Compact RF Chopper for High-Energy Heavy-Ion Beams

Radioactive ion beams have a major impact on studies of nuclear structure and nuclear physics far from stability. During projectile fragmentation, which is used for rare isotope production, along with the desired isotope, many other isotope species are produced. A key part of the Facility for Rare Isotope Beams (FRIB) will be a magnetic fragment separator, specifically designed to remove the unwanted fragments of high power beams. However, for the best beam purification on the proton-rich side, it is necessary to have an additional separation method. This can be realized through the time of flight separation, using an RF fragment separator. The RF separator, however, requires the beam to be chopped by a factor of four. In response to this problem, RadiaBeam in collaboration with FRIB proposes to develop and build a compact RF chopping system to reduce the bunched ion beam frequency from about 80.5 MHz to 20.125 MHz at the energy of 500 keV/u. The proposed chopper uses an innovative combination of an RF cavity with an integrated deflecting magnet to reduce the required power and size. The system will be capable of operating near a superconducting RF cryomodule and dump up to 1 kW of heavy ion beam power. In Phase I, we will design the RF chopper system, which includes an RF cavity, magnet and a beam dump, to provide 30 mrad deflection of a 500 keV/u beam. We will optimize cavity parameters such as frequency and length to ensure the most efficient performance. We will consider novel operation with two harmonics to improve beam parameters. The electromagnet design will be performed and the whole chopper system performance will be verified with beam dynamics simulations in the FRIB lattice. The results of this work will be of immediate benefit to FRIB and other heavy-ion machines, such as ATLAS at ANL. Another applications of the developed resonator is as a beam-multiplexor system. RF choppers are also of interest for mass-spectrometry, which is critical for security, defense, and environmental applications.