Application of the Bead Perturbation Technique to a Study of a Tunable 5 GHz Annular Cavity

TL;DR

This paper demonstrates the novel application of the bead perturbation technique to analyze and diagnose a tunable 5 GHz annular cavity used in axion search experiments, providing insights into mode behavior and cavity properties.

Contribution

It is the first to apply the bead perturbation method to cavities in axion searches, enabling detailed mode analysis and identification of cavity characteristics.

Findings

Identified effects of rod misalignment on mode localization

Analyzed mode-mixing at avoided crossings of TM/TE modes

Provided initial results for cavity diagnostics in axion experiments

Abstract

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.