Stability improvements of the XFEL intensity at SACLA

For stable user operation of SACLA, an XFEL intensity is necessary to be stable within 10% RMS for a short term jitter and 10% pk-pk for long term drift. In the early stage of XFEL lasing at SACLA, however, the XFEL intensity decreased to about half of the demand, if we did not adjust the accelerator condition. The XFEL intensity is mainly affected by the peak current fluctuation and drift of an electron beam and the demanded peak current is achieved by multi-stage bunch compression system. From a simulation study, the acceleration rf phase stability for the bunch compression stage is 50 fs RMS for short term jitter and a few 100 fs pk-pk for long term drift. However, we found that the rf phase of an injector part had a large drift of 10 ps. Therefore, we improved the temperature stability of the acceleration cavity in the injector from 0.08K pk-pk to less than 0.01K pk-pk by using a further precise temperature regulation system. In addition, the temperature coefficient of low-level rf electronics was reduced. As a result, the long term XFEL intensity drift was reduced to 10% pk-pk level, if an operator sometimes finely adjusted some rf parameters of the injector. (author)