Broad frequency tuning of a Nb$_{3}$Sn superconducting microwave cavity for dark matter searches

TL;DR

This paper introduces a novel broad-frequency tuning method for superconducting microwave cavities used in dark matter searches, enabling over 1 GHz of continuous tuning without degrading the quality factor.

Contribution

The work presents a new mechanical tuning technique for Nb3Sn superconducting cavities that maintains high quality factors across a wide frequency range, suitable for axion haloscope experiments.

Findings

Achieved over 1 GHz continuous frequency tuning with high Q factor.

Finite-element simulations show minimal Q degradation up to 9 mm aperture.

Experimental results confirm high Q across the tuning range despite imperfections.

Abstract

We demonstrate a novel broad-frequency tuning mechanism for superconducting microwave cavities designed for dark matter searches. Using a Nb$_3$Sn-coated cigar-shaped cavity operating at approximately 9\,GHz, we achieve continuous frequency tuning exceeding 1\,GHz by mechanically separating the two cavity halves: a "tuning-by-opening" technique. Finite-element method simulations predict that radiative losses do not degrade the quality factor even for large openings, as a closed cavity with an intrinsic quality factor of $10^7$ maintains this value for apertures up to 9\,mm, corresponding to a tuning range from 9.0 to 7.5\,GHz. Experimental validation using both copper ring spacers and a continuous sliding mechanism confirms $Q_0$ values exceeding the dark matter quality factor across the entire explored frequency range, despite mechanical imperfections and film non-uniformities. This tuning approach avoids inserting elements into the resonant volume, making it particularly suitable for high-Q superconducting cavities in axion haloscope experiments and readily applicable to REBCO-based implementations capable of operating in multi-tesla magnetic fields.