Emittance growth due to decoherence and wakefields

Suddenly induced coherent centroid oscillations about the closed orbit will decohere due to nonlinearities in the magnetic optics-at the expense of a stored beam's emittance. Collective effects mediated by the vacuum chamber wakefield and dependent on the beam current, can however damp the coherent oscillations-ameliorating the emittance growth. Closed form expressions for both the beam centroid and the beam size are obtained in the absence of collective effects. Simultaneous turn-by-turn measurements of beam centroid and size in the SLC damping ring are presented, and the importance and intricacy of collective effects is discussed.