Influence of negative momentum compaction factors on longitudinal beam dynamics in an electron synchrotron

For the development of new synchrotron light sources, new operation modes are considered. For a successful application of these schemes, however, an understanding of the effects involved is essential. The KIT storage ring, Karlsruhe Research Accelerator (KARA), allows the implementation of new operating modes, including tests using a variety of high-performance beam diagnostics devices. Negative momentum compaction optics has been established at KARA at various beam energies. Systematic studies of the resulting beam dynamics showed significant differences to the dynamics at equivalent positive momentum compaction factors. Moreover, a longitudinal instability occurs at low absolute values of the momentum compaction factor, caused by coherent synchrotron radiation, which is the dominant longitudinal impedance at KARA. This work outlines the status of the operation with negative momentum compaction factors at KARA and presents the current-dependent bunch length at negative momentum compaction. Furthermore, the observation of the instability and its dynamics, including significant deviations from threshold predictions, is shown and compared to the equivalent instability at positive momentum compaction.