RF focusing methods for heavy ions in low energy accelerators

A new type of axially symmetric RF field focusing (ARF) in a drift-tube accelerating structure is proposed and studied. Beam stability in both transverse and longitudinal directions is achieved by the variation of drift tube aperture and appropriate phasing. An analysis of 3D beam dynamics is performed using the smooth approximation technique. Beam dynamics in an APF structure and in a conventional RFQ are compared. A universal method is suggested to choose a voltage V and aperture in the APF structure designed for heavy-ions with charge-to-mass ratio q/A ≥ 1/66 in the energy range above 20 keV/u. The proposed structure and analyzing technique can be useful in the design of a post-accelerator for the Rare Isotope Accelerator Facility being designed in the U.S.