Emittance Exchange by Crossing a Coupling Resonance

When the working point of a synchrotron is shifted across a second-order difference resonance in the presence of weak linear coupling, it is possible to exchange the transverse emittances. This result goes beyond the classical theory, where only emittance sharing is predicted. Systematic experimental investigations of the phenomenon have been performed at the CERN PS Complex. In the PS-Booster, the measurement of the horizontal emittance is hampered by the contribution of the momentum spread to the beam size through dispersion. Based on the new mechanism, the horizontal emittance was measured in the vertical plane, where no dispersion is present, thus improving the overall accuracy. In the PS, detailed measurements of the emittance exchange process were performed. In parallel, an analytical theory was derived. The agreement between theory and experimental results is remarkable.