Compact 110 MW modulator for C-band high gradient accelerator

The C-band high gradient accelerator of SACLA is driven by 72 units of 50 MW klystrons and 110 MW modulators. They are tightly placed in each 4 m space intervals in a 400 m-long klystron gallery. Therefore the modulator has to be very compact and reliable. SACLA is the X-ray free electron laser (XFEL) facility, which requests very tight tolerances of 100 ppm on amplitude and 0.2 degree on phase of the accelerating field. Then the voltage fluctuation and timing jitter of the high-voltage pulse on the klystron cathode should be minimized. In order to meet these demands, we developed a compact, reliable, and stable modulator, which is a conventional linetype modulator with a novel architecture design. All the highvoltage components, including a PFN circuit, a thyratron tube, a pulse transformer, and reverse protection circuits, were installed in a compact steel tank, filled with insulation oil. This design provides good EM noise-shield performance, and superior operational stability. Timing jitter of the thyratron is less than 1 ns, and flatness of the high-voltage pulse is 2%, which provides acceptable effect on the RF phase and amplitude fluctuations. In order to stabilize the charging voltage of PFN capacitors, we developed an extremely stable inverter-type high-voltage charger. The pulse-to-pulse stability of the charging voltage is about 10 ppm in standard deviation, which leads sufficient stability on the accelerating field. Since 2011, 72 modulators have been operated in 8000–9000 hours, to provide a stable electron beam for XFEL. Thus the technology of the innovatively compact and stable modulator system was established.