An RF beam Sweeper for Purifying In-Flight Produced Rare Isotope Beams

Radioactive ion beams (RIB) are produced via two-step processes and several production techniques have been developed. At ATLAS, the production of such beams is provided via in-flight method and grants access to more than 100 short-lived isotopes in the mass range up to A~60. However, after traversing the production target, the primary beam is energy degraded and acquires a long low-intensity energy tail from the multiple-scattering processes in the target material. At ATLAS, the RF sweeper, operating 6 MHz with maximum voltage of 55kV is used. However, this voltage is not sufficient for a clean separation of many beams such as 37K at 275 MeV, where at least twice as high of a voltage is required. In response to this problem, RadiaBeam is developing an RF sweeper operating at ATLAS fundamental harmonic frequency (12 MHz) with a 150 kV kicks that is triple of the voltage achieved with the existing ATLAS sweeper. During Phase I, we found that for lower energy beams, ~ 5 MeV/u, the frequency of 6 MHz would be preferable because of the flight time, so the design must include two frequencies to cover the whole range of the beam species. The switch of frequency regimes can be performed manually in between the tests. In Phase II we had to completely reconsider the design of the sweeper due to the requirements for two frequency operation with twice higher voltage than planned initially. To date, we resolved all challenges corresponding to these new requirements and finished the engineering design. We started the sweeper resonator fabrication to be completed by the end of the phase. We have also designed and fabricated a 2 kW solid-state generator module, sufficient for initially planned single-frequency 75 kV operation, which is currently being tested. The complexity of the design initially proposed for Phase II has increased significantly due to the requirements for RF power, reliability, fabrication feasibility of the large vacuum vessel, the development frequency switch, and the need for highly efficient solid-state RF power sources for two frequencies. In view of these challenges, we propose the Phase IIA continuation of this project is to successfully complete the originally planned initial Phase II prototype. In this sequential Phase II project, we will finalize the design and fabricate all components that are essential for the sweeper operation, including high power switch and full-power RF power sources for both frequencies. The sweeper prototype will then be delivered to ATLAS for high-power and beam tests. The results of this work will be of immediate benefit to the ATLAS facility. In addition, a similar separator could be used at FRIB for radioactive beam separation. RF sweepers are also of interest for mass- spectrometry as they can provide very precise isotope separation, which is critical for security, defense, and environmental applications.