Development of a low charge-to-mass ratio injector for a RIB linac Original Research Article

A post-accelerator for rare isotopes (RIB linac) which must produce high-quality beams of radioactive ions over the full mass range, including uranium, at energies above the coulomb barrier is being developed for the U.S. RIA Facility. The linac will accept all ions in the 1+ charge state to provide the highest possible intensity of rare isotopes with masses from 6 to 240. A high-resolution separator to purify beams at the isobaric level precedes the RIB linac. Charge stripping in the linac takes place at two stages: helium gas stripping at 7 keV/u for masses below 132 and 22 keV/u for higher masses, and an additional foil stripping at ∼ 1.1-2.0 MeV/u for the heavier ions. Most of the accelerating voltage in the RIB linac is provided by superconducting resonators. A very-low-charge-state injector, which is needed for the first ∼ 9 MV of the accelerator, will be based on room temperature accelerating structures. This section consists of a pre-buncher followed by three sections of cw RFQ. The first section is a split-coaxial RFQ operating at 12 MHz. The following two sections are hybrid RFQs (H-RFQ) operating at 12 and 24 MHz. A 1:2 scale cold model of the 12 MHz H-RFQ has been built and studied, and has resulted in the development of final specifications for the construction of the full power 12 MHz H-RFQ. Economic design of the RFQs calls for three different types of resonant structures. This paper reports on the present status of the injector design including beam optics and studies of resonant structures.