Numerical investigations of single bunch collective instabilities in synchrotron storage ring

Thesis

Electron beam traversing in vacuum chamber can be severely affected by collective instabilities generated from an interaction of beam with its surrounding, geometric wakefield, and the synchrotron radiation emitted, radiation wakefield. The beam dynamics is significantly influenced by such beam-wakefield interactions eventually limiting the maximum stored current. Especially in short bunch consideration which is a primary criterion for the generation of coherent synchrotron radiation (CSR), the beam instability is strongly amplified by an enhanced wakefield. The understanding of beam intensity limitation mechanism has still been underdeveloped. Therefore, the dynamical development of a bunch with a presence of collective instability needs to be scrutinized.

A numerical evaluation tool, sbtrack, is developed to be able to calculate the collective effects due to both radiation and geometric wakefields. The impedance characterization with Haissinski equation is also proposed and evidently shows that a broad-band impedance model is able to reproduce most features of intensity dependent bunch development under particular wakefield. Evaluation of longitudinal beam dynamic verifies that each kind of wakefield contributes characteristic effect to the bunch development and essentially influences resulting CSR bursting behaviour. In positive alpha, the bunch transforms from a confined barred-spiral shape to sawtooth behaviour while in negative alpha, spur-like local fluctuation is induced on the bunch phase space and enhancing the semi-chaotic fluctuation of the entire bunch when the chaotic threshold is exceeded. Beyond the chaotic threshold, bunch profile of both alpha cases are found to evolve with similar systematic development. However, such mechanism in negative alpha seems to be able to consistently occurred at higher current than positive alpha. Unexpectedly, the temporal occurrence of chaotic-harmonic evolution of a bunch is numerically discovered in negative alpha. The bunch appears to be recovered to the state of steady CSR bursting even when it has already exceeded the chaotic threshold. This phenomenon is likely suggesting a novel technique on the generation of THz CSR at much higher current than any conventional methods.

Authors: Puntree, J

• Type of award: MSc by Research
• Level of award: Masters
• Awarding institution: University of Oxford
• Keywords: beam instability
• Subjects: Particle accelerators