Mode identification and cavity stretching for the prototype storage ring cavity

Abstract

The Advanced Photon Source (APS) storage ring uses single cell cavities. As described in the paper ``Coupled Bunch Instabilities in the APS Ring`` by L. Emery, several of the higher order modes (HOM) in these cavities cause instability thresholds below the desired operating level. In order to clearly identify these modes, an experimental method of measuring the fields is necessary. A well- known technique is measurement with probes or loops extending in from the walls, but this cannot be used in the interior. A different approach is the use of small perturbations which cause a frequency shift that is related in a known way to the local field. This perturbation can be calculated for objects of needle-shaped, spherical, and disk-shaped form. With proper use this method can give very accurate measurements of the direction and magnitude of the electric and magnetic fields. This report gives examples of a method of measuring electric field strengths in a resonant cavity. It is shown that insertion of a metallic bead (needle), whose dimensions are small compared to the wavelength, perturbs the frequency of a resonant electromagnetic cavity by an amount that depends upon the local electric field at the position of the perturbingmore » object.« less