Characterization of the HAYSTAC dark matter detector cavity: microwave measurement and simulation*

HAYSTAC utilizes a tunable microwave cavitiy in the 3-6 GHz range to resonantly enhance electromagnetic fluctuations sourced from dark matter axions. Non-ideal behavior of the cavity can hinder the operation of the experiment; two non-idealities of the modes that are of interest are mode localization and mode mixing. In this work, we present results from the first comprehensive characterization of an axion haloscope cavity. We discuss the effects of design imperfections as they primarily relate to these two phenomena. To study the behavior of the modes, we performed measurements on the cavity by perturbing the frequency of the modes via a small dielectric bead. Electromagnetic simulations of the same setup were also carried out. These results have allowed us to determine the range of misalignments in our cavity that allows for acceptable operating conditions in the experiment. The high accuracy of our simulations indicates that they can be used both to improve our understanding of existing cavities and to optimize future designs.