Field-axis measurement of an AC-powered magnet by a single stretched wire

Magnetic measurements of superconducting magnets at room temperature are performed for quality control and alignment, aiming to position the cold mass accurately, namely the assembly of one or more magnets in its cryo-vessel. The main challenge in performing these measurements is related to the sensitivity of the transducer, as the field values achievable in the magnet bore are in the 1 mT range. The single stretched-wire method is a proven technique for measuring DC-powered normal-conducting and superconducting accelerator magnets. It is vastly used to locate the magnetic axis in quadrupole magnets and higher-order magnets for their alignment and positioning in the accelerator. However, it fails to provide accurate measurements when used to measure superconducting magnets at room temperature because the typical magnet current levels are three orders of magnitude lower than the nominal level used in operation, leading to field values scaling by the same order of magnitude. The problem of the alignment of superconducting quadrupole magnets at room temperature has already been addressed in the literature by adopting alternating current (AC) magnet powering to enhance measurement sensitivity. This paper presents a comprehensive overview of the single stretched-wire method with AC magnet powering, showcasing the different performance compared with the DC powering technique. To further validate the method, a case study of a superconducting quadrupole magnet is presented, and the results are compared against those from a rotating coil scanner.