Observations of bunch-lengthening effects in the APS 7-GeV storage ring

Measurements of the bunch length and horizontal beam size at a bending magnet source point in the lattice versus single-bunch current have been done on the Advanced Photon Source (APS) 7-GeV storage ring. These data are relevant to issues (limits) of obtaining higher volume charge densities for storage-ring-based FELs. Bunch lengths from στ ∼ 25 to 70 ps were measured using a Hamamatsu C5680 dual-sweep streak camera. Additional complementary data on energy spread deduced from horizontal beam size at a dispersive point in the lattice were also tracked versus single-bunch current. Both optical synchrotron radiation (OSR) and X-ray synchrotron radiation (XSR) techniques were used. The significant bunch lengthening observed without a comparable horizontal size change (ΔE growth) is consistent with the potential-well distortion model rather than the predictions of a microwave instability calculation. With higher RF gap voltage, peak currents up to 400 A were observed.