Beam loss position monitor using Cerenkov radiation in optical fibers

The VUV FEL in TESLA technology at DESY provides Giga-Watt output power in laser pulses. The SASE single pass Free Electron Laser (FEL) has been developed for high-brightness user applications. At the design parameters the average power of the electron beam is about 72 kW. To avoid vacuum breakdown and high radiation levels caused by electron losses a machine protection system is required. Collimators are installed upstream of the radiation sensitive undulators [1]. However, the proper operation of the collimator system needs to be measured with a beam loss monitor. Conventional radiation sensor systems are not suited for the VUV-FEL undulators, because of the restricted free space in the undulator gap. A Beam Loss Position Monitor (BLPM) based on Cerenkov light in optical fibers allows real time monitoring of loss location and loss intensity. Electrons with energies above 175 keV generate Cerenkov light during their penetration of the optical fiber. The fast response o f the Cerenkov signal is detected with photomultipliers at the end of the irradiated fibers. The reconstruction of the particle loss trace in 3 space dimensions became possible with four sensors.