Streak Camera Studies of Vertical Synchro-Betatron-Coupled Beam Motion in the APS Storage Ring

We present experimental studies of synchro-betatron-coupled electron beam motion in the Advanced Photon Source (APS) storage ring. We used a vertical kicker to start the beam motion. When the vertical chromaticity is nonzero, electrons with different initial synchrotron phases have slightly different betatron frequencies from the synchronous particle, resulting in a dramatic progression of bunch-shape distortion. Depending on the chromaticity and the time following the kick, images ranging from a simple tilt in the bunch to more complicated twists and bends are seen with a visible light streak camera. We found that most of the experimental observations can be described by the synchro-betatron-coupled equations of motion. Also note that the fast apparent increase in vertical beam size after the kick is dominated by the internal synchro-betatron-coupled motion of the electron bunch. Experimentally this increase could be easily confused with decoherence of vertical motion if the bunch is only imaged head-on.