Simulations of the Fast Transverse Instability in the SPS

In 2002 and 2003 measurements carried out at the CERN SPS on beams with low longitudinal emittance have revealed, probably for the first time in a proton machine, the crossing of the transverse mode coupling instability (TMCI) threshold far from transition. According to the present understanding, this threshold could be reached due to the increase of the microwave longitudinal instability threshold, as a result of the longitudinal impedance reduction program at the SPS in recent years. Comparing measured thresholds with those found in numerical simulations we estimate the transverse broad-band impedance parameters in 2002, and how they possibly changed in 2003 after the installation of 5 new MKE kickers. The expected variation in 2006, when 4 more MKE kickers will be installed, is extrapolated and a new threshold proposed. Simulations show the important role of the space charge which can on the one hand push the threshold to higher intensity, but on the other hand cause the emittance to blow-up well below this higher value.