Design of 57.5 MHz cw RFQ for medium energy heavy ion superconducting linac

The nuclear science community considers the construction of the Rare Isotope Accelerator (RIA) facility as a top priority. The RIA includes a 1.4 GV superconducting linac for production of 400 kW cw heavy ion beams. The initial acceleration of heavy ions delivered from an electron cyclotron resonance ion source can be effectively performed by a 57.5 MHz 4-m long room temperature RFQ. The principal specifications of the RFQ are (i) formation of extremely low longitudinal emittance, (ii) stable operation over a wide range of voltage for acceleration of various ion species needed for RIA operation, and (iii) simultaneous acceleration of two-charge states of uranium ions. cw operation of an accelerating structure leads to a number of requirements for the resonators such as high shunt impedance, efficient water cooling of all parts of the resonant cavity, mechanical stability together with precise alignment, reliable rf contacts, a stable operating mode, and fine tuning of the resonant frequency during operation. To satisfy these requirements a new resonant structure has been developed. This paper discusses the beam dynamics and electrodynamics design of the RFQ cavity, as well as some aspects of the mechanical design of the low-frequency cw RFQ.