LLRF Control Techniques for the European XFEL Continuous Wave Upgrade

In this doctoral thesis, possible solutions for superconducting cavity control systems operating in the Continuous Wave(CW) regime are discussed and analyzed. The paper focuses on the requirements for adding a CW mode of operationto European XFEL alongside the existing pulsed mode of operation. The main challenge from a control system point of view stems from the need to use a loaded quality factor (QL) in the range of 10e7-10e8. Such a (QL) is an order of magnitude greater than the actual value that is used in the pulsed mode of operation. As aresult, the cavity tune is more sensitive to microphonic effects. For this reason, active noise compensation techniques are required in order to limit the overall power consumption of the accelerating system, and to increase the energy stability of the accelerated beam. Another effect present in the future upgrade are ponderomotive instabilities. These instabilities result from the nonlinear coupling between the Lorentz force-induced detuning and the cavity acceleration field. These in-stabilities are then analyzed and a possible compensation scheme is proposed. In order to obtain a precise estimate of the cavity detuning, a real-time detuning estimator has been developed. This component, necessary for resonance control systems, is also used for the estimation of the cavity quality factor in order to realize a quench detection system. The last topic is about the linearization of radio frequency amplifiers based on vacuum tubes used to produce power the superconducting cavities. The reason for doing a linearization of these amplifiers is the improvement of the field stability and the simplification of setup procedures of the RF control system. For this study, Klystrons and Induction Output Tube (IOT) amplifiers were used