Multibunch operation of CESR

The Cornell storage ring, CESR, is designed to collide one bunch of e+ with one bunch of e- at energies up to 8 GeV each, is used almost exclusively for research on b-quark physics (the T region, 4.5 - 5.6 GeV per beam). Here the radiation loss is less than one-quarter the design maximum - low enough to relax beam-current limitations due to total rf power, yet still high enough to provide a comfortable amount of radiation damping, which helps control beam instabilities. To raise the luminosity, CESR has since 1983 operated with multiple bunches in each beam. At the unwanted crossing points within the guide-field arcs, these bunches are separated in the horizontal plane: e+ and e- orbits are oppositely distorted, "pretzel" fashion, by a pair of electrostatic separators near the ends of each arc. The orbits still coincide around the two interaction points, and must do so very accurately of course. We report our progress here, as an example of the considerations applying when two beams are bedded side-by-side in the same aperture, separated by electrostatic steering elements.