Observation of transverse quadrupole mode instabilities in intense cooled antiproton beams in the AA

In the CERN Antiproton Accumulator (AA) a breathing mode type of instability has been identified as an intensity-limiting mechanism in cooled stacks. The more well-known dipole-mode instabilities are adequately controlled by the existing damper system. With the aid of a quadrupole pickup it was possible to observe transverse modes in the beam at frequencies (n-2Q). The instabilities occur only at certain emittance and intensity thresholds and are believed to be caused by uncleared pockets of ions trapped in the beam potential. A quadrupole kicker was added to the machine so that these modes could be excited, and beam transfer functions were measured for each of the possible modes. Feedback can be applied to actively damp the quadrupole modes. The best solution has been to rid the machine of the remaining ion pockets gradually by improving the clearing, by the careful choice of tunes, and even by shaking the beam.<>