Development of a fourfold dielectric-filled reentrant cavity as a beam position monitor (BPM) in a proton therapy facility

At the Paul Scherrer Institute (PSI), the superconducting cyclotron “COMET” delivers a 250 MeV proton beam for radiation therapy in pulses of 1ns at the cyclotron-RF frequency of 72.85 MHz. Accurate measurement of the beam position at proton beam currents of 0.1-10 nA in the beam transport line downstream of the degrader is of crucial importance for the treatment safety and quality, beam alignment and feedback systems. This is essential for efficient operation and beam delivery. These measurements are usually performed with intercepting monitors such as ionization chambers (ICs). In this paper, we present a novel non-intercepting position sensitive cavity resonator. The resonant monitor, tuned to the second harmonic of the cyclotron's RF, is based on the detection of the transverse magnetic dipole mode of the EM field generated by the beam. This mode is only excited for off-center beam positions and is measured with the help of four floating cavities within a common grounded cylinder. This paper discusses the BPM fundamental characteristics, design optimization and the underlying parametric investigations involving the contribution of the different modes and crosstalk. We estimate the expected signals from the prototype BPM for position offsets from simulations and compare them with test-bench measurements and beam measurements with the prototype and the improvised BPM design. We conclude by summarizing the achieved position sensitivity, precision, and measurement bandwidth.