LLRF Controls and Feedback: Free-Electron Lasers and Energy Recovery Linacs

Abstract

The low-level radio frequency (LLRF) system generates the drive sent to the high-power components. In this paper, we give the basics of LLRF controls and feedback. This includes a brief introduction to the two concepts for this school, i.e., the energy recovery linac and the free-electron laser, and their main differences, from a LLRF perspective. The physical behaviour of the subcomponents (detector, amplifier, cavity, etc.) within an LLRF feedback loop is outlined, together with its system modelling in the time and frequency domains. System identification using special input–output signals is introduced. Furthermore, stability checks for the closed-loop operation and the feedback controller design in the time and frequency domains are briefly discussed. This paper concludes with some examples for system identification, its feedback design, and the achieved stability at different facilities, showing the need for LLRF control and feedback eration.