Reduction and compensation of the transient beam loading effect in a double rf system of synchrotron light sources

Double rf systems are used to lengthen the beam bunches in synchrotron light sources. In such a system, the performance of the bunch lengthening is limited by the transient beam-loading effect, which is induced by gaps in the fill pattern. To improve its performance, we investigate an application of a normal-conducting harmonic cavity, which is based on the TM020 resonant mode. By using the TM020 mode with low R/Q and high Q, fluctuation of the rf voltage due to the transient beam loading can be reduced significantly. The remaining small fluctuation of the rf voltage can be compensated by using an active feedforward technique. Using these measures, we expect to realize a bunch-lengthening performance that is comparable to that obtained with superconducting cavities under realistic operational parameters of a proposed 3-GeV next-generation light source. We estimate the bunch-lengthening performances using macroparticle tracking simulations together with semianalytical calculations.