Design of half-reentrant SRF cavities for heavy ion linacs.

The Spallation Neutron Source (Oak Ridge), the proposed 8 GeV Proton Driver (Fermilab), and the proposed Rare Isotope Accelerator use multi-cell elliptical SRF cavities to provide much of the accelerating voltage. This makes the elliptical cavity segment the most expensive part of the linac. A new type of accelerating structure called a half-reentrant elliptical cavity can potentially improve upon existing elliptical designs by reducing the cryogenic load by as much as 40% for the same accelerating gradient. Alternatively, with the same peak surface magnetic field as traditional elliptical cavities, it is anticipated that half-reentrant designs could operate at up to 25% higher accelerating gradient. With a half-reentrant shape, liquids can drain easily during chemical etching and high pressure rinsing, which allows standard multi-cell processing techniques to be used. In this paper, electromagnetic designs are presented for three half-reentrant cell shapes suitable for an ion or proton linac (beta = 0.47, 0.61 and 0.81, frequency = 805 MHz or 1.3 GHz). The mechanical designs for single-cell prototypes have also been completed.