Design and test of double PI-control loop to generate flat-top output of RF pulse compressor

Abstract

BackgroundShanghai Soft X-ray Free-electron Laser (SXFEL) is upgrading and requires to operate in double-bunch mode instead of original mode to transport uniform energy along the bunch chain.PurposeThis study aims to satisfy the requirement of double-bunch mode by designing and implementing low level radio frequency (LLRF) system to flatten the output of RF pulse compressor through amplitude modulation.MethodsBased on double PI-control loops, the LLRF control system was designed and implemented with one PI-feedback loop for keeping the long-term stability of amplitude and phase, and the additional operating-outside-LLRF PI-feedback loop for flattening the output pulse shape. Furthermore, an experimental setup, including the compressor, was built for the verification of the above method.ResultsThe 2-h continuous test results show that the double PI-control loops can flatten and keep the compressor output, and maintain the long-term stability of amplitude and phase within 0.09% and 0.1° (Root Mean Square, RMS) respectively. In addition, the algorithm can restore the flat-top output automatically when a large manual frequency offset occurs.ConclusionsThe double PI-control loop applied to the LLRF system preserves satisfactory static and dynamic behaviors, meeting the requirement of the SXFEL upgrade.

Keywords

• low level radio frequency
• rf pulse compressor
• pi control
• amplitude modulation