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.
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.