The magnetic quadrupole pick-ups in the CERN PS

The idea of using non-linearities of beam position monitors to measure the second moment sx2 - sy2 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 optimising 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 60dB. Two pick-ups have been installed in the machine. When the data from these pick-ups is combined, it is possible to measure both the matching parameters and the emittance of each bunch in the injected beam. This paper gives an overview of the pick-up design, describes the methods used to analyse the data, and presents some measurement results, including comparisons with other instruments in the machine.