Stretched bunch shapes in the NSLS VUV ring

For bunches stretched with a higher-harmonic cavity, longitudinal bunch shapes are influenced by small variations in the potential wells of the buckets of the bunch train. This paper discusses three things that perturb the bunch shapes in the NSLS VUV ring: the -90° (non-optimal) phase of the harmonic cavity maintained by the RF controls [1], higher-order modes (HOM) of the main cavity, and the effect of unfilled buckets on the RF fields in the accelerating modes of the two cavities. The suppression of the lowest-lying HOM in an experiment using RF feedback applied to two HOM damping-probe ports is described and results of calculations of the effect of unfilled buckets are given. Measured bunch shapes are shown and discussed