To perform a proof of principle experiment of a SASE based Free Electron Laser operating a permanent magnet undulator has been installed in the TESLA Test Facility (TTF) linac. The type of permanent magnets (NdFeB) used is known to be sensitive to irradiation. Already losses of the order of 10−6 at nominal TTF beam current can cause a degradation of the undulator magnets after a few month of operation. To protect the undulator against radiation a collimation system in front of the undulator removes the electrons with large betatron motion. To detect beam haloor dark current escaping the collimators a beam loss monitor (BLM) system based on photomultiplier has been developed. During the past two years the BLM system has been improved in its electronic components and detectors. It has become a standard tool for linac operation and is nowintegrated part of the linac protect system. In this paper the design, the operation experiences and the performance limits of the system are presented
