title: Modeling and Optimization of Barrier-Bucket RF Systems
Creators: name: Schweickhardt, Jens
Publisher: name: UNSPECIFIED
Year: 2021

description: Particle accelerators are machines designed to accelerate charged particles to high energies using electromagnetic fields. Besides a variety of industrial and medical applications, they are indispensable tools in multiple fields of basic research, as experiments using accelerated particles allow to investigate fundamental physical interactions. In order to achieve certain beam parameters, such as energy, intensity and temperature, necessary to reach uncharted research areas, a complex interplay between different accelerators and storage rings is necessary. In some cases, this interplay requires the ability to flexibly control the length of the accelerated particle beams. For some experiments planned at GSI (Helmholtz center for heavy ion research) and the future accelerator complex FAIR (Facility for Antiproton and Ion Research), both located in Darmstadt, Germany, barrier-bucket radio frequency systems are required, as they make it possible to almost freely control the longitudinal distribution of charged particle beams by generating pulsed electric fields. Due to the high complexity of barrier-bucket systems combined with the high demands on the quality of the output pulses, the system design as well as the input signal computation still pose large challenges. For the barrier-bucket system recently installed inside the ESR (Experimental Storage Ring) at GSI, different strategies to tackle these challenges have been developed during the work on this thesis. It is shown that the frequency bandwidth of barrier-bucket systems is - as expected - a crucial factor for the accessible output signal quality. Since the behaviour in the demanded frequency range is determined by the properties of different system components and their intricate mutual coupling, simulation tools are necessary for a reliable design process, especially for the design of the cavity. Therefore, an equivalent-circuit model is explored for the barrier-bucket cavity, enabling the adaption of different cavity components during the design process. The model allows to accurately simulate the future cavity characteristics and to investigate the coupling between cavity and power amplifier stage. Even for systems with sufficiently high frequency bandwidth, frequency-dependent properties of different system components as well as saturation effects in the power amplifier lead to high output signal distortion, resulting in undesired impact on the particle beam. In order to counteract these distortions, different strategies are investigated to compute the required input signal for the desired output pulses, based on different black-box models and dedicated identification processes. Additionally, different technical concepts of generating the computed input signals and controlling amplitude and timing as required for the planned experiments are analyzed. Lastly, the results of a machine experiment performed to demonstrate the functionality of the developed ESR barrier-bucket system are presented. During the experiment, a particle beam of 56Fe25+ ions was successfully captured between two barrier pulses and compressed and de-compressed afterwards. The analysis of the RF output signals shows that the output signal quality is maintained during the RF manipulations and that both, amplitude and phase jitter are sufficiently small to have negligible impact on the beam quality. Even though the computations and models in this thesis were focused on the ESR barrier-bucket system in the first instance, they are widely applicable to other barrier-bucket systems. Specifically, they are also well suited to improve the output signal quality of existing barrier-bucket systems and to support the development of future barrier-bucket systems such as the SIS100 barrier-bucket system currently under development for FAIR.