A New Optimised Quadrupole Pick-up Design Using Magnetic Coupling

The idea of quadrupole pick-ups, sensitive to beam size, dates back several decades. Such pick-ups measure the quantity $\sigma^2_x-\sigma^2_y$, where $\sigma_x$ and $\sigma_y$ are the horizontal and vertical r.m.s. beam sizes. Thus, a quadrupole pick-up is a candidate for non-invasive study of processes such as coherent beam width oscillations due to injection mismatch. Up to now, quadrupole pick-ups have been produced essentially by enhancing the electronics of normal position pick-ups to produce the so-called quadrupole signal, with little or no effort being put into the design of the pick-up itself. In developing a quadrupole pick-up for the CERN PS, however, it has been found necessary to optimise the pick-up design. The result is a somewhat unconventional pick-up, where magnetic coupling is employed to suppress the otherwise very strong, and undesired, common mode-signal. In this paper, the basic design idea and the final optimised design are presented, together with simulations, test bench measurements and real beam data.