DAΦNE injection kicker: electromagnetic analysis of trapped modes and damping antenna design

During the commissioning of the Frascati Φ-Factory DAΦNE, vertical and longitudinal multibunch coupled bunch oscillations have been observed. Modal analysis and measurement of the instability thresholds as a function of local orbit bumps have shown that, with high probability, the coupled bunch oscillations can be attributed to the beam interaction with parasitic resonant modes trapped in the injection kickers. By means of theoretical models and computer simulations (based on HFSS code by HP) an electromagnetic characterization of the kickers HOMs has been carried out and the obtained results have been compared with the impedance measurements made on a prototype. The study and design of a damping antenna coupled with the structure resonant modes is described. Measurements on a prototype equipped with two antennas confirm the effectiveness of the proposed solution to strongly reduce the kicker longitudinal and vertical coupling impedances, i.e. to eliminate the instability sources. (author)