Emittance Reduction via Dynamic RF Frequency Shift at the SLC Damping Rings

The generation of small emittance beams is a key issue determining the luminosity at a future linear collider. Dedicated damping rings have been designed to produce such beams, but techniques to further reduce the design emittances would yield immediate improvements in terms of higher luminosity. At injection into the damping rings, the transverse beam emittances are large and the beam fills a large fraction of the dynamic aperture. At later times during the store, after the beam has radiation damped, the horizontal damping time and equilibrium emittance may be reduced by shifting the rf frequency, such that the particle orbit moves inwards. By passing off-center through the quadrupoles in regions of nonzero dispersion, the horizontal partition number Jx is changed. This reduces both the horizontal damping time and equilibrium emittance. In addition, to the extent that the vertical emittance is determined by betatron coupling, the reduction in horizontal emittance may be accompanied by a corresponding reduction in vertical emittance. We report experience with such a scheme at the Stanford Linear Collider damping rings.