Symmetrically Tuned Large-Volume Conic Shell-Cavities for Axion Searches

TL;DR

This paper introduces a new class of tunable large-volume conic shell-cavities for axion searches, significantly improving scan rates and maintaining high axion coupling efficiency, thus enhancing the potential for high-frequency axion detection.

Contribution

It generalizes conic shell-cavity design for robust frequency tuning, enabling large-volume, high-efficiency axion haloscopes at higher frequencies.

Findings

Achieved 20% tunable frequency range in large-volume cavities.

Scan rate is three orders of magnitude higher than current cylindrical cavities.

Proposed experimental setup at 20 GHz with an array of brain cavities.

Abstract

In an earlier paper, a new class of thin-shell cavities were proposed to evade the steep frequency scaling of conventional axion haloscopes. In this follow-up work, we see that a generalized conic geometry enables robust frequency-tuning for these large-volume cm-wave cavities. The frequency-defining dimension of a conic shell-cavity changes symmetrically and uniformly during tuning, maintaining a high axion coupling efficiency (the form factor) to an external solenoid field. It is further shown that such tunable geometry is not restricted to circular cones. A general prescription for arbitrary volume-filling conic shell-cavities is developed and direct solutions are obtained for the created numerical models. The largest of the realized designs is a meandering "brain" cavity that is tunable over a frequency range of 20%. The scan rate of this cavity is three orders of magnitude larger than that of a scaled cylindrical cavity used in the current generation experiments. The prospects for such a large improvement in the scan rate should motivate R & D efforts in fabrication and other implementation techniques. If these engineering challenges can be met, cavity-based axion haloscopes can stay competitive at frequencies higher than a few GHz. We propose an experimental configuration at 20 GHz (~ 80 $\mu$eV) using an array of brain cavities and compare it with other proposals for similar frequencies.