-based approach was used to characterize (attenuation and signal strength) different fibers (diameter, index profile, and numerical aperture) and to select the most appropriate one. The response of the detector has also been studied numerically for different geometries (vacuum pipe and in-vacuum type undulators), beam energies, and beam loss scenarios, to determine the optimum number of fibers and their position in order to achieve the required detection limit. The results of the first few months of operation show that the SPring-8 CBLM can detect beam losses of less than 0.5 pC over the full 150 m length of the fiber.