Non-Invasive Measurement of Emittance and Optical Parameters for High-Brightness Hadron Beams in a Synchrotron

For hadron colliders, such as the Large Hadron Collider at CERN, emittance preservation is of prime importance to achieve a high luminosity. Since there are no significant effects (apart from active cooling) that decrease the emittance of a hadron beam, the smallest possible emittance is set by the source, and has to be preserved along the entire injector chain. One possible source of emittance increase is due to optical mismatch at beam transfer between two machines in this chain. To verify the matching on-line requires a non-invasive instrument capable of measuring the optical parameters of the injected beam. Such instruments are very rare. A quadrupole pick-up is a non-invasive instrument sensitive to beam size. It is basically a beam position monitor, where the non-linear response to particle position is used to extract information on the second moment of the transverse beam distribution. The basic idea was proposed a long time ago, and have been successfully used at a few occasions, but have not found any widespread application. This is largely due to the fact that in the quadrupole pick-ups used so far the very small interesting signal was embedded in a massive common-mode background. The use of quadrupole pick-ups therefore required very good rejection of this background to extract the beam size information. This report describes a new design concept where, due to the pick-up geometry, there is no common-mode background. In practice, this is achieved by coupling to the radial component of the magnetic field. Analytic formulae for the transfer impedances of such a pick-up have been derived and compared with simulations and measurements on a series of prototypes. Based on these results, a quadrupole pick-up have been developed for the CERN Proton Synchrotron (PS), and its performance is discussed in detail. It is shown that installing two such instruments in well-chosen locations in a circular machine like the PS enables the measurement of matching and emittance of a single, selected bunch, with an accuracy comparable to destructive methods. Some further possible applications of quadrupole pick-ups in circular machines are also discussed.