Design of a superconducting interdigital accelerator structure for radioactive heavy ions

An on-line isotope separator (ISOL) has been installed on one of the beamlines at the TRIUMF cyclotron. Assuming a 60 keV/u, 50 MHz RFQ preaccelerator, a drift-tube linac design based on the superconducting interdigital structure developed at the Argonne National Laboratory, has been studied for acceleration of the ISOL radioactive beams. The linac consists of a first stage with 12 four-gap 50 MHz resonator tanks and 6 inter-tank superconducting solenoid lenses, followed by a second stage operating at 100 MHz, with 21 tanks and 7 inter-tank solenoids. This then can deliver a 1.6 MeV/u beam of ions with q/A ≥ .05. Including solenoid lens space, the equivalent average accelerating gradient for singly charged ions is 1.9 MeV/m, about 60% higher than for a room temperature design reported earlier. The design was developed using a modified version of the PARMELA code, called PARMION, which dynamically varies the cell lengths to maintain a specified synchronous phase. The optimum synchronous phase was found to be -15 degree