Efficient continuous wave accelerating structure for ion beams

The Facility for Rare Isotope Beams driver linac was designed for acceleration of multiple-charge-state beams after the stripping at ion beam energies of 17 to 20 MeV/u depending on the ion species. The linac includes a 180 degrees achromatic bend to select multiple charge states for further acceleration and to dump unwanted charge states after the first 45 degrees magnet. Two rf rebunchers between the stripper and the linac segment 2 (LS2) are required to minimize the effective emittance growth of the multiple-charge-state beams and provide matching to the LS2. Recent studies have shown that the best choice for these two rebunchers is a room-temperature (RT) accelerating structure capable of providing robust operation in the presence of the stripped heavy ion beam contaminants. The latter can result in uncontrolled losses of contaminant ions after the passage off the stripper. Therefore, using a superconducting (SC) rebuncher after the stripper is not rational due to possible contamination and performance degradation of SC cavities over a long period of operation. For the beam bunching in the energy range from 13 to 22 MeV/u, two 161 MHz rebunchers based on interdigital H-type (IH) structure were developed, built, installed and commissioned with beam at the FRIB linac. This is the first experimental demonstration for application of a cw RT drift tube accelerating structure in this energy range.