Studies of beam instabilities and associated beam-loss on a high-intensity proton synchroton

Thesis

High-intensity hadron accelerators provide beams for many different applications around the world: from high energy particle physics to drivers of spallation neutron sources such as ISIS. These facilities face a common, significant challenge: the control and mitigation of beam-loss, often defining the limit to the beam intensity achievable.

Beam-loss may be driven by many different processes including transitions between accelerators, RF capture and non-linearities in the accelerator’s magnetic lattice. Collective effects, such as space charge and beam instabilities, may also be a major source of beam-loss, particularly in accelerators pushing for higher intensities. The interaction of space charge with beam instabilities is a key topic of current research and development.

The head-tail instability drives beam-loss on a number of high intensity hadron accelerators around the world. Measurements of the instability are shown from the ISIS Neutron and Muon Source at the Rutherford Appleton Laboratory in the UK. These are shown in normal, Rapid Cycling Synchrotron, operation as well as in a new operating mode: bunched beams stored at injection energy. The new mode is utilised to explore the instability on ISIS systematically, with a simplified experimental setup for comparison with established theory, as a function of the vertical betatron tune and the vertical beam size.

Particle-in-cell tracking simulations of ISIS in the new operating mode are performed, varying the associated beam and accelerator parameters, and the results compared to observed instability characteristics. Further simulations include a self-consistent model of the beam’s transverse space charge field, allowing analyses of its effect on the instability characteristics.

Experimental and simulation results are compared with predictions derived from a well established theory of head-tail dynamics which does not include the effects of space charge. Recent theories including these effects are summarised and the results discussed in the light of those theories.

Key agreements and discrepancies between experiment, simulation and theory are presented together with directions for future research. Finally, the impact of this research on the operation of current and future high intensity hadron accelerators is discussed with particular focus on current ISIS operations.

Authors: Williamson, R

• DOI: 10.5287/ora-6gak4za6b
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: beam; ISIS; space charge; instability; proton; beam-loss; head-tail; RAL; accelerator
• Subjects: Proton accelerators; Particle beam instabilities; Space charge