Design challenges for high current storage rings

Abstract

High luminosity e+ e -and pp colliders stand to gain from the use of superconducting RF systems (SRF).To achieve luminosities> 3 x 10 33 in a B-factory, 1 -2 amps of current must be stored in several 100 bunches, spaced a few meters apart.Similarly the LHC p-p ring aims to store rv 1 amp in nearly 5000 bunches, spaced 5 meters apart.Such high currents make it imperative to lower the impedance of the rings, of which RF cavities are a major source.The tight bunch spacing makes control of multibunch instabilities one of the most important issues.The advantage of using SRF cavities for high luminosity machines is that they can economically provide higher gradients than copper cavities.Hence, by minimizing the structure length, they can sizably reduce the machine impedance.In superconducting cavities, the need to use small beam holes and re-entrant iris shapes to maximize the shunt impedance of the accelerating mode is eliminated by virtue of the high Q.SRF cavity shapes can be chosen to tolerate reduced impedance in the accelerating mode, while substantially cutting the impedance of all the higher order modes.As an added bonus, capital and operating cost savings can be realized from the reduced RF installation which no longer needs to provide for RF power dissipation in the cavity walls.To operate in future high current machines, SRF cavities developed for the present generatioan of e+ e -storage rings need to be advanced in several respects, in particular their input and output power handling capabilities, and the damping of higher order modes.Progress in meeting these challenges in the context of specific application of SRF to the Cornell B-factory, CESR-B is discussed.