Calibrating the sensing-coil radius by feed-down from a harmonic reference

A method for calibrating the radius of rotating, harmonic sensing coils is proposed that allows relaxing constraints on alignment and field errors of the reference quadrupole magnet. Radius calibration considering roll angle misalignment between the measurement bench, the magnet, and the motor-drive unit is studied first. We then study the calibration error when a harmonic field error of higher order is present in the calibration magnet. This also yields a calibration when a sextupole magnet is used, for example, when an insitu calibration [1] is required. The proposed calibration method has been validated by simulations with the CERN numerical field computation program ROXIE.