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- H. Maesaka, T. Asaka, T. Hara, T. Hasegawa, T. Inagaki, T. Ohshima, Y. Otake, H. Tanaka, K. Togawa
RIKEN SPring-8 Center, Sayo-cho, Sayo-gun, Hyogo, Japan
- T. Hasegawa, Y. Kano, T. Morinaga, Y. Tajiri, S. Tanaka, R. Yamamoto
SES, Hyogo-pref., Japan
- S. Matsubara
JASRI/SPring-8, Hyogo, Japan
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The XFEL facility, SACLA, achieved first x-ray lasing in June 2011 and started public user operation in March 2012. In the early days after the first x-ray lasing, large drift of FEL intensity was observed and the period of FEL lasing condition to keep within acceptable intensity variation was only about an hour. We found that this short period mainly came from drifts of the rf phases and amplitudes of sub-harmonic buncher cavities and accelerator cavities in an injector section (238, 476, 1428, 5712 MHz). These rf drifts caused drifts of a peak current, a beam energy and a beam trajectory. As a result, the FEL gain was significantly degraded. Since the rf field in the cavity had a strong correlation with the cavity temperature, we improved a cavity temperature regulation system by a factor of 2 or 3 and the temperature stability was reduced to be 0.08 K peak-to-peak. In addition, we introduced an energy feedback loop for a C-band main accelerator and an orbit feedback loop for an undulator beamline. After these improvements, the FEL intensity was maintained within 10% for longer than a day.
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