DESIGN OF A C-BAND DISK-LOADED TYPE ACCELERATING STRUCTURE FOR A HIGHER PULSE REPETITION RATE IN THE SACLA ACCELERATOR

It is expected that the high pulse-repetition rate of the SACLA accelerator provide 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 the higher pulse rate above 120 pps than the present rate of 60 pps. Since the higher repetition rate operation is inclined to increase vacuum electrical discharges, it is required to reduce the surface electric fields in the structure without decrease in accelerating voltage. We designed a cross sectional shape of ellipsoidal curvature, which reduces the maximum surface electric or rf field by 20%. The designed structure adopts a TM01-2π/3 mode disk-loaded type with a quasi-constant gradient. Since the high repetition rate also increases the heat load to the structure, in simulation, we optimized cooling channels. As a result of the design, an accelerating gradient of more than 40 MV/m will be expected, when an input RF power of 80 MW is fed into the structure.