Studies of Higher Order Mode Couplers for the Upgraded Travelling Wave Acceleration System in the CERN SPS

Future high energy particle physics research based on accelerators requires high beam intensity. Parasitic interaction of the charged particle beam with its environment can deteriorate the beam quality and limit the peak intensity for which safe machine operation can be ensured. The beam induces electromagnetic fields in the accelerator components surrounding it which in turn affect the particle motion and can lead to beam instabilities. This interaction is typically described by the concept of beam coupling impedance. At CERN, the Large Hadron Collider is at the end of an accelerator chain in which particles are accelerated to ever higher energies in transferring the beam from one accelerator to the next. This pre-accelerator chain is presently upgraded to ensure delivery of the demanding high intensity proton beam required by the Large Hadron Collider in the future. The Super Proton Synchrotron is the last pre-accelerator in the chain and utilises a travelling wave system operating at a fundamental frequency of 200MHz for acceleration to a proton energy of 450GeV. The travelling wave structures of this broadband acceleration system are cylindrical waveguides periodically loaded with drift-tubes and stems. One main obstacle for higher beam intensity in the Super Proton Synchrotron is presently a multi-bunch instability which is triggered by higher order standing wave modes around 630MHz in the accelerating structures. The beam coupling impedance of these modes is already heavily damped by resistive higher order mode couplers, but particle-tracking simulations suggest that additional impedance mitigation by a factor of three in the ideal case can cure the instability for the beam intensity required in the future. The goal of this thesis is to properly identify and characterise the deteriorating higher order modes and to significantly improve on the present impedance mitigation of these modes. The improvement is achieved most notably by a revision of the existing HOM coupler and a newly developed mitigation technique that consists of subtle changes in the accelerating structure by the introduction of posts which are resonant at 630MHz. The practical feasibility of the proposed improvements is imperative and the impact on the fundamental accelerating mode must stay within acceptable limits. In this regard, a broad understanding of the acceleration system, which was implemented more than four decades ago, is required and is therefore also developed throughout this thesis. The proposed solutions for achieving the increased damping are developed by theoretical analysis together with electromagnetic simulations and their performance is validated by RF measurements. Measurements are also used to quantify in unprecedented detail the impact of the numerous higher order mode mitigation devices on the available fundamental accelerating voltage. Thus, this thesis represents a significant contribution to the future operation of the Super Proton Synchrotron and to the numerous efforts that ensure the long-term physics performance of the Large Hadron Collider and its experiments at CERN.