Application of the Bead Perturbation Technique to a Study of a Tunable 5 GHz Annular Cavity
Microwave cavities for a Sikivie-type axion search are subject to several constraints. In the fabrication and operation of such cavities, often used at frequencies where the resonator is highly overmoded, it is important to be able to reliably identify several properties of the cavity. Those include identifying the symmetry of the mode of interest, confirming its form factor, and determining the frequency ranges where mode crossings with intruder levels cause unacceptable admixture, thus leading to the loss of purity of the mode of interest. A simple and powerful diagnostic for mapping out the electric field of a cavity is the bead perturbation technique. While a standard tool in accelerator physics, we have, for the first time, applied this technique to cavities used in the axion search. We report initial results from an extensive study for the initial cavity used in the HAYSTAC experiment. Two effects have been investigated: the role of rod misalignment in mode localization, and mode-mixing at avoided crossings of TM/TE modes. Future work will extend these results by incorporating precision metrology and high-fidelity simulations.
KeywordsAxion Bead-pull Mode mixing Mode crossing Simulation Perturbation technique Dielectric TM modes TE modes
This work was supported under the auspices of the National Science Foundation, under grant PHY-1607417, and the Heising-Simons Foundation under grant 2014-182.