Longitudinal single bunch stability in LHC with phase loop on

The MD was devoted to studies of longitudinal beam stability on the flat bottom and during the ramp in the LHC continuing the studies performed during a previous MD on May 14-15th 2011. We used a filling scheme with eight single bunches in nine equally spaced buckets, the bunches being produced in the SPS with and withouti controlled longitudinal emittance blowup. We varied the capture voltage in the LHC in the range (3.5 - 8) MV and observed bunch oscillations on the flat bottom. The main difference to the previous MD with single bunches was in the phase loop setting since this time input from all bunches was used for a reference phase (as opposed to using only one, the first bunch, in the previous MD). During the last fill these bunches were accelerated to 3.5 TeV without intentional longitudinal blow-up. The onset of dipole oscillations during the ramp was observed on all bunches of Beam 2 while bunches of Beam 1 became unstable only on the flat top. Individual phases and bunch lengths of all 8 bunches measured at different flat bottom voltages are presented for the fills with and without ramp.