Abstract
Abstract For a low-to-medium energy 3rd generation synchrotron light source, the beam lifetime is dominated by Touschek scattering which can be improved by lengthening the bunch length. Physically, the longitudinal restoring force can be reduced by introducing a harmonic voltage generated by passive harmonic cavities. The gap transient induced by an asymmetric filling pattern of bunch train can cause a considerable shift of synchronous phase that result in a huge variation of the harmonic voltage along the bunch train. Therefore, the bunch lengthening as well as the lifetime improvement is greatly degraded owing to deviation of the harmonic voltage from its optimal target in a degree strongly dependent on the size of filling gap. In this thesis, a tracking code is developed which reproduces the simulation results of previous studies. An analytic model, under assumption of stationary circulation of bunch trains around the storage ring, is conceived benefiting from the tracking code as a test bench for probing the degradation mechanism of bunch lengthening. The analytic model explain that the amplification of synchronous phase shift by implementing the passive harmonic cavity with an asymmetric filling pattern of bunch train reflects a competition between the variation of beam induced harmonic voltage owing to transient beam loading and the required shift of synchronous beam phase to gain opposite amount of rf generator voltage, in addition for maintaining the energy balance of individual stationary bunches along the bunch train.