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Measurement of the longitudinal microwave instability in the APS storage ring

We studied a longitudinal, single-bunch instability in the Advanced Photon Source (APS) storage ring. Previous measurements showed significant bunch lengthening without a comparable growth in energy spread. Because the small dispersion at a bending magnet source point made it difficult to resolve the small change in the energy spread, we adjusted the lattice to make a dispersion bump within a sector, providing a large dispersion at the source point. The enhanced component from dispersion allowed us to measure the energy spread with greater accuracy. We observed anomalous growth in the energy spread beyond a threshold current of 7.2 mA with an rf voltage of 9.4 MV. The energy spread increased from 9.0E-4 to 9.8E-4 above the threshold current up to 10 mA. We also observed the appearance of the coherent signal at four times the synchrotron frequency beyond the threshold current, which indicates that the growth in energy spread is caused by a longitudinal instability.