A read-out system for online monitoring of intensity and position of beam losses in electron linacs

In particle accelerators, beam loss position monitors (BLPM) are diagnostic systems revealing an interaction of the beam with the vacuum chamber or with other obstacles. They primarily measure the intensity of the beam loss by detecting particles from the electromagnetic shower in the proximity of the beam pipe. Through an appropriate detector configuration, this measurement also provides information about the longitudinal position of the beam loss along the beam line. This paper presents the design and performance of the BLPM system originally proposed and tested [1] at the SPARC accelerator at Frascati National Laboratory (LNF) of INFN and, later, further developed in the framework of a collaboration with the FERMI@Elettra project at Sincrotrone Trieste. For both facilities, the monitoring of beam losses in permanent magnet undulators is of particular interest. The BLPM consists of quartz fibers stretched in parallel to the beam axis which collect the Cherenkov light generated by charged particles traversing them. Multi-pixel photon counters are used to transform the light into a proportional time dependent electric signal. The use of these simple but highly sensitive photon detectors instead of traditional photomultiplier tubes is an innovation over previous BLPM implementations. It has allowed the development of a compact and reliable read-out system at low cost. In daily use, the system provides information to machine protection and control systems for beam transport monitoring and optimization.