Abstract
RADES (Relic Axion Detector Exploratory Setup) is a project with the goal of directly searching for axion dark matter above the 30μeV scale employing custom-made microwave filters in magnetic dipole fields. Currently RADES is taking data at the LHC dipole of the CAST experiment. In the long term, the RADES cavities are envisioned to take data in the BabyIAXO magnet. In this article we report on the modelling, building and characterisation of an optimised microwave-filter design with alternating irises that exploits maximal coupling to axions while being scalable in length without suffering from mode-mixing. We develop the mathematical formalism and theoretical study which justifies the performance of the chosen design. We also point towards the applicability of this formalism to optimise the MADMAX dielectric haloscopes.
Highlights
RADES (Relic Axion Detector Exploratory Setup) is a project with the goal of directly searching for axion dark matter above the 30μeV scale employing custom-made microwave filters in magnetic dipole fields
In this article we have discussed the concept and realisation of a new cavity design useful in the search for > 30 μeV axions with microwave filters. As such filters can be used in any dipole magnetic field, this is of general interest for ‘axion hunters’ all over the world and a prime example of a ‘physics beyond colliders’ initiative [47], combining accelerator technology at CERN for particle search outside colliders
At the time of writing, a ∼ 1 meter long cavity of this type is installed in the CAST magnet and is foreseen to take physics data in 2020
Summary
The first RADES prototype built and tested in 2017/2018 (described extensively in [31]) was a ∼ 15cm long H-plane filter consisting of N = 5 sub-cavities connected through inductive irises. This structure was designed to fully couple its first (lowest frequency). Resonant mode to the axion field, but it is not optimal when going to a large number of sub-cavities because of the larger mode-mixing among adjacent resonant modes of the filter We have solved this problem by designing an optimal filter with minimal modemixing of the resonant mode to the axion field, which consists on a cascade of cavities connected by alternating inductive (H-plane) and capacitive (E-plane) irises. We show by simulations that the new design is robust to mechanical tolerances
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