A Rotating-Coil Scanner for the Precise Magnetic Characterization of Superconducting Accelerator Magnets at Ambient Temperature

This article presents a versatile scanning system for the magnetic measurements of the accelerator magnets. This system, based on a rotating-coil magnetometer, has been developed to meet the accuracy requirements imposed by the inner-triplet quadrupoles for the High Luminosity Large Hadron Collider (HL-LHC) project at CERN. The main field strength of the magnet must be measured with an accuracy of 100 ppm, and the required accuracy for the angle and the axis are 0.1 mrad and 0.1 mm, respectively. All these parameters must be measured both locally and integrated over the entire magnet length at various stages of production. In this way, it is possible to intercept the manufacturing errors at an early stage of production. Moreover, these measurements are used for the alignment of the magnet assembly in its cryostat. The measurements are performed at an ambient temperature, with low excitation currents. The presented system provides a full set of data for the characterization of the magnet in a single measurement run, including the field quality (multipole field errors) and the magnetic axis location at several longitudinal positions in the magnet bore. The system is able to achieve the required accuracy by using induction coils based on the printed circuit board (PCB) technology, a high-resolution encoder, and retro-reflectors for the laser tracker positioned directly on the PCB. The system is also equipped with a motor unit that allows a high degree of automation in the measurements.