Vibrating Wire Technique and Phase Lock Loop for Finding the Magnetic Axis of Quadrupoles

The tolerance in the alignment of the quadrupoles in the linacs and in the undulator lines of the Swiss Free Electron Laser (SwissFEL), the next project at the Paul Scherrer Institute (PSI), will be about 1
$\mu{\rm m}$
. This accuracy will be reached using beam alignment techniques. To minimize the commissioning time, it is also requested to install the quadrupoles with a precision of
$\pm 50\ \mu{\rm m}$
. To achieve this goal, the vibrating wire technique will be used in association with other systems to determine the position of the magnetic axis of the quadrupoles with respect to external fiducials. After a general introduction to the project and to the instrumentation principle, the novel approach developed at PSI is presented. The main innovation consists in the use of a phase lock loop (PLL) for maintaining the wire in its resonance condition. This approach simplifies the operation of the system in case the wire is moved or replaced and increases substantially the reliability of the measurement outcomes in the frame of a series test campaign.