Simulation and analysis of wake fields and trapped rf modes in insertion device vacuum chambers at the canadian light source

The Canadian Light Source (CLS) synchrotron operates with four in-vacuum insertion devices, three in-vacuum undulators, and one in-vacuum wiggler. Presently, each of the devices occupies half of a straight section. The wiggler is unique in our ring as it is both in-vacuum and shares a straight section with an in-vacuum undulator. We have observed gap dependent beam instabilities in the undulator located in the straight section. To better understand the problem, the cause of the instabilities was investigated using 3D electromagnetic modelling. First, the 'trapped' RF modes (natural resonances) for this undulator chamber, their Q value, and their peak frequencies were analyzed using eigenmode simulation. Secondly, beam excitation of the eigenmodes was simulated with the Wakefield solver. Herein we present the results of this electromagnetic modelling.