Bunch Length along the Batch of the LHC Beam at Extraction from the PS

During machine development experiments with multi-bunch LHC beams in the PS, it has been observed that the bunch length at extraction increases slightly towards the end of the batch. This effect does not only get stronger with increasing intensity, but it is also strongly amplified when instead of the usual two all three 80 MHz cavities are operated to produce bunches significantly shorter than nominal. Systematic measurements show that the bunch length gradient along the batch can be attributed to the effect of the impedance of the 80 MHz cavities.