The Magnetic Quadrupole Pick-Ups in the CERN PS

The idea of using the non‐linearities of beam position monitors to measure the second moment σx2 − σy2 of the transverse beam distribution is almost as old as the synchrotron. However, although a few successful experiments have been reported, the method has not become widely accepted. One reason for this has been that little or no effort was put into optimizing the pick‐ups that were used for the new purpose. In a standard beam position pick‐up, the signal from the second moment is extremely weak and embedded in a strong common‐mode background. Separating the signal from the background has therefore been a major stumbling block. Driven by the need for a non‐destructive measurement of injection matching to preserve the small emittance of the LHC beam, a dedicated quadrupole pick‐up has been developed for the CERN PS. The design employs magnetic coupling in a special pick‐up geometry to remove the otherwise dominating background signal, thereby reducing the common‐mode rejection requirements by about 60 dB....