Reaching the 5--9 $\mu$eV Range with ADMX: Multi-Cavity Array

Axions are particles that arise from the Peccei-Quinn solution to the strong charge-parity problem in quantum chromodynamics. Axions with a few $\mu$eV mass are a prominent cold dark matter (CDM) candidate. The aim of the Axion Dark Matter eXperiment (ADMX) is to detect CDM axions in the halo of our Galaxy. ADMX seeks to detect axions by observing the conversion of axions to microwave photons in a high-$Q$ resonant cavity in a strong magnetic field (an Axion haloscope). ADMX recently has completed successfully a search over the 2.66--2.81 $\mu$eV mass range with unprecedented sensitivity. For higher mass range searches, ADMX has developed multi-cavity arrays as the heart of a haloscope for axion masses in the 5--9 $\mu$eV range. We will present design aspects and preliminary study results of a 4-cavity array prototype. We will also discuss the performance of a Pound locking method adapted to synchronize the resonant frequencies of the multiple cavities.