Cavity Field Control for High-Intensity Linear Proton Accelerators

Abstract: The European Spallation Source will, once fully operational in 2025, be the world's brightest neutron source. The neutrons will be generated by bombarding a tungsten target with protons accelerated to 96\% the speed of light by electromagnetic fields confined in 155 radio-frequency cavities along the world's most powerful linear accelerator.This thesis has been motivated by the strict control specifications on the amplitudes and phases of the accelerating electromagnetic fields. By considering the field control problem from an automatic control perspective, the thesis aims at improving the understanding of the problem and to explain important aspects of the control design.Throughout the thesis it has been helpful to model the cavity and RF system by complex-coefficient single-input single-output systems. The complex-coefficient representation was particularly useful for discussing: (1) the control design for cavities with parasitic resonance modes; (2) the effect of loop phase variations on feedback stability; (3) the directionality of the disturbances and the objective function.The thesis presents a non-standard parametrization and derivation of the cavity field dynamics that make it easier to relate the physical cavity process to the model, and simplifies the understanding of how the cavity parameters affect the achievable control performance.The control performance of simple PI(D)-controllers and general linear time-invariant controller was compared using the Youla parametrization and convex optimization; it was found that PI(D)-controllers in many cases achieved performance similar to the more general linear time-invariant controller; this indicates that simple PI(D)-controllers in many cases are an excellent choice for cavity field control.Lastly, the energy-optimal strategy to build up the electromagnetic cavity fields is derived, allowing the sustainability of the European Spallation Source to be further improved.