Increasing instability thresholds in the SPS by lowering transition energy

A new optics for the SPS with lower transition energy was tested experimentally during 2010-2011, showing a significant increase of the single bunch instability thresholds at injection due to the 3-fold increase of the slip factor. The performed machine studies for different LHC beams and intensities are summarized in this paper. In particular, the search for the single bunch TMCI threshold in the new optics is presented. Observations on the longitudinal multibunch stability are compared between the nominal and the low-transition optics. Finally, optics variants with higher vertical tunes are discussed, which could allow to further increase the thresholds for TMCI and other vertical instabilities by reducing the vertical beta function.