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Superconducting intermediate-velocity drift-tube cavities for the RIA driver linac

This paper reports the status of prototypes for two types of niobium superconducting drift-tube cavity. The structures are intended for use in the ion-linac driver-accelerator for the proposed Rare Isotope Accelerator (RIA) facility, and cover an intermediate range of particle velocities, 0.18 < beta < 0.5. Both structures fit within a cylindrical housing of roughly 18 inch diameter and 15 inch length, and incorporate two drift tubes to define the accelerating field. The cavity types are a 172.5 MHz lollipop-loaded cavity for (peak) beta = v/c = 0.19, and a 345 MHz spoke-loaded cavity for beta = 0.4. The results of numerical electromagnetic and mechanical simulations will be presented and several evolving design features discussed. These include, for example, a design for an integral stainless-steel helium jacket which provides for very good frequency stability against changes in helium pressure. We also discuss the reduction of beam steering effects using small, asymmetric changes in the drift-tube geometry.