z www.nature.com/scientificreports/ commercially available CCs have shown lower R s than 300 µm thick Cu colaminated on stainless steel (equivalent of FCC beam screen) at T = 50 K and ν = 8 GHz up to 9 T. In addition, we demonstrated compatibility with thin a-C layers to mitigate the secondary electron yield 11. In this work, we extend the temperature range of measured surface resistances to T = 20−70 K. It allows a discussion about the surface resistance and vortex physics of CCs with different microstructures in a wide range of temperatures. The vortex parameters depinning frequency ν 0 , vortex viscosity η and Labusch parameter k p are derived within the Gittleman-Rosenblum and Bardeen-Stephen models. The model proposed by Gittleman and Rosenblum 12 is a mean-field theory for vortices in a periodic pinning potential driven by high frequency oscillating, subcritical currents without thermal activation. It modulates both the resistive and reactive response of vortices to the driving field. In our case, the lack of complete surface reactance data sets has to be compensated by additional model confinements. With a modified Bardeen-Stephen model 13 , which describes the motion of vortices in a type II superconductor, we can estimate the vortex viscosity η. It reduces the Gittleman-Rosenblum model parameters to only one, the depinning frequency ν 0 , and thus makes it available through fitting the measured surface resistance R s. We validate the two model approach for one sample with the ratio of surface reactance and surface resistance at T = 20 K , which lets us determine the depinning frequency sticking exclusively to the Gittleman-Rosenblum model. Finally, having established the microwave vortex parameters, we extrapolate the surface impedance of CCs down to 1 GHz , up to 16 T and compare it to the microwave response of Cu at FCC-hh conditions.

To assess the feasibility of using high-temperature superconductors for the beam screens of future circular colliders, we have undertaken a study of the power dependence of the microwave surface resistance in state-of-the-art REBCO coated conductors at about 8GHz and 50K. We have employed a dielectric resonator to produce radio-frequency electromagnetic fields on the surface of the coated conductors having amplitudes similar to those generated by proton bunches circulating in the vacuum chamber of the proposed hadron-hadron Future Circular Collider at CERN. We show that surface resistances in REBCO coated conductors without artificial pinning centers are more affected by a radio-frequency magnetic field than those containing nano-inclusions. Despite that, at 8GHz, 50K, and 9T, most REBCO coated conductors studied outperform copper in terms of surface resistance, with the best sample having a 2.3mΩ surface resistance while being subject to an RF field 2.5 times stronger than that in the FCC-hh. We also extrapolate the measured data to 16T and 1GHz, the actual FCC-hh dipole magnetic field, and mid beam frequency spectrum, demonstrating the possibility of lowering the surface resistance of the vacuum chamber by up to two orders of magnitude compared to copper. Further, we discuss the correlation between the time structure of the electromagnetic fields provided by vector network analyzers compared to the proton bunches' time structure in the collider and present the effect of low alternating magnetic fields on vortex displacement and the possibility of demagnetization of superconducting samples.

The 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 µeV. 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 REBa2Cu3O7−x, Nb3Sn 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.

To ensure beam stability in CERN’s future circular hadron collider, RE(=Y, Gd, Eu)Ba2Cu3O7−x high-temperature superconductor was proposed as a low-surface impedance coating for its beam screen. Unfortunately, persistent currents in the superconductor will degrade the magnetic field homogeneity inside the beam chamber, endangering the stability of the beam trajectory. To counteract this effect, we have explored the possibility to use a highly conductive hybrid coating made of Cu and REBa2Cu3O7−x. This decreases the surface impedance when compared to that of pure copper, while maintaining high magnetic field quality inside the beam screen chamber. This work formulates guidelines for hybrid coating geometries to comply with the field quality criterion of a circular accelerator during operation with dipole magnets by means of finite elements numerical analysis. We produced hybrid coating samples with compliant geometries via photolithography. Scanning Hall microscopy and radiofrequency characterization have given the first experimental confirmation that these hybrid coatings offer high field quality and present a surface resistance lower than that of copper for the beam screen of the future circular hadron collider. The excellent agreement shown between experimental results and simulations validate that the numerical analysis performed throughout this work can be used as a prediction tool for future proposed geometries.

No abstract