Magnet alignment monitoring system with eigenfrequency-based wire sag correction

The precision of magnet alignment in next-generation light sources is critical. To achieve this, the multipole magnets on a common girder of SPring-8-II, an upgrade of SPring-8, are planned to be aligned using the vibrating-wire technique. We developed an alignment monitoring system to monitor the magnet positions in the tunnel where the vibrating-wire technique cannot be executed while the magnets are not energized or when there are vacuum chambers in the magnet center. The alignment monitoring system utilizes a non-contact capacitive sensor embedded in a ceramic ball to measure the wire position relative to the center of the ball and derives the wire sag by measuring a higher mode eigenfrequency. The constitution of this system is illustrated for relevant studies, including the investigation of wire sag against the eigenfrequency, observation of the actual wire sag at a test bench, and validation of the system on the girders for the prototype magnets of SPring-8-II. The system measured the positions of a series of magnets with a precision of +/- 4 mu m (peak to peak) in a 4 m range, in both the horizontal and vertical directions, meeting the requirement of SPring-8-II.