Development of a helix-based beam position monitor calibration system for sub-relativistic charged-particle beams

Accurate calibration of beam position monitor (BPM) responses is a prerequisite for precise beam position measurements. Traditional methods such as the stretched-wire or antenna method can accurately calibrate BPM responses for relativistic beams, but there has been a lack of effective calibration methods for non-relativistic beams, limiting the measurement accuracy of BPMs in low-energy proton or heavy ion linear accelerators. In this study, we developed a BPM calibration platform based on helical slow-wave structures, capable of effectively simulating the electromagnetic fields generated by non-relativistic beams and calibrating the BPM response to these beams. We introduced a period averaging method, which reduces calibration errors caused by impedance mismatches in the helix. We achieved offline calibration of button-type BPMs for non-relativistic beam response characteristics with beta as low as 0.122 using this platform. BPM calibration data were compared with simulations and then on-line measurements in the medium beam transport line at the Xi'an Proton Application Facility. Within the central linear response region of the BPM (similar to 16% of the BPM aperture), the calibrated linear sensitivity of 0.0668 +/- 0.0003 is in close agreement with the actual measurement result of 0.0669 +/- 0.0009. In the nonlinear response region further from the center, extending to at least 60% of the BPM aperture, the relative measurement error of beam position can be controlled to within 1.2% after calibration. This advance significantly expands the measurement range and enhances the measurement accuracy of BPMs for non-relativistic beams.