Modeling the Optical Coupling Efficiency of the Linac Coherent Light Source Beam Loss Monitor Radiator*

A large-solid-angle, Cherenkov detector beam loss monitor has been built and tested as part of the Linac Coherent Light Source machine protection system (MPS). The MPS is used to protect the undulator magnets by detecting high-energy electron beam loss. These electrons produce other forms of radiation that can lead to demagnetization of the undulator magnets. Cherenkov light is generated when primary electrons, lost from the beam, create a shower of secondary electrons that transit through the Cherenkov radiator medium. The radiator consists of an Al-coated plate of high-purity, fused-silica 12.77 cm wide, 6.29 cm high, and 0.64 cm thick, which is formed into a tuning fork geometry that envelopes the beam pipe preceding each undulator. The radiator transports Cherenkov photons via internal reflection through a tapered region and stem into the photocathode of a compact photomultiplier tube (PMT). We calculate the optical efficiency of the radiator ηc, that is, the probability that a photon generated within the fused silica will reach the exit aperture adjacent to the PMT. A simple model based on line sources summed across image planes is compared for the case of normally incident electrons with a more detailed Monte Carlo random-walk simulation called RIBO[1]. Both analytical and numerical models show the efficiency to be relatively uniform over the full range of transverse locations in the radiator. This is encouraging for the MPS detection scheme, which seeks to protect the undulator magnets over their entire cross section. As we expect, both analyses show ηc to be a strong function of the surface reflectivity Rf; ηc ~0.0084 for Rf=0.95, but drops to 0.0033 for Rf=0.90.