Longitudinal quadrupole instability and control in the Frascati DAPHNE electron ring

A longitudinal quadrupole (q-pole) instability was limiting the maximum stable current in the DA{Phi}NE e{sup -} ring at a level of 700-800 mA. In order to investigate the phenomenon, the instability threshold has been measured as a function of various machine parameters as radio frequency voltage (V{sub rf}), momentum compaction ({alpha}{sub c}), number of bunches, fill pattern, etc. An unexpected interaction with the longitudinal feedback system, built to control the dipole motion, has been found and a proper feedback tuning has allowed increasing the threshold. The maximum stable beam current has now exceeded 1.80 A and it is no longer limited by the quadrupole instability.

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