BEAM MEASUREMENTS OF A LARGE SOLID-ANGLE BEAM LOSS MONITOR IN THE APS

For reliable radiation dosimetry of undulator magnets, a beam loss monitor (BLM) covering a large solid angle from the point of beam losses is highly desirable. A BLM that uses a Cherenkov radiator plate wrapping around the beam pipe is utilized in the Linac Coherent Light Source (LCLS) undulators, and a similar BLM geometry has been tested for the Advanced Photon Source (APS) undulators. We report on measurements made with these BLMs recently installed in the APS storage ring and the booster-to-storage-ring transfer line. A two-order-of-magnitude variation in the relative sensitivity of the Cherenkov detector is observed as a function of incident electron position in the quartz radiator. A factor of 10 variation in signal sensitivity was observed with the change of particle entry angle. The introduction of tungsten and lead shields enhances count rates by 30 - 40%. When the detector is moved along the insertion device chamber, the signal intensity peaks 1 m from the chamber entrance. The measured data are compared with numerical simulation of the beam loss processes.