Design of reentrant cavity BPM for low-intensity ion beam and high-current proton measurement simultaneously at Xi’an Proton Application Facility

Purpose Beam position monitors (BPMs) are among the most crucial detectors in particle accelerators. In the Xi’an Proton Application Facility (XiPAF) upgrade project, the medium-energy beam transport line will transmit both high-current proton and low-intensity heavy ion beams. The current variation between the two types of particles is as large as two to three orders of magnitude, which poses challenges for accurate simultaneous measurement using broadband button-type BPMs.
Methods We introduce a dielectric-filled reentrant cavity BPM with a resonant frequency of 108 MHz for accurately measuring both high-current proton beams and low-intensity heavy ion beams. The cavity BPM operates in resonant mode when measuring heavy ion beams, whereas it operates in non-resonant mode for proton beams.
Results For a heavy ion beam with a current of 200 e \mu A and an energy of 2 MeV/u, the BPM signal amplitude variation is approximately 7.7 mV when the beam is offset by 3 mm from the BPM center, which is 14 times larger than that of the existing button BPM of XiPAF. For a 7 MeV/20 mA proton beam, the cavity BPM functions like a capacitive BPM, with the BPM signal amplitude controllable within 170 mV across 80% of the BPM aperture.
Conclusion The detailed design, response simulation, and mechanical design of the cavity BPM are presented in this paper.