New concept for acceleration of slow, low-charge state heavy ion beams

A modern post-accelerator for radioactive beam facilities requires acceleration of heavy ions beginning with charge state 1+ in order to maximize the intensity of the beams available for experiments. The acceleration of unstable nuclides with mass number up to 240 is required. For acceleration of low-velocity heavy ions very low frequency RFQs are appropriate. However, the accelerating efficiency of such RFQs is low. To overcome this drawback we propose a hybrid accelerating structure that is formed by an alternating series of drift tubes (DTL) and RFQ sections. In such a structure the accelerating and focusing functions are decoupled. A hybrid RFQ (H-RFQ) operating at 12 MHz was designed for acceleration of heavy ion beams with q/A=1/240. The accelerating structure is ~3.3 m long and consists of three sections of DTL and two sections of RFQ. Each section of the DTL comprises 10 to 14 drift tubes. The RFQ sections comprise five βλ/2 cells and form a focusing triplet. The average accelerating gradient of the H-RFQ is about twice that of a conventional RFQ This paper describes a complete study of beam dynamics in the new H-RFQ accelerating structure. A project to carry out a cold-model test of the H-RFQ will be pursued