Author: Inagaki, T.
Paper Title Page
MOPB005 High Gradient Operation of 8 GeV C-Band Accelerator in SACLA 186
  • T. Inagaki, C. Kondo, Y. Otake, T. Sakurai
    RIKEN SPring-8 Center, Sayo-cho, Sayo-gun, Hyogo, Japan
  SACLA (SPring-8 angstrom compact free electron laser) is the X-ray free electron laser (XFEL) facility. In order to shorten the 8 GeV accelerator length, a C-band (5712 MHz) accelerator was employed. Since the accelerating gradient of C-band accelerating structure is 35 MV/m in nominal, the active accelerator length is 230 m. In total, 64 klystrons, 64 pulse compressors, and 128 accelerating structures are used. In order to withstand the high surface field (~ 100 MV/m), and to reduce the amount of dark current, which decreases the demagnetization effect of undulators, the accelerating structures are carefully fabricated in the factory. After high power RF conditioning of 500 hours, the beam commissioning was started in February 2011. For night time of the commissioning, we continued the RF conditioning. The RF breakdown rate of the structure was steadily decreased. Now we operate the accelerator with the beam energy as much as 8.3 GeV, and the accelerating gradient of 37 MV/m in average. We found the amount of dark current is small enough. So far no trouble occurred in C-band RF components of 64 sets.  
MOPB084 Design of a C-band Disk-loaded Type Accelerating Structure for a Higher Pulse Repetition Rate in the SACLA Accelerator. 372
  • T. Sakurai, T. Inagaki, Y. Otake
    RIKEN SPring-8 Center, Sayo-cho, Sayo-gun, Hyogo, Japan
  • H. Ego
    JASRI/SPring-8, Hyogo-ken, Japan
  The higher pulse repetition rate of the SACLA accelerator provides a higher rate of X-ray laser pulses to expand ability of user experiments, such as simultaneously providing the laser to several beamlines and reducing a measuring time in the experiment. Therefore, we studied on a C-band accelerating structure for a higher pulse rate above 120 pps than that of the present case of 60 pps. The designed structure adopts a TM01-2π/3 mode disk-loaded type with a quasi-constant gradient . Since higher repetition rate operation is inclined to increase a number of vacuum electrical discharges, it is required to reduce the surface electric field in the structure. We designed an ellipsoidal curvature shape around an iris aperture, which reduces the maximum surface field by 20%. Since the higher repetition rate also increases the heat load of the structure, in simulation, we optimized cooling channels to obtain acceptable frequency detuning. As the results of the design, an accelerating gradient of more than 40 MV/m will be expected, when an input RF power of 80 MW is applied to the structure. In this paper, we report the design of the C-band accelerating structure and its rf properties.  
TUPB006 Stability Performance of the Injector for SACLA/XFEL at SPring-8 486
  • T. Asaka, T. Hasegawa, T. Inagaki, H. Maesaka, T. Ohshima, Y. Otake, S. Takahashi, K. Togawa
    RIKEN SPring-8 Center, Sayo-cho, Sayo-gun, Hyogo, Japan
  To realize the SACLA, it is necessary to obtain stabilities of 10-4 and 50 fs in the amplitude and time of an acceleration voltage, respectively. The achievement of the rf stabilities were almost satisfactory for the target values. Consequently, the 7 GeV beam energy stability was 0.02% (std.) or less. However, there was XFEL power variation caused by a variation of a beam position in a 40 MeV injector section. A periodically changed beam position of 40 μm (std.) was found out at a cycle of 2 s by Fourier transform method using BPM data. The temperatures of all the injector rf cavities are controlled within 28±0.04˚C by a controller using the cooling water. The AC power supplies of the controller to heat the cooling water are operated at 0.5 Hz by pulse width modulation control with alternatively turning on or off. The strong correlation between laser intensity variation and the modulation frequency of the AC power supplies was found out. We are planning to improve the cavity temperature variation in the order of less than 0.01˚C with DC power supplies to establish continuously regulated the cavity temperature. This plan will reduce the XFEL power variation.