Measurements of the APS Storage Ring Beam Stability at 225 mA

Abstract

The Advanced Photon Source (APS) is a third‐generation hard x‐ray user facility that operates nominally at 100‐mA stored beam current. As part of our probing of future operation regimes we have run at 130 mA for a user run, and we are on the path to test operations at up to 300 mA. Our most recent experiments have been at 225 mA in December 2003. One sector in the ring is dedicated to diagnosing the electron beam transverse and longitudinal parameters. We use a combination of imaging tools, including an x‐ray pinhole camera and optical streak camera for the bending magnet source and x‐ray monochromator measurements of divergence on the insertion device source. The onset of longitudinal instabilities due to rf cavity higher‐order modes (HOMs) is one of the limitations in going to higher currents. These effects are seen in the horizontal plane of the pinhole camera images since the dipole source is at a dispersive point in the lattice. We observed about a factor of two increase in horizontal beam size and integrated bunch length in the unstable condition. By correlating the size changes with the temperature readouts of the 16 rf cavity cells, we identified the source of the instability. The cavity temperature set points were adjusted to avoid an HOM.