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 Bfactories and state of the art synchrotron radiatron sources. Following the idea of Conciauro-Arcioni [1] 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. I. INTRODUCTION BESSY II is a 1.7 GeV third generation electron storage ring with 16 straight sections to produce highly brillant synchrotron radiation beams in the spectral range of soft x-ray and vacuum ultraviolet. One of the design goals is a low emittance of 6 nm#rad and a maximum beam current of 200 mA [2], [3]. Therefore it is n..