Longitudinal quadrupole instability and control in the Frascati DAΦNE electron ring

Abstract

A longitudinal quadrupole (q-pole) instability was limiting the maximum stable current in the DAΦNE e^{-} 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_{rf}), momentum compaction (α_{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.