High-volume tunable resonator for axion searches above 7 GHz

We present results from the first experimental demonstration of a tunable thin-shell axion haloscope. This novel geometry decouples the overall volume of the cavity-based resonator from its resonant frequency, thereby evading the steep sensitivity degradation at high-frequencies. An aluminum $2.6$ L ($41$ ${\lambda }^3$) prototype which tunes from $7.1$ to $8.0$ GHz was fabricated and characterized at room temperature. An axion-sensitive, straightforwardly tunable $\mathrm{T}\mathrm{M}$${}_{010}$ mode is clearly identified with a room temperature quality factor, $Q$, of $\sim$$5,000$. The on-resonance $E$-field distribution is mapped and found to agree with numerical calculations. Anticipating future cryogenic operation, we develop an alignment protocol relying only on rf measurements of the cavity, maintaining a form factor of $0.57$ across the full tuning range. These measurements demonstrate the feasibility of cavity-based haloscopes with operating volume $V\gg {\lambda }^3$. We discuss plans for future development and the parameters required for a thin-shell haloscope exploring the post-inflationary axion parameter space ($\sim$$4$ to $\sim$$30$ GHz) at DFSZ sensitivity.