Tunable Photonic Band Gap Resonator Studies for HAYSTAC

The axion is a promising dark matter candidate that can be detected by the Primakoff effect by which it can resonantly convert to a single photon in a microwave cavity permeated by a magnetic field. Currently, Haloscope at Yale Sensitive to Axion CDM (HAYSTAC) uses a cylindrical microwave cavity with a single tuning rod; by translating the tuning rod radially, the most favorable search mode, TM010 can be tuned over a wide dynamic range in frequency (mass). A major challenge for the experiment is mode crossing and hybridization with TE modes which proliferate at higher frequencies. We have investigated the utility of Photonic Band Gap (PBG) structures designed to eliminate these TE modes while confining TM010 modes. A tunable single-rod prototype was constructed with a PBG lattice replacing the conventional cylindrical barrel boundary condition; its characteristics were studied over the 7.4 - 9.4 GHz range in one configuration, and 5 - 7 GHz in another. This talk will present results demonstrating successful tuning of the TM010 over its entire dynamic range free of TE modes, with only the periodic TEM modes remaining. We will briefly discuss the practical adaptation of PBG structures into actual haloscope resonators.