  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.
