High-frequency Cavity Haloscopes for the ADMX-VERA Program

Post-inflationary axions are predicted to have m_a > 16.5 μeV (4 GHz), a regime in which unfavorable volume scaling drastically reduces the sensitivity of conventional cavity-based haloscopes. In this talk, I present on the design and characterization of high-volume (V ≫ λ³) cavity geometries in this frequency range developed by the volume-enhanced resonating axion (VERA) program, an R&D effort of the ADMX collaboration. Among the VERA resonators is the single-wedge, which is currently employed in a tabletop dark photon search projected to produce leading limits from 7 to 8 GHz. A larger-scale single-wedge cavity is currently being lapped to a tight flatness tolerance for optimal form factor, and will be characterized cryogenically. The triple-wedge design can efficiently fill the volume of an O(1 m³) solenoid magnet but requires custom flexure stages for the relative alignment of the three wedges. This resonator is currently undergoing metrology prior to RF studies. To achieve greater coupling with the novel resonators without degrading their form factor, an array of slot antennas combined via phased summing tree is under development. I will also discuss the regimes in which microwave single photon counters are advantageous compared to continuous-wave amplification.