Axion Searches with Microwave Filters: the RADES project

TL;DR

This paper introduces a novel microwave filter-based axion haloscope design, the RADES project, capable of exploring higher mass ranges of dark matter axions with improved sensitivity using an array of coupled cavities.

Contribution

It proposes a new cavity array design for axion detection, develops the theoretical framework, and presents the construction and initial testing of a prototype at CERN.

Findings

Electromagnetic characterization shows promising QCD axion sensitivity.

Prototype operates at 8.4 GHz in a 9 T magnetic field.

Design optimizes detection volume and frequency for axion searches.

Abstract

We propose, design and construct a variant of the conventional axion haloscope concept that could be competitive in the search for dark matter axions of masses in the decade 10-100 $\mu$eV. Theses masses are located somewhat above the mass range in which existing experiments have reached sensitivity to benchmark QCD axion models. Our haloscope consists of an array of small microwave cavities connected by rectangular irises, in an arrangement commonly used in radio-frequency filters. The size of the unit cavity determines the main resonant frequency, while the possibility to connect a {large} number of cavities allows to reach large detection volumes. We develop the theoretical framework of the detection concept, and present design prescriptions to optimize detection capabilities. We describe the design and realization of a first small-scale prototype of this concept, called Relic Axion Detector Exploratory Setup (RADES). It consists of a copper-coated stainless steel five-cavities microwave filter with the detecting mode operating at around 8.4 GHz. This structure has been electromagnetically characterized at 2 K and 298 K, and it is now placed in ultra-high vacuum in one of the twin-bores of the 9 T CAST dipole magnet at CERN. We describe the data acquisition system developed for relic axion detection, and present preliminary results of the electromagnetic properties of the microwave filter, which show the potential of filters to reach QCD axion window sensitivity at X-band frequencies.