Theories predict that Superconducting-Insulating-Superconducting (SIS) multilayers delay vortex penetration allowing for operation gradients more than twice of bulk Nb cavities and significantly higher Q-values [1]. The University of Hamburg focuses on Atomic Layer Deposition (ALD) as the most promising technique to coat SIS multilayers. A proof-of-principle experiment to coat cavities with an insulator has been successfully carried out, and the complex coating process was numerically modelled, which resulted in a further process time reduction while maintaining the high film quality [2,3]. For SIS multilayer deposition, plasma-enhanced ALD (PEALD) is used to deposit AlN and NbTiN as dielectric and superconducting material, respectively. The deposition process and post-deposition treatments have been optimized by studying the superconducting properties of the NbTiN thin film [4]. Moreover, properties such as flux-trapping behaviour and thermal transmittance of SIS multilayers have been measured. Furthermore, various material characterization techniques were applied to investigate the contribution of vacancy densities, recrystallization eVects due to the annealing past the deposition and the impact of the insulating layer on the properties of SIS multilayers. This talk will show the aggregated results of all those measurements and present the status of the PEALD single-cell cavity coating device at the University of Hamburg.

The native oxides of niobium cause surface losses during cavity operation arising from two-level systems/defects (TLS). These losses dominate the quality factor at low accelerating gradients (Eacc < 0.1 MV/m). In particular, the amorphous Nb2O5 is identified as a prominent host for the TLS. Nb2O5 dissociates when the material is baked above 200 °C for several hours in vacuum (the so-called Mid-T Bake), allowing for the modification or reduction of these losses. However, due to the inevitable exposure to air after the annealing, the surface reoxidizes and Nb2O5 regrows. When the cavity is already coated with Al2O3 or Ta2O5 and then subjected to the Mid-T Bake, this subsequent reoxidation of the niobium is inhibited. It is still unclear how the TLS losses are modified when the surface undergoes a passivating coating, and this study aims at possibly finding a correlation between the different passivating layers. Herein, we studied the quality factor of several superconducting radio frequency cavities in the low gradient range (Eacc < 0.1 MV/m) at 1.5 K and analyzed the data using TLS models like the standard TLS model and the non-interacting TLS (one species and two species). Specifically, we used cavities that had undergone the standard “European XFEL” treatment, followed by an atomic layer depositing coating with a passivating layer and the subsequent Mid-T Bake.

In the last years, low frequency gravitational waves (GWs) have been consistently measured by the LIGO-Virgo collaboration, but little to no attention has been paid to higher frequencies GWs in the range of 10 kHz to 100 MHz, at which confirmation for current theories or even new physics could be hidden. The MAGO 2.0 project aims at filling this gap in the parameters space using superconducting radio-frequency (SRF) cavities. Exploiting the excellent Q-factors of these resonators, we plan to detect tiny harmonic deformations induced by GWs which change the boundary conditions of the oscillating electromagnetic field. We present the results of the first cold tests ran at DESY and FNAL using the cavity prototype built 20 years ago at the end of the MAGO collaboration, characterizing the RF spectrum, Q-factor and surface resistance. In particular we present the mechanical vibration spectrum characterization and the RF response of the cavity with the injection of a “fake GW” signal using piezoelectric actuators.

A program of quantitative measurements of magnetic flux expulsion on flat macroscopic samples has been used to assess and categorise magnetic expulsion efficiency. The measurement setup is a magnetic flux lens based on closed-topological heating/cooling through the material’s superconducting transition. This offers systematic and repeatable expulsion measurements for bulk, thin film and multilayer samples. Of particular interest is the magnetic response of superconductor-insulator-superconductor (SIS) multilayer structures, which can exhibit a response that is characteristically different to that of bulk Niobium, if thermally manipulated in a specific way - this process we term “flux ratcheting”. Flux ratcheting is the incremental expulsion of trapped magnetic flux with repeated, controlled thermal cycles on a SIS sample, such that the trapped flux is incrementally moved (“ratcheted”) out, with limited magnetic relaxation. Measurements indicate flux ratcheting is particular to the SIS structure, and requires the Tc of the surface thin film to be greater than that of the substrate. To assess the impact of flux ratcheting on cavity performance, the application of an SIS structure to a 1.3 GHz bulk Nb cavity has been prepared, and referenced to the baseline performance of the bare Nb cavity. The RF performance with and without flux ratcheting is compared, and first implications of magnetic flux ratcheting to RF cavity performance are discussed.

The successful development of Nb3Sn/Cu coatings for the SRF cavities of next generation particle accelerators would result in the reduction of the needed cryogenic power by a factor 3 with respect to what normally needed for bulk Nb cavities, while maintaining operation at 4.5 K. In the framework of the I.FAST and ISAS collaborations, research activities are carried out at INFN-LNL to develop new technologies for the application of Nb3Sn on Cu, including seamless spinning of cavity prototypes, surface chemical preparation, cavity coating and testing. At the same time, an optimized recipe for Nb3Sn films deposited via DCMS has been established on small samples and is discussed in this work. The recipe delivers films showing a Tc ≈ 17 K, at deposition temperatures ≤ 650 °C, on a Cu substrate pre-coated with a 30-micron thick buffer layer of Nb. The deposition recipe is validated on bulk Nb by measuring the RF properties on a QPR sample, with the results being also discussed in this work. A surface resistance of 23 nΩ at 4.5 K (at 20 mT, 417 MHz, with quench field ~ 70 mT) is measured, which is about 5 times larger than the baseline specifications for the LHC Nb/Cu cavities and already fulfills the requirements for the FCC-ee. Finally, the expected challenges toward the scalability of the coating recipe to an elliptical cavity prototype, and the perspectives for further recipe refinement are discussed.

Superconducting–Insulating–Superconducting (SIS) multilayers offer a promising approach to surpass the accelerating gradients and quality factors of standard bulk-Nb SRF cavities†. Plasma-enhanced atomic layer deposition (PEALD) stands out as a key technique for the next-generation thin-film-based SRF cavities, providing conformal coatings on highly structured, three-dimensional substrates without shadowing effects and with sub-nm thickness precision. This poster contributes to thin-film SRF R&D through dedicated material studies. The results presented correspond to Nb–AlN–NbTiN multilayers grown by PEALD, focusing on the S–I and I–S interfaces. Depth-resolved X-ray photoelectron spectroscopy (XPS) and cross-sectional energy-dispersive X-ray spectroscopy (EDX) are employed to assess the film stoichiometry and detect any interdiffusion or deposition residues. Side effects induced by high-temperature post-deposition annealing–required to obtain high-Tc NbTiN‡–are systematically investigated. Lastly, complementary studies on Superconducting–Superconducting (SS) Nb–NbTiN bilayers–grown without the AlN interlayer–underscore the crucial role of AlN as an effective diffusion barrier.

Today, apart from some isolated R&D efforts, there are no GW experiments, yet which explore a large part of the vast frequency range above the LIGO/Virgo band. It is planned to establish an experiment at DESY and FNAL to search for high-frequency GWs in the frequency range of 10 kHz to 100 MHz. The basic idea is to use superconducting radiofrequency (SRF) cavities to detect tiny harmonic deformations induced by GWs which change the boundary conditions of the oscillating electromagnetic field. This talk deals with a brief motivation of this search, which dictates the challenging environmental boundary requirements, and the R&D to operate a cavity using a LLRF system which pushes beyond state-of-the-art accuracy and resolutions and a seismic noise mitigated cryostat at 1.8 K. A focus of the presentation will be the warm and cold commissioning of a prototype cavity, built 20 years ago during the MAGO collaboration, and its first measurement in our collaborative research project. We will address the questions “What can we learn from this prototype?” and “how to design and build an optimized cavity for high-f GW search?” and the preparations for a first physics run in 2026 to explore an uncharted phase space in GW physics.”