Optimising Re-entrant Cavity Designs for Low Mass Axion Haloscopes

TL;DR

The authors tackle the challenge of detecting low-mass QCD axions with haloscopes by optimizing re-entrant cavity geometries in the 100-500 MHz range using finite-element simulations. They formalize an effective scan-time metric based on $d\nu/dt \propto C^2 V^2 G$ and compare several rod configurations in a fixed outer cavity ($R=0.2\ \mathrm{m}$, $H=1\ \mathrm{m}$) to maximize scanning speed. The standout result is the double-attack design with two opposing rods ($r \approx 0.09\ \mathrm{m}$) achieving $T \approx 2.03\times10^5$, more than a factor of three faster than the single-rod baseline, albeit with mode-coupling challenges. To ease implementation, a hybrid one-fixed-rod-plus-one-tunable approach is proposed, maintaining most gains and enabling full 100-500 MHz coverage by swapping fixed-rod heights to mitigate avoided crossings, guiding near-term prototype development.

Abstract

Axion haloscopes provide a leading experimental approach to detecting QCD axion dark matter through resonant axion-photon conversion in microwave cavities. Extending haloscope sensitivity to low axion masses remains challenging due to the large resonator volumes required at sub-GHz frequencies. Re-entrant cavities offer a compact solution, but their performance depends strongly on geometric optimisation. We present a comprehensive finite-element study of re-entrant cavity haloscope designs operating in the 100 to 500 MHz range, comparing their performance using effective scan time as a figure of merit. Among the configurations studied, we identify a double attack geometry that achieves a roughly threefold improvement in effective scan time compared to a conventional single-rod re-entrant cavity. We further investigate practical implementation strategies, including a hybrid design employing one fixed rod and one tunable rod, which preserves a scan time gain while reducing mechanical complexity. These results demonstrate a pathway to enhanced low-mass axion haloscope sensitivity.

Figures (10)

• Figure 1: (Top left) Geometry of the single re-entrant tuning rod, showing the rod radius $r$ and the tuning parameter $\mathrm{Z}$ defined as the axial displacement of the rod. The dashed line in cyan is the central axis of symmetry for the setup. (Top right) Resonant frequency $\nu$ as a function of the tuning parameter $\mathrm{Z}$ tuned at a step size of $2 \ \mathrm{cm}$. (Bottom left) Cavity form factor $\mathrm{C}$ as a function of resonant frequency. (Bottom right) Inverse scan rate $\mathrm{1/S}$ as a function of frequency, with the shaded region indicating the contribution to the total scan time relation.
• Figure 2: (Top left) Schematic of the double attack cavity design, showing two tunable rods inserted from the opposite ends. The parameters $r$ and $\mathrm{g}$ denote the rod radius and inter-rod gap, respectively. The remaining panels follow the same layout and definitions as in Fig. \ref{['fig:SINGLE ROD REENTRANT']}
• Figure 3: (Left) Inverse scan rate ($\mathrm{1/S}$) as a function of frequency $\nu$ for a range of tuning rod radii ($r = 0.03$ to $0.15$ m) for the double attack design. The configuration with $r = 0.09$ m, highlighted by the red stars, consistently yields the minimum $\mathrm{1/S}$ values, identifying it as the optimal geometry for minimising scan time across the $100–500 \ \mathrm{MHz}$ frequency range. (Right) Area under the curve (scan-time relation) of $\mathrm{1/S}$ vs $\nu$ plotted against radii $r$ of tuning rods, confirming the optimum performance at $r = 0.09 \ \mathrm{m}$
• Figure 4: Schematic of hybrid double attack design showing the primary fixed rod (left) and the secondary fixed rod (right), along with the tuning rod and the tuning parameter $Z$.
• Figure 5: Resonant frequency $\nu$ as a function of the parameter $\mathrm{Z}$ when using a fixed rod of height $h = 44 \ \mathrm{cm}$. A clear interaction between the axion-sensitive tuning mode and non-sensitive intruder mode is observed, resulting in an avoided level crossing. This interaction creates an un-scannable frequency band of approximately 46 MHz (shaded brown region), spanning from 146.6 MHz to 191.8 MHz. The filtered data in Regions 1 (yellow) and 2 (cyan) highlight the operational tuning range, while the transition gap ($\mathrm{Z} \approx 0.67$–$0.68$ m) marks the beginning of mode mixing zone where sensitivity is significantly degraded.