The 1991 dynamic aperture experiment at the CERN SPS

This year’s dynamic aperture experiments at the SPS concentrated on the study of the diffusion enhancement by tune modulation in presence of strong nonlinear fields. The aim of this experiment is to specify an upper limit for the power supply ripple in view of the LHC, the proposed hadron collider in the LEP tunnel. Due to strong, unavoidable non‐linearities of the superconducting magnets we expect this accelerator to be very sensitive to power supply ripple. An additional tune modulation, large in comparison with the natural one, was therefore introduced in the SPS and a particle diffusion was meaured for several modulation amplitudes. The combined effect of two frequencies with equal ampltiude was also tested. In parallel long‐term tracking studies have been performed, which allow to find the dynamic aperture for a model of the SPS with the added non‐linearities and tune modulations.