Recent progress of the rf and timing system of xfel/spring-8

To provide precise rf and timing signals for the XFEL facility at SPring-8, we have developed a stable optical rf and timing distribution system and a precise low-level rf control system. The phase stability of an acceleration rf field is required to be 0.1 degree (rms) of 5712 MHz, corresponding to 50 fs, and the amplitude stability is required to be 0.01% (rms). For the optical system, we use a phase-stabilized optical fiber and a water-cooled cable duct to suppress its path-length drift. In addition, the path length is monitored by an optical interferometer and regulated by a variable delay line. The phase and amplitude fluctuations of the optical system, including E/O and O/E converters were measured to be 0.71 degree (p-p) and 0.86% (p-p). For the low-level rf system, we use an IQ (in-phase and quadrature) modulator to generate an acceleration rf signal and an IQ demodulator to detect the rf field. These circuits are enclosed in a water-cooled 19-inch rack to keep the temperature within ± 0.2 K. The stabilities of the rf phase and the amplitude of these instruments were measured to be 0.30 degree (pp) and 0.56% (p-p), respectively. Although the amplitude stability is still worse than the requirement, this can be reduced by a high-power klystron that is operated at the saturation point. Consequently, the phase and amplitude stabilities are sufficient for XFEL/SPring-8.