Influence of transient beam loading on the longitudinal beam dynamics at BESSY VSR

BESSY VSR, a scheme where 1.5 ps and 15 ps long bunches (rms) can be stored simultaneously in the BESSY II storage ring has recently been proposed[1]. The strong longitudinal bunch focusing is achieved by superconducting high gradient RF cavities. If the fill pattern exhibits a significant inhomogeneity, e.g. due to gaps, transient beam loading causes a distortion of the longitudinal phase space which is different for each bunch. The result are variations along the fill pattern in synchronous phase, synchrotron frequency and bunch shape. This paper presents investigations of transient beam loading and depicts the consequences on bunch length, phase stability and longitudinal multi-bunch oscillations for the projected setup of BESSY VSR.