• H. Tomizawa, H. Dewa, H. Hanaki, S. Matsubara, A. Mizuno, T. Taniuchi, K. Yanagida
  JASRI/SPring-8, Hyogo-ken
• T. Ishikawa, N. Kumagai
  RIKEN/SPring-8, Hyogo
• K. Lee, A. Maekawa, M. Uesaka
  The University of Tokyo, Nuclear Professional School, Ibaraki-ken

We are developing a single-shot and non-destructive 3D bunch charge distribution (BCD) monitor based on Electro-Optical (EO) sampling with a manner of spectral decoding for XFEL/SPring-8. For fine beam tuning, 3D-BCD is often required to measure in real-time. The main function of this bunch monitor can be divided into longitudinal and transverse detection. For the transverse detection, eight EO-crystals surround the beam axis azimuthally, and a linear-chirped probe laser pulse with a hollow shape passes thorough the crystal. The polarization axis of the probe laser should be radially distributed as well as the Coulomb field of the electron bunches. Since the signal intensity encoded at each crystal depends on the strength of the Coulomb field at each point, we can detect the transverse BCD. In the longitudinal detection, we utilize a broadband square spectrum (> 400 nm at 800 nm of a central wavelength) so that the temporal resolution is < 30 fs if the pulse width of probe laser is 500 fs. In order to achieve 30-fs temporal resolution, we use an organic EO material, DAST crystal, which is transparent up to 30 THz. We report the first experimental results of this 3D-BCD monitor.

• H. Maesaka, S.I. Inoue, S. Matsubara, Y. Otake
  RIKEN/SPring-8, Hyogo

The x-ray FEL facility at SPring-8 produces a very short-bunch beam by using bunch compressors (BC) consisting of magnetic chicanes. Since the bunch compression ratio is strongly depends on the beam energy and the energy chirp, we need to monitor the energy from the beam position at the dispersive part of the BC with a 0.1% resolution. However, a beam profile at the dispersive part is horizontally flat and wide, maximally 50 mm, due to the large energy chirp of the beam. Therefore, we designed a multi-stripline beam position monitor. This monitor has a flat rectangular duct with a 70 mm width and a 10 mm height. Six stripline electrodes at individual intervals of 10 mm are equipped on each of the top and the bottom surface. Due to the small height of the monitor, each electrode is sensitive to the electron position within 10 mm in the horizontal. Therefore, the monitor provides a rough charge profile and the beam position which is calculated from the gravity center of the signals. We prepared a prototype of the monitor and tested it at the SCSS test accelerator. We confirmed that the position sensitivity was better than 0.1 mm, which corresponds to 0.1 % energy resolution.

• S. Matsubara, A. Higashiya, H. Maesaka, T. Ohshima, Y. Otake, T. Shintake, H. Tanaka, K. Togawa, M. Yabashi
  RIKEN/SPring-8, Hyogo
• H. Ego, S. Inoue, K. Tamasaku, T. Togashi, H. Tomizawa, K. Yanagida
  JASRI/SPring-8, Hyogo-ken

We report the design, performance, and installation of the beam diagnostic system of XFEL/SPring-8. The electron beam bunches of an XFEL accelerator are compressed from 1 ns to 30 fs by bunch compressors without emittance growth and peak-current fluctuation which directly cause SASE fluctuation. To maintain the stable bunch compression process, the accelerator requires rf caivty beam position monitors (BPM) with 100 nm resolution, OTR screen monitors (SCM) with a few micro-meter resolution, fast beam current monitors (CT) and temporal structure measurement systems with resolution under picosecond. The performance of the developed monitor instruments, such as the BPM, the SCM, and the CT, was tested at the SCSS test accelerator and satisfied with the requirements. To measure the temporal structure of the electron bunch, three type measurement systems, which are a streak camera, an EO sampling measurement, and a transverse deflecting cavity with a resolution of few-tens femtosecond, are being prepared. The streak camera and EO sampling shows the resolution of sub-picosecond. The installation of these beam diagnostic systems is going on smoothly.

• 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.