• T. Ohshima, N. Hosoda, H. Maesaka, S. Matsubara, Y. Otake
  RIKEN/SPring-8, Hyogo

The pulse width of an X-ray laser at XFEL/SPring-8 is several tens femto-seconds, which requires reference rf signals to have the same time-stability. The reference signals with a low phase-noise oscillator are sent to instruments in 19" racks developed along an accelerator by an optical fiber system. The temperature drift of the fiber makes phase shifts of the reference signals. Therefore, the fiber is put in a thermal-insulated duct. By feeding temperature-controlled water (26.1 ± 0.1 deg. C) in a pipe attached to the duct, the fiber temperature was kept to be 26.2 ± 0.08 deg. C at the ambient temperature change of 29.1 ± 1.7 deg. C. From this temperature controllability, the phase shifts of the signals through a 400 m fiber of a thermal coefficient of 5 ps/km/K are 160 fs. Further reduction of the shifts is required and will be achieved by a fiber-length feedback control in a future plan. Vibration of the fiber also degrades the quality of the signals. The fiber is embedded on a vibration buffer material. A test to evaluate the effect of the vibration to the transmitted signal phase was carried out. The test result will be also shown in this paper.

• N. Hosoda, H. Maesaka, S. Matsubara, T. Ohshima, Y. Otake, K. Tamasaku
  RIKEN/SPring-8, Hyogo
• M. Musha
  University of electro-communications, Tokyo

The intensity of SASE generated by undulators is sensitive to the peak intensity fluctuation of an electron bunch. The bunch is formed by velocity bunching in an injector and magnetic bunching in bunch compressors (BC). The peak intensity is sensitive to rf phase and amplitude of off-crest acceleration at injector cavities and 5712 MHz cavities before the BCs. Thus, demanded stabilities of the rf phase and amplitude for stable SASE generation are very tight. These are 0.6 degree (p-p) and 0.06 % (p-p) at the 5712 MHz cavities, respectively. We are constructing a low-level rf (LLRF) system comprising a master oscillator, an optical rf signal transmission system, and a digital rf control system using IQ modulator/demodulator to drive klystrons. To realize the demands, much attention was paid to temperature stabilization for the system. A water-cooled 19-inch rack and a water-cooled cable ducts are employed for almost all part of the system. Temperature stability of the rack was 0.4 K (p-p) even though outside was 4 K (p-p). The phase and amplitude stabilities of the LLRF modules were measured to be 0.30 degree (p-p) and 0.56 % (p-p). These stabilities are sufficient for our demands.