Thin Film (High Temperature) Superconducting Radiofrequency Cavities for the Search of Axion Dark Matter

5 pages, 6 figures. v2: minor updates after referee comments, matches published version in IEEEThe axion is a hypothetical particle which is a candidate for cold dark matter. Haloscope experiments directly search for these particles in strong magnetic fields with RF cavities as detectors. The Relic Axion Detector Exploratory Setup (RADES) at CERN in particular is searching for axion dark matter in a mass range above 30 $\mu$eV. The figure of merit of our detector depends linearly on the quality factor of the cavity and therefore we are researching the possibility of coating our cavities with different superconducting materials to increase the quality factor. Since the experiment operates in strong magnetic fields of 11 T and more, superconductors with high critical magnetic fields are necessary. Suitable materials for this application are for example REBa$_2$Cu$_3$O$_{7-x}$, Nb$_3$Sn or NbN. We designed a microwave cavity which resonates at around 9~GHz, with a geometry optimized to facilitate superconducting coating and designed to fit in the bore of available high-field accelerator magnets at CERN. Several prototypes of this cavity were coated with different superconducting materials, employing different coating techniques. These prototypes were characterized in strong magnetic fields at 4.2 K.This project has received funding from the European Union’s Horizon 2020 Research and Innovation programme under Grant Agreement No 730871 (ARIES-TNA). BD and JG acknowledge funding through the European Research Council under grant ERC-2018-StG-802836 (AxScale). We also acknowledge funding via the Spanish Agencia Estatal de Investigacion (AEI) and Fondo Europeo de Desarrollo Regional (FEDER) under project PID2019- 108122GB-C33, and the grant FPI BES-2017-079787 (under project FPA2016-76978-C3-2-P). Furthermore we acknowledge support from SuMaTe RTI2018-095853-B-C21 from MICINN co-financed by the European Regional Development Fund, Center of Excellence award Severo Ochoa CEX2019- 000917-S and CERN under Grant FCCGOV-CC-0208 (KE4947/ATS).With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Thermodynamic Design of a Cryostat for a Radiofrequency Cavity Detector in BabyIAXO in Search of Dark Matter

Despite compelling evidence, dark matter (DM) has still not been directly detected yet. The promising DM candidate, the axion particle, is searched for by the International Axion Observatory (IAXO). BabyIAXO is the demonstrator of IAXO and will be built in the near future. The Relic Axion Detector Exploratory Setup (RADES) plans on installing a 10 m long resonant cavity detector at BabyIAXO. This serves as an additional experiment in BabyIAXO, which probes unexplored axion parameter space and for which low temperatures are stringent for the success of the operation. Thus, the need for a cryostat arises that fulfills the requirements of a dry cryostat, the operation in a low-temperature and magnetic field environment, and the low-interference with the sensitive measurements. This work develops a first design of the cryostat which is based on the cryocooler remote cooling via a closed circulation loop of helium. The heat loads of the components are estimated and a thermodynamic model of the cooling system is compiled. An experiment imitating the present system is conducted at the European Organization for Nuclear Research (CERN) as proof-of-concept and compared with the model. The operation of the experiment reveals a substantial background heat load and a susceptibility for thermal acoustic oscillations (TAOs), which is why the envisioned low temperature regime is not reached. Nevertheless, the remaining data suggest a satisfactory operation of the cooling system and an acceptable agreement of the temperature levels of the experimental setup and the thermodynamic model. Especially, the intermediate temperatures and the cryogenic circulator can be portrayed sufficiently. Thus, for now, the predictions of the thermodynamic model are cautiously trusted. The latter enables the optimization of the layout and operating parameters of the cooling system. The findings of the optimization are discussed in the present thesis. The present cooling system, consisting of two cryocoolers, a cryogenic circulator, a counter-flow heat exchanger (CFHX), and connecting cooling pipes, can reach cavity and low noise amplifier (LNA) temperatures of about 4.6 K. The results demonstrate the feasibility of the application of the resonant cavity detector in BabyIAXO. A more detailed design of the resonant cavity, the tuning mechanism, and the support structure inside the BabyIAXO bore seem exciting and are needed to progress in the cryostat design and to increase the accuracy of the model. Additionally, an electromagnetic and mechanical analysis must assess the influence of a quench of the BabyIAXO magnet on the present system

A proposal for a low‐frequency axion search in the 1–2 μ eV range and below with the BabyIAXO magnet

In the near future BabyIAXO will be the most powerful axion helioscope, relying on a custom‐made magnet of two bores of 70 cm diameter and 10 m long, with a total available magnetic volume of more than 7 m³. In this document, it proposes and describe the implementation of low‐frequency axion haloscope setups suitable for operation inside the BabyIAXO magnet. The RADES proposal has a potential sensitivity to the axion‐photon coupling gaγ down to values corresponding to the KSVZ model, in the (currently unexplored) mass range between 1 and 2 μ eV, after a total effective exposure of 440 days. This mass range is covered by the use of four differently dimensioned 5‐meter‐long cavities, equipped with a tuning mechanism based on inner turning plates. A setup like the one proposed will also allow an exploration of the same mass range for hidden photons coupled to photons. An additional complementary apparatus is proposed using LC circuits and exploring the low energy range (≈10⁻⁴ − 10⁻¹ μ eV). The setup includes a cryostat and cooling system to cool down the BabyIAXO bore down to about 5 K, as well as an appropriate low‐noise signal amplification and detection chain

A Proposal for a Low‐Frequency Axion Search in the 1–2 μ eV Range and Below with the BabyIAXO Magnet

In the near future BabyIAXO will be the most powerful axion helioscope, relying on a custom‐made magnet of two bores of 70 cm diameter and 10 m long, with a total available magnetic volume of more than 7 m³. In this document, it proposes and describe the implementation of low‐frequency axion haloscope setups suitable for operation inside the BabyIAXO magnet. The RADES proposal has a potential sensitivity to the axion‐photon coupling gaγ down to values corresponding to the KSVZ model, in the (currently unexplored) mass range between 1 and 2 μ eV, after a total effective exposure of 440 days. This mass range is covered by the use of four differently dimensioned 5‐meter‐long cavities, equipped with a tuning mechanism based on inner turning plates. A setup like the one proposed will also allow an exploration of the same mass range for hidden photons coupled to photons. An additional complementary apparatus is proposed using LC circuits and exploring the low energy range (≈10⁻⁴ − 10⁻¹ μ eV). The setup includes a cryostat and cooling system to cool down the BabyIAXO bore down to about 5 K, as well as an appropriate low‐noise signal amplification and detection chain