Measuring field multipoles in accelerator magnets with small-apertures by an oscillating wire moved on a circular trajectory

A method based on an oscillating wire for measuring the field quality in accelerator magnets with small apertures of the order of 10 mm is proposed. The wire is positioned step-by-step on the generators of a cylindrical domain inside the magnet aperture, i.e. its end-points at the stages are moved on a circular trajectory. The amplitudes of the wire's forced oscillations are measured and related to field harmonics by a suitable analytical model. In this paper, the analytical model, the measurement procedure, and the measurement system architecture of the oscillating wire method are presented. The method is validated by comparison with the standard rotating-coil system. A case study on small-aperture, permanent-magnet quadrupoles constructed for the Linac4 injector at CERN is illustrated.