Developing Centimeter-scale-cavity Arrays for Axion Dark Matter Detection in the 100 Micro-electron-volt Range
The cavity haloscope technique has been the most successful approach to date in searching for axion dark matter, owing to a confluence of factors at the GHz scale including the macroscopic size of the axion-to-photon converting cavity volume, the sophistication of present radio-frequency/microwave technologies including quantum amplifiers, and the location of the quantum limit temperature. These factors scale in a disadvantageous way overall as searches move up the axion mass/frequency scale, with the quantum limit noise temperature scaling linearly with frequency $T_{\text{SQL}} \sim f$, the effective single cavity volume scaling as the inverse frequency cubed $C V \sim f^{-3}$, and the axion-coupled cavity mode quality factor shrinking as $Q \sim f^{-2/3}$ for copper cavities, necessitating the search for remedies. One approach is to make up the loss in volume using an array of efficiently packed matched cavities coordinated in space and time to act as a single axion-to-photon converting array. This paper presents PNNL's progress in developing technologies for cavity array axion haloscope in the $m_a \sim 100$ micro-eV mass range including the design of moderate scale cm-diameter cavities and their fabrication process using electric discharge machining, the development of mode tuning mechanisms towards a re-entrant style combination tuning rod and coupler, mode matching, and RF readout. The result is the first demonstration of a tunable array of matched cavities with axion-coupling modes in the $f_0 \in [22.88,22.93]$ GHz ($94.62-94.83$ micro-eV) range. Prospects for future larger arrays leading to viable axion DM searches of this type in this mass range are discussed.