Higher harmonic cavities at 3rd generation synchrotron light sources

University dissertation from MAX-lab, Lund University

Abstract: Synchrotron radiation is a tool widely used in science. It is produced by electron accelerators called storage rings. With the high performance of the 3rd generation synchrotron light sources, the scientific results are excellent. It is important to identify and overcome the limitations of a storage ring in order to enhance its performance. This will enable research facilities to withstand competition and serve science for many years to come. Typical limitations are short electron beam lifetimes and induced energy spreads originating from instabilities. Short beam lifetime is due to the small transverse and longitudinal beam sizes. Increasing the transverse beam size would spoil the small emittance of the electron beam. However, increasing the longitudinal beam size, the bunch length, is an attractive option. This can be realised by adding a passive higher harmonic radio frequency, RF, system. Longitudinal and transverse feedback systems are often installed to damp coupled bunch instabilities and these feedback systems functions efficiently. A higher harmonic cavity also adds Landau damping. This damping can be so strong that it can be an alternative to the feedback systems in some cases. Higher harmonic cavities are therefore also called Landau cavities. By installing a Landau cavity system, two performance limitations can be solved. It lengthens the bunches, and with this the lifetime, and it introduces Landau damping that damps instabilities. In storage rings that already have installed feedback systems Landau cavity systems are useful for lifetime improvement. In this thesis I present the work at four different electron storage rings where Landau cavities were installed: Firstly, Landau cavities were installed at MAX II where they are used both for damping of instabilities and for lifetime enhancement. Secondly, feedback systems were installed at both ALS and Bessy, and the higher harmonic cavities installed were mainly for lifetime improvement. Finally, at Super-ACO, a fifth harmonic cavity, which is usually used for bunch shortening, was instead tuned for bunch lengthening and damping of all instabilities was obtained. I also present a paper where the lifetime increase is studied numerically, with ALS as a model, depending on different modes of operation and if normal conducting or superconducting cavities are used.

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