In search of trapped modes in the single-cell cavity prototype for CESR-B

Plans for CESR-B, the proposed upgrade of the CESR e⁺e ^- storage ring to a B-factory, call for single-cell superconducting cavities with heavily damped higher-order modes (HOMs); Q⩽100 is required for the dangerous modes in order to avoid multi-bunch instabilities. The cavity is designed to enable all HOMs to propagate into the beam pipe, where they are damped by a layer of microwave-absorbing ferrite. Other “single-mode cavity” designs have been shown to have trapped modes, i.e. HOMs with low fields in the beam tube even though they are above cutoff. Trapped modes in the CESR-B cavity would be poorly damped by the ferrite loads and might have excessively high Q´s. Several different approaches have been taken in assessing the effectiveness of the CESR-B cavity design: (i) the Q´s of monopole and dipole modes have been measured in a full-size copper cavity with ferrite loads; (ii) the Q´s of monopole modes have been predicted using SEAFISH to model the cavity and loads; (iii) the current-voltage method has been used to search for trapped monopole modes. The results of (iii) are discussed in this paper. An explanation of the current-voltage method and some recent improvements to it are also given