Studies of beam impedance with related beam stability and loss effects on a high intensity proton synchrotron

Thesis

Neutrons have been used to probe the structure of matter for applications as wideranging as fundamental magnetism and archaeological investigation, and the particle accelerators which drive short-pulsed neutron scattering facilities are pushing the limits of beam intensity. For future machines to operate safely and effectively, they must rise to the challenge of reducing beam-loss whilst simultaneously increasing intensity.

Whilst beam-loss can be driven by a variety of processes, losses due to coherent transverse beam instabilities are typically expected to worsen as beam intensity increases. One such instability currently limits the operational performance of the ISIS Neutron and Muon Source, and similar instabilities have been observed on at least three other facilities. In order to limit the impact of these instabilities on future machines, it is necessary to understand and mitigate them on those that are currently operating. This thesis will focus on the head-tail instability at ISIS, which was first observed some decades ago but is still not well understood.

Fundamental concepts of beam instabilities will first be reviewed, and then used to identify key requirements for the study. A transverse driving impedance model will be developed, guided by earlier studies to seek-out sources of a low frequency narrowband impedance. Computational tools will be developed to produce this model; taking the form of a new resistive-wall impedance calculation code, and a technique to apply commercial finite-element software to this application.

A new semi-analytical code, based on well established theory, will be developed to compute the properties of beam instabilities. It will be benchmarked against particle tracking simulations and applied to the case of ISIS operating in Rapid Cycling Synchrotron mode and Bunched Storage Ring mode, with the newly developed impedance model. Predictions will be compared with experimental observations, and comments made as to the applicability of this theory to ISIS, despite neglecting space charge.

Conclusions will be drawn on whether the newly developed impedance model can explain the head-tail instability, and observations made in earlier studies. Modifications to key impedance sources will be discussed, with the aim of improving beam stability on ISIS and guiding future equipment designs. Opportunities for future study will also be briefly discussed.

Authors: Posthuma De Boer, DW

• DOI: 10.5287/ora-ppajjqz29
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: RF Shield; Vlasov; instability; RF Screen; resistive wall; Beam Coupling Impedance; head-tail; TMCI; transverse impedance; ISIS
• Subjects: Particle accelerators; Particle beam instabilities; High Intensity Proton Beams