A Tunable Photonic Band Gap Resonator for HAYSTAC

Haloscopes search for dark matter axions via their conversion to photons in an applied magnetic field. The Haloscope at Yale Sensitive to Axion CDM (HAYSTAC) uses a copper cylindrical cavity with an off-axis tuning rod to resonantly enhance the converted photon signal. The lowest order transverse magnetic mode (the TM$_{010}$) is tuned to search over a range of potential axion masses. This process is complicated by other fundamental cavity modes which interfere with the TM$_{010}$ mode, reducing the achievable signal power and sensitivity. Current experiments tolerate these mode crossings, but the problem worsens at higher frequencies. Photonic Band Gap (PBG)-based resonators allow for the confinement of TM modes while eliminating unwanted modes. We have developed a tunable PBG resonator which eliminates transverse electric (TE) modes and tunes the TM$_{010}$ from 7.3 to 9.3 GHz. This work will present results from an improved prototype aluminum structure and will discuss ongoing research on future designs.