Observations of “effective” transverse beam-size instabilities for a high current per bunch fill pattern in the APS storage ring

The X-ray pinhole camera diagnostics on the Advanced Photon Source (APS) storage ring have recorded an “effective” transverse beam size instability during operations with a sextuplet plus 22 singlets fill pattern. These instabilities were not observed with the sextuplet plus 25 triplets fill pattern that has been the standard fill pattern in FY´98. The instability threshold is at 82-85 mA with positrons. The features include an increased average (few seconds) transverse size both horizontally anti vertically for stored currents above the threshold with a correlated effect on the beam lifetime. The horizontal transverse emittance is 25-30% larger at 100 mA than below the threshold. There is a related horizontal beam centroid motion as well, but this does not explain the vertical size change nor the lifetime effect. Complementary data were also taken with the diagnostic undulator, and a similar threshold effect on divergence was observed. The cross-comparison of the data and possible mechanisms are presented