Study of Electron Beam Instabilities in the Storage Ring at the Canadian Light Source Using the Transverse Feedback System

The Canadian Light Source (CLS) 2.9 GeV electron storage ring circulates hundreds of approximately 1 nC bunches of charge in an electromagnetic trap. The oscillatory motion of these electron bunches is coupled through the electromagnetic interaction with the vacuum chamber in which they circulate. If this motion is left uncontrolled, this coupling can result in unstable motion. These so called coupled bunch instabilities can lead to the beam size enlarging or worst case beam loss. Consequently, mitigation strategies against these coupled bunch instabilities have become a critical element of modern synchrotron design. As part of their mitigation strategy, the CLS utilizes a Transverse Feedback System (TFBS), to identify and correct against these instabilities via active damping where they arise. The TFBS also doubles as a diagnostic tool, enabling the study of these coupled bunch instabilities. This research project studied the properties of the coupled bunch instabilities in the CLS storage ring using simulations and experiments. Experiments have been performed using the TFBS to measure the exponential damping rates of the induced beam oscillations. To study these beam instabilities, experiments were largely done via adjustment of in-vacuum insertion device gap heights, which changes the vertical vacuum chamber profile height. Early experiments focused on grow-damp methods, where the feedback loop is briefly disabled to allow instabilities to grow before being damped by the system. To yield new results, later experiments involved excite-damp methods, where the beam is deliberately excited to study damping rates. To better understand and compare against experiment results, eigenmode and equivalent-circuit simulations of the Brockhouse beamline’s in-vacuum wiggler insertion device have been performed. Characterizing and controlling the instabilities found in this project will be a limiting factor for higher storage ring beam current or the addition of new insertion devices at the CLS in the future.