The Interaction between a Beam and a Superconducting Cavity Module: Measurements in CESR and CESR-Phase III Goals*

Plans for the next generation of electron-positron colliders (B-factories and B-factory-like machines) call for high beam currents to produce luminosities of the order of 1033. To store these high currents in a machine, special attention must be paid to the interaction of the beam with discontinuities in the surrounding vacuum chamber. RF cavities are among the biggest perturbations in accelerator vacuum chambers and are therefore among the biggest sources of beam instabilities. Accelerating structures for new machines are being designed to have smaller impedance to reduce the beam-cavity interaction. Several new designs for both normal and superconducting cavities are now being considered at various laboratories [1]. The phased luminosity upgrade program for CESR calls for a total current of 1A in two beams [2]. The existing normal conducting copper cavities are to be replaced with superconducting niobium cavities. Quality factors of less than 100 are required for the dangerous cavity higher-order modes (HOMs) [3, 4]. Figure 1 shows a schematic of the entire module which includes the cavity, a 24 cm round beam pipe, a fluted beam pipe, two ferrite HOM loads, sliding joints, gate valves, and tapers to the CESR beam pipe. The beam tubes were designed so that all of the HOMs propagate out of the cavity and are damped by the ferrite HOM loads, which are located outside the cryostat and which are an integral part of the beam tube. Prototypes for the cavity, input coupler, cryostat, and HOM loads were subjected to a beam test in CESR in August 1994. Figures 2 4 show photographs of the HOM load, cavity, and cryostat being installed in CESR. A superconducting (SRF) cavity was installed in addition to the four five-cell normal conducting (NRF) cavities. The results of the test have been reported at the PAC’95 conference [5-7]. In this paper we review the results from the perspective of the CESR Phase III upgrade plan.