Sub-micron resolution rf cavity beam position monitor system at the SACLA XFEL facility

We have developed and constructed a C-band (4.760 GHz) rf cavity beam position monitor (RF-BPM) system for the XFEL facility at Spring-8, SACLA. The demanded position resolution of the RF-BPM is less than 1 mu m, because an electron beam and x-rays must be overlapped within 4 mu m precision in the undulator section for sufficient FEL interaction between the electrons and x-rays. In total, 57 RF-BPMs, including IQ demodulators and high-speed waveform digitizers for signal processing, were produced and installed into SACLA. We evaluated the position resolutions of 20 RF-BPMs in the undulator section by using a 7 GeV electron beam having a 0.1 nC bunch charge. The position resolution was measured to be less than 0.6 mu m, which was sufficient for the XFEL lasing in the wavelength region of 0.1 nm, or shorter. (C) 2012 Elsevier B.V. All rights reserved.