Determination of resonant frequency and external Q values for the BESSY II HOM-damped cavity

Damping of higher order cavity modes (HOM´s) is one way to increase the intensity thresholds for multibunch instabilities in B-factories and state of the art synchrotron radiatron sources. Following the idea of Conciauro-Arcioni (1990) one of the simplest geometries is the threefold-symmetry cavity-waveguide structure with a broadband absorber terminating the end of every guide. The iris coupling between the cavity and the waveguide has to be designed so that the fundamental cavity-mode remains nearly unperturbed. The values of the external Q und the shift resonant frequencies due to the waveguide loads is determined and tested by known methods with MAFIA