LONGITUDINAL HEAD-TAIL INSTABILITY IN THE CERN-SPS COLLIDER

The first observation of a single bunch longitudinal head-tail instability in the CERN SPS collider is reported An evaluation of the growth rate for the dipole mode is presented, which agrees with the experimental results. A slow growing longitudinal instability has been observed in the CERN-SPS collider, along the 26 GeV/c injection energy “flat bottom”. The 7 ns long single bunches captured and held by the 100 MHz RF system oscillate in the dipole and quadrupole modes, with growth times of the order of a few seconds (fig. 1). The observations made exclude long range instabilities, like coupled-bunch or Robinson (m=l) types. They rather suggest a single bunch effect excited by short range wake fields. A head-tail instability can exist in the longitudinal plane as well as in the transverse plane provided chromatic effects are important. This is actually the case close t