Bunch-by-bunch measurements of the betatron tune and the synchronous phase and their applications to beam dynamics at KEKB

Abstract

The betatron tune and the synchronous phase of individual bunches were measured at KEKB, an asymmetric electron/positron circular collider. The measurement was performed under three conditions: a single bunch, and multibunches with and without collision. The coupling impedance of the rings can be estimated from the current dependence of the parameters of a single bunch. The betatron tune along a positron-bunch train indicates variations of the electron cloud density that cause electron cloud instability. Comparing the betatron tune between colliding and noncolliding bunches, the beam-beam tune shift was investigated in regular operation. The coherent beam-beam parameter was evaluated in the two-ring collider with unequal tunes. Transient beam-loading effects were experimentally demonstrated by a phase shift along a bunch train in both rings. A rapid advance in the beam phase was observed, which is considered to be due to the effects of longitudinal wakes.