FIRST BEAM COMMISSIONING EXPERIENCE WITH THE SwissFEL CAVITY BPM SYSTEM

SwissFEL is a free electron laser facility designed to produce FEL radiation at wavelengths from 0.1 to 7 nm. The beam commissioning of its hard X-ray undulator line ("Aramis") started 10/2016, and first lasing was observed in 12/2016. Presently, a 2nd undulator line ("Athos") for soft X-rays is being constructed, with 1st beam scheduled for mid-2019. In the injector, linac and Aramis beam transfer lines, 95 low-Q cavity BPMs operating at 3.3 GHz are used that were designed to support the future two-bunch operation mode with 28 ns bunch spacing and 100 Hz repetition rate. The Aramis (and future Athos) undulator lines will only be operated with single bunches by means of a fast beam distribution kicker system, and are thus equipped with high-Q cavity BPMs operating at 4.9 GHz. The BPMs are not only used for beam trajectory optimization, but also for beam energy measurements (using standard cavity BPMs in the bunch compressors and beam dumps), beam charge and transmission measurements, or improvement of the performance of other monitors like wire scanners for profile measurement. This paper will report about the first operation experience with the BPM system, including a performance comparison of low-Q and high-Q BPMs.