RIKEN/SPring-8, Hyogo
JASRI/SPring-8, Hyogo-ken
We present the design and performance of the beam diagnostic system of XFEL/SPring-8. The XFEL accelerator requires sub-um resolution beam position monitors (BPM), few-um resolution screen monitors, high-speed beam current monitors and a ten femtosecond resolution temporal structure measurement system. We designed an rf cavity BPM which has a resonant frequency of 4760 MHz and a position resolution of less than 1 um. For the screen monitor, thin stainless-steel foil (0.1 mm thick) is used to reduce beam divergence. In addition, a custom-made lens system having few-micron resolution was designed. For the beam measurement, we developed a differential current transformer (CT) with four ports, two are positive and the others are negative, to reduce common-mode noise. The rise time of the CT output pulse is 0.1 ns. To measure the temporal structure of a beam, we developed a C-band (5712 MHz) transverse deflecting cavity that has a disk-loaded backward traveling wave structure. The iris shape of the cavity is a race-track to separate x- and y-mode. This cavity can resolve a beam into femtosecond fragments. Thus, the beam diagnostic system satisfies the demands of the XFEL machine.
DESY, Hamburg
CERN, Geneva
RIKEN/SPring-8, Hyogo
PSI, Villigen
XFELs require high precision orbit control in the long undulator sections. Due to the pulsed operation of these systems the high precision has to be reached by single bunch measurements. So far cavity BPMs achieve the required performance and will be used at the European XFEL between each of the 116 undulators. Coupling between the orthogonal planes limits the precision of beam position measurements. A first prototype build at DESY shows a coupling between orthogonal planes of about -20 dB, but the requirement is lower than -40 dB (1%). The next generation Cavity BPM was build with tighter tolerances and mechanical changes, the orthogonal coupling is measured to be lower than -43 dB. This report discusses the various observations, measurements and improvements which were done.
RIKEN/SPring-8, Hyogo
JASRI/SPring-8, Hyogo-ken
In the XFEL project at SPring-8, the resolution of a beam position monitor (BPM) is required to be less than 1 um. Therefore, we developed an rf cavity BPM (RF-BPM) to achieve a precise position resolution. The RF-BPM has two cavities: one is a TM110 cavity for position detection and the other is a TM010 cavity for phase reference and charge normalization. The resonant frequency is 4760 MHz and the loaded Q factor is approximately 50 for both cavities. The designed performance of the RF-BPM cavity was confirmed by low-level rf measurement. The rf signal is detected by an IQ demodulator to obtain the intensity and the phase. Although the BPM signal is a mixture of a position signal and a slope signal, the IQ demodulator can easily distinguish them because the phases of these signals are 90 degrees different from each other. We developed a new circuit that has small errors: the intensity error is 1 % and the phase error is 0.5 degree. The RF-BPM system has been tested by using a 250 MeV electron beam at the SCSS test accelerator. We report results of confirmed RF-BPM performances; position resolution, xy coupling, linearity, dynamic range, beam arrival timing measurements etc.