Combined disturbance-observer-based control and iterative learning control design for pulsed superconducting radio frequency cavities

The development of iterative learning control combined with disturbance-observer-based (DOB) control for the digital low-level radio frequency (LLRF) system of the International Linear Collider project is presented. The objective of this study is to compensate for both repetitive (or predictable) and unpredictable disturbances in a radio frequency system (e.g., beam loading, Lorentz force detuning, and microphonics). The DOB control approach was verified using the LLRF system at the Superconducting Test Facility (STF) at KEK in the absence of a beam. The method comprising DOB control combined with an iterative learning control algorithm was then demonstrated in a cavity-simulator-based test bench, where a simulated beam was available. The results showed that the performance of the LLRF system was improved, as expected by this combined control approach. We plan to further generalize this approach to LLRF systems at the STF and the future International Linear Collider project.