Design of helix based calibration setup for low-[formula omitted] beam position monitors

Abstract

Responses of beam position monitors (BPMs) in low-energy beamlines are sensitive to beam velocity and bunching frequency due to non-relativistic effects. However, traditional BPM calibration methods based on the coaxial wire or wire antenna can only simulate electromagnetic fields of high-energy beams with β close to 1. A helix with an inner conductor was adopted to simulate low-β beams. The dispersion relation and characteristic impedance of the helix are analyzed based on the sheath-helix model. The phase velocity and characteristic impedance of the helix have a low dependency on the frequency and helix position inside a BPM, which is crucial for calibrating low-β BPMs. A helix with a phase velocity of 0.12c and characteristic impedance of 50 Ω is designed to calibrate the button BPMs at the Xi’an Proton Application Facility. Responses of the BPMs to the helix and a beam with β = 0.12 are simulated with CST Studio Suite. The maximum relative calibration error of the helix compared to the beam is 0.47 mm in the transverse positions within three-quarters of the BPM aperture, which shows the remarkable capability of the helix to calibrate low-β BPMs offline.