Dielectric-Loaded Cavities for ADMX Low-Frequency Searches

Axion dark matter searches using the resonant cavity technique are limited at low mass by the physical cavity size. Previous experiments by the ADMX collaboration have reached masses as low as 1.9 μeV using a 50 cm diameter cylindrical cavity. Because of the fixed size magnet limitation, a large empty cavity is not generally a possibility at lower frequencies. Reentrant cavities can operate at a lower frequency for a given diameter and allow access to a lower axion mass range. However, this comes with a penalty on the form factor. Moreover, tuning a reentrant cavity would likely involve moving a tuning rod in and out of the cavity, which could complicate the design of the tuning system. This talk will show an elegant alternative technique based on loading the cavity with dielectric rods, which brings the frequency down depending on the dielectric loading. Furthermore, the dielectric rods can be arranged to allow a rotational movement of a metallic tuning rod, which is a well-established tuning mechanism. The form factor, although reduced, in this case, depending on the dielectric loading, is better than what it is for a comparable reentrant cavity. We will present conceptual designs for cavities with different dielectric loadings that enable ADMX to search for axions in the range of 0.8-2.1 μeV.