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12th International Workshop on Thin Films and New Ideas for Pushing the Limits of RF Superconductivity
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Europe/London
Prince of Wales Suite (Crowne Plaza Chester, Chester, United Kingdom)
Prince of Wales Suite
Crowne Plaza Chester, Chester, United Kingdom
Description
The 12th Workshop in this series will further the goal of providing a Forum for new initiatives in innovative thin films and related technology to advance future generations of superconducting RF accelerators. Present superconducting RF accelerator technology is based on predominantly bulk niobium, for which the state of the art in performance is reaching the theoretical limit. Thin film technology offers the prospect of considerable savings in fabrication costs and opens the way with innovative technologies to the use of alternative superconducting materials with enhanced intrinsic properties such as critical temperature and critical field. Intensive and coordinated R&D effort is of decisive importance for the scientific community.
The primary aim of the workshop is to support this initiative by providing an opportunity to bring together individuals and institutions working in this effort and infusing expertise of specialists from related disciplines (superconductivity, plasma physics, material science, nanotechnology, RF engineering and industry). Reports on work from each participating group and extensive discussions on existing problems, new ideas and programs for the future constitute the primary focus of the program.
This edition will be held in: The Crowne Plaza Chester by IGH United Kingdom (21st-25th September 2026)
Crowne Plaza Chester by IHG - Google Maps
Registration deadline is August 7th, 2026.
Abstract submission deadline is August 7th, 2026.
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WelcomeConvener: Reza Valizadeh (STFC)
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Welcome Address 8mSpeaker: Peter McIntosh (STFC)
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Workshop Information 10m
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Setting the Horizon: Global Perspectives & Technical RoadmapsConvener: Reza Valizadeh (STFC)
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Coordinated thin film SRF activities in Europe 35m
Over last 10 years, the EU funded Horizon 2020 and Horizon Europe programmes helped with setting up cross-European collaboration on developing superconducting thin (TF) film R&D for SRF applications. Four programmes were including work packages desiccated to this activity.
ARIES (May 2017 – Apr. 2021) WP15 on Thin film for Superconducting RF Cavities was aiming to study an effect of substrate preparation on a quality of superconducting TF, primarily of Nb, however, other materials (NbN, NbTiN, Nb3Sn) as well as SS and SIS structures were studied.
IFAST (May 2021 – Oct. 2025) WP9 on Innovative superconducting thin film coated cavities was aiming to further develop apply this knowledge to 6 and 1.3 GHz cavities. The main result was the first in the world bulk Nb cavity PVD coated with Nb3Sn film. The R&D includes deposition target development, optimising PVD and ALD deposition, functional layers of TF, post deposition annealing with laser and FLA. Another important outcome was producing the European strategy on TF SRF.
ISAS (Feb. 2024 – Jan. 2028) WP3 on Innovative superconducting thin film coated cavities is focused on specific problems: producing several copper cavities coated with Nb3Sn and SRF testing, studying magnetic field trapping and its mitigation, exploring Nb3Sn coated cavity tunability.
EPITA (May 2027 – Apr. 2031) WP4: Superconducting Thin Films coated RF cavities aims industrialisation of cavity production. The programme includes producing several copper cavities coated with Nb3Sn and SRF testing in vertical and horizontal cryostats, exploring conduction colling cryostats, and ongoing R&D on target development and thin film deposition optimisation.
These programmes enabled a close collaboration and coordination between HEI with TF SRF activities as well as academics from laboratories specialised in other technologies such as superconducting materials, surface science, plasma science, laser-matter interaction and FLA.
The TRL level has been increased after completing each programme from 3 in ARIES to 6 in EPITA. The ultimate aim of these programmes is to develop the technology of Nb3Sn coated cavity production and transfer it to industry. A number of industrial partners has grown from zero in ARIES to 3-5 involved in EPITA.Speaker: Dr Oleg Malyshev (STFC Daresbury Laboratory) -
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Ongoing DOE RF Roadmap Update Exercise 35m
Generation SRF Thin Film Technologies
As conventional bulk niobium (Nb) superconducting radio-frequency (SRF) technology approaches its intrinsic physical limits (Bc1 / Bsh), thin film deposition and novel material engineering have emerged as the paramount path forward to expand accelerator energy reach, increase quality factors (Q0), and significantly lower cryogenic operating costs. Under the U.S. Department of Energy’s General Accelerator R&D (GARD) framework, an updated strategic roadmap is being formulated to align research thrusts across national laboratories, universities, and international collaborating institutions.
This presentation outlines the ongoing update to the SRF GARD thin-film R&D strategy. We highlight primary research thrusts and technological milestones, including heterogeneous Nb-on-Cu structures aimed at mitigating Q-slope at elevated accelerating gradients (Eacc) through energetic condensation techniques (e.g., ECR, HiPIMS) and refined substrate preparation to match or exceed bulk Nb performance with superior thermal stability; higher-Tc and Hc2 superconducting materials, most notably Nb3Sn, Nb1-xTixN, and MgB2, to enable high-Q operation at 4.2 K, dramatically simplifying cryoplant requirements for future high-duty-cycle and compact industrial accelerators; and multilayer superconductor-insulator-superconductor (SIS) coatings utilizing nanometer-scale magnetic screening layers to delay flux penetration beyond Hc1bulk, opening unprecedented parameter spaces for high-field cavity operations.
In addition to core material physics, the updated roadmap places strong emphasis on cross-cutting capabilities: seamless cavity fabrication, specialized surface diagnostics, substrate functionalization, and scalable deposition technologies suitable for complex 3D structures and full-scale cryomodule integration. By consolidating current experimental achievements and identifying remaining technological bottlenecks, the updated GARD strategy provides a unified framework to accelerate the transition of advanced SRF thin films from fundamental laboratory coupons to mature accelerator infrastructure.Speaker: Dr Anne-Marie Valente-Feliciano (Jefferson Lab)
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Coffee Break 30m
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Pushing SRF Thin Films Limits: Theoretical Advances, Deposition Modeling & DFTConvener: Claire Antoine
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Modelling flux vortex dynamics in type-II superconductors under a thermal gradient-driven phase transition with closed cooling topology 20m
While perfect type-II superconductors are expected to completely expel their internal magnetic flux after transitioning into the Meissner state, impurities and defects may cause some level magnetic flux to be trapped as vortices. Such trapped vortices can be problematic for SRF cavities, causing unwanted localized heating from vortex interaction with the applied RF field, while the trapped flux itself is impacted by cool down dynamics [1] and thermal topology.
As part of dedicated magnetic flux expulsion measurements performed on bulk, thin-film and multilayer superconductor samples using CERN’s Magnetic Flux Lens (MFL) [2], thermally driven flux dynamics have been studied, and thermo-electric current and recursive flux ratcheting effects observed. To better understand and isolate contributions of the observed flux dynamics, a detailed multiphysics simulation model of the MFL setup has been implemented, based on time-dependent Ginzburg-Landau equations coupled with thermodynamics. These numerical methods, in combination with measurement data, are used to investigate flux trapping mechanisms during the superconducting transition. Results show a clear and physical understanding of the dynamics, with connections to the order parameter and magnetic flux density distributions, while detailing the temperature-dependent critical field evolution across the closed cooling topology of the MFL sample geometry, in line with recent theoretical models [3].[1] O. Kugeler et al., Manipulating the intrinsic quality factor by thermal cycling and magnetic fields, Proc. 14th Int. Conf. RF Supercond. (SRF'09) p. TUPPO053 (2009).
[2] D. Turner et al. Flux expulsion lens: concept and measurements, Proc. 21st Int. Conf. RF Supercond. (SRF'23) p. 56—61 (2023)
[3] T. Kubo, Flux trapping in superconducting accelerating cavities during cooling down with a spatial temperature gradient, Prog. Theor. Exp. Phys, Volume 2016, Issue 5, 053G01.Speaker: Guilherme Theophilo Telles (CERN) -
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Chemical Adsorption of Water on Niobium and Its Surface Oxides in the Medium-Temperature Baking Regime: An ab initio Study 20m
The surface chemistry of superconducting radio-frequency (SRF) niobium cavities during medium-temperature baking (MTB, ~250–400 °C) has been widely investigated. However, atomic-scale insight into how water molecules—inevitably introduced by high-pressure rinsing (HPR)—interact with niobium and its surface oxides remains largely absent. Here, using first-principles calculations, we systematically study water adsorption on clean Nb and on its representative oxide surfaces. Our preliminary results show that water physisorbs weakly on metallic niobium, whereas on oxidized surfaces it spontaneously dissociates with the aid of lattice oxygen, forming two surface hydroxyl groups in a significantly more stable chemisorbed state. We further assess the desorption kinetics of these hydroxyl groups through ab initio molecular dynamics (AIMD) simulations at selected temperatures. These adsorption and desorption characteristics may change the loss tangent of the surface oxides. They may also correlate with the precipitation of dissolved hydrogen during cavity cooldown, which leads to hydride-related Q degradation. These findings offer new insight into the microscopic origin of the MTB window.
Speaker: Shantong Chen (Institute of Modern Physics,CAS) -
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Preliminary Particle-in-Cell Simulation Results of Plasma Discharge from a DC Magnetron Sputtering Device 20m
In this study, we report on preliminary results from plasma-kinetic particle-in-cell (PIC) simulations, investigating the plasma discharge phenomenon in a DC magnetron sputtering device for thin film deposition. As the quality of the sputtered films depend on the plasma chemical composition and interaction with the target material, it is important to understand and characterise the underlying plasma discharge process for different operating conditions like magnetic field, pressure, choice of background gas etc. Of particular interest is also the nature of plasma instabilities for different operating conditions and their influence on the sputtering process. We will also explore possibilities of comparing PIC simulation results with experiments in the future.
Speaker: Dr Benzi John (STFC-UKRI) -
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Multilayer model for coatings with arbitrary number of planar layers for SRF applications 20m
We extend the model of Kubo et al. [1] in two ways: first, by generalizing it to a arbitrary sequences of planar layers of arbitrary type, e.g. superconducting (S), normal conducting and insulating (I); and second,
by accounting for all contributions, including Ohmic losses and dielectric effects. We examine the maximum applicable field for (SI)$^n$S structures and find that the optimum configuration corresponds to $n=1$. However, allowing for $n>1$, the thickness of the superconducting coatings can be chosen such that they are below the penetration depth with minor performance penalty. We further discuss the ability to model transitions in SS bilayers by introducing a set of virtual layers that represent the transition region through interpolated parameters. We find a degradation of the maximum applicable field and a larger effective penetration depth of the electromagnetic fields with thicker transition layers. Finally, we determine the surface impedance of such multilayer structures in a formulation suitable for integration into numerical electromagnetic field solvers, which we applied in a recent work [2] currently under the peer-review process at Physical Review: Accelerator and Beams. The discussed work is currently available on arXiv [3].[1] T. Kubo, Multilayer coating for higher accelerating fields in superconducting-radio-frequency cavities: a review of theoretical aspects, Supercond. Sci. Technol. 30(2), 023001 (2016).
[2] A. Gobeyn, W. Ackermann, H. De Gersem, Numerical quality factor statistics for SRF cavities with spatially inhomogeneous multilayer coatings modeled by Gaussian random fields, arXiv:2605.05505, (2026).
[3] A. Gobeyn, W. Ackermann, H. De Gersem, Multilayer model for coatings with arbitrary layers for superconducting radio-frequency applications, arXiv:2605.07958 (2026).Speaker: Mr Aaron Gobeyn (TU Darmstadt) -
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Pushing SRF Thin Films Limits: Theoretical Advances, Deposition Modeling & DFT - Discussion 10m
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Lunch 1h 30m
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Latest Advances in Nb Thin Film TechnologyConvener: Walter Venturini Delsolaro (CERN)
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Toward Taming Topographical Vulnerabilities in Nb/Cu Thin Films for Next-Generation SRF Accelerator 30m
Deposition of high-purity niobium (Nb) thin films onto copper (Cu) substrates provides a direct path to Deposition of high-purity niobium thin films onto copper substrates bypasses the high material costs and thermal stability limits of bulk niobium for superconducting radio-frequency cavities, yet achievable accelerating gradients remain hindered by premature vortex entry and high-field quality-factor degradation. Although energetic condensation techniques optimize film growth kinetics, conventional average roughness metrics fail to account for field limitations; symmetric triangular surface morphologies maintain an average roughness that is entirely invariant to slope angle, concealing steep nanoscale topographies on surfaces that appear exceptionally smooth under standard profiling. Atomic force microscopy reveals corrugations whose physical dimensions and steep facet slopes directly correspond to the characteristic London penetration depth and superconducting coherence length of clean niobium. Within the London theoretical model, these geometries induce localized magnetic field enhancement at protrusion ridges and significantly degrade the Bean–Livingston surface barrier within adjacent troughs, providing a quantitative origin for the peak magnetic field plateaus observed in state-of-the-art coated structures. Optimizing deposition energetics alongside near-surface impurity engineering offers a promising path to mitigate these geometric vulnerabilities and enable high-gradient cavity operation.
Speaker: Anne-Marie Valente-Feliciano (Jefferson Lab) -
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Recent RF test results of Nb/Cu full-seamless 1.3 GHz cavities 30m
KEK and CERN are conducting collaboration research to fabricate superconducting RF cavities by hydroforming. To date, several 1.3 GHz single‑cell cavities have been produced and their performance measured. It is well known that hydroforming an elliptical cavity causes substantial plastic deformation, which degrades the surface roughness. We are investigating how improving the surface finish of this substrate affects RF performance. In the cavities previously manufactured, several approaches were taken: one was coated with a niobium thin film without any prior reduction of surface roughness, while the other involved polishing the copper surface before deposition using several methods. Mechanical polishing of the copper tube prior to hydroforming has been applied to two cavities; however, this treatment did not appreciably improve the roughness. RF measurements revealed that one cavity achieved an excellent result, maintaining $Q_0 > 1 \times 10^{10}$ at 18 MV/m at 1.85 K, while the second exhibited markedly poorer performance. In addition, a method of mechanically polishing the copper surface after hydroforming has also been investigated. This technique yielded significant improvement in surface roughness, and RF performance measurement is scheduled in the near future. We will present the ongoing status of our collaboration and the results with a focus on the roughness improvement.
Speaker: Hayato Araki (KEK) -
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From 1.3 GHz Demonstrators to 400 MHz FCC-ee Cavities: Challenges in Process Scale-Up 30m
Superconducting radiofrequency (SRF) niobium-on-copper (Nb/Cu) cavities are a key enabling technology for meeting the performance requirements of the Future Circular Collider (FCC). Within the ongoing CERN R&D programme, substantial progress has been made on 1.3 GHz test cavities through optimisation of Nb film deposition via high power impulse magnetron sputtering (HiPIMS), including the use of DC substrate bias and synchronised pulsed biasing. These approaches provide control over ion energy and ionic-species flux during film growth, enabling 1.3 GHz Nb/Cu cavities not only to meet, but to exceed, the FCC-ee performance targets when scaled to this frequency. The next challenge, and the focus of this talk, is the transfer of this process route to the FCC-ee operational 400 MHz cavity geometry. The larger cavity scale introduces additional constraints, making copper substrate preparation a critical element of the technology scale-up. In particular, electropolishing of large copper surfaces requires tight control of electrolyte chemistry, temperature, flow conditions, material-removal uniformity, rinsing, and passivation before coating. These steps determine the quality of the Nb-Cu interface and are therefore central to film adhesion, suppression of local defects, mitigation of field-limiting features and reproducible RF performance.
Speaker: Caroline Hain (CERN)
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Coffe Break 30m
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Latest Advances in Nb Thin Film TechnologyConvener: Walter Venturini Delsolaro (CERN)
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Latest Advances in Nb Thin Film Technology - Discussion 1h
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Nb3Sn Technology: Recent Technical Advances & Scaling HorizonsConvener: Uttar Pudasaini (Jefferson Lab)
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From Nb to Nb3Sn: Complexity in SRF Coating Development 30m
Nb$_3$Sn coatings on copper offer a promising route beyond established niobium-on-copper (Nb/Cu) thin film superconducting radiofrequency (SRF) cavities, particularly for future accelerators where reducing cryogenic power demand is a key objective. In the Future Circular Collider (FCC) context, Nb$_3$Sn/Cu is being explored as a potential pathway towards operation above 2 K, by exploiting the higher critical temperature of Nb$_3$Sn and its lower expected BCS surface resistance compared with Nb. However, Nb$_3$Sn is not a direct replacement for elemental Nb. Its performance depends on the formation of the ordered A15 phase, near-stoichiometric composition, controlled microstructure, low surface roughness, and stable film–substrate interfaces. Work at CERN has shown that dense Nb$_3$Sn films can be deposited on Cu via high power impulse magnetron sputtering (HiPIMS). However, their superconducting properties remain highly sensitive to substrate effects and processing conditions. The thermal expansion mismatch between Nb$_3$Sn and Cu can generate residual stress, potentially affecting long-range atomic order and reducing the critical temperature relative to the 18.3 K bulk value. Cu interdiffusion is another critical parameter. While it may assist A15 phase formation, diffusion towards the RF surface can create normal-conducting regions and degrade performance. Diffusion barriers such as Ta are therefore being investigated, although their effectiveness depends on obtaining the appropriate crystalline phase. To better understand the conditions required for high-performance Nb$_3$Sn films, this work draws insight from the deposition of Nb and Ta coatings, by investigating deposition conditions with and without in situ substrate heating on different substrate materials, enabling the specific influence of the Cu substrate to be more clearly isolated and understood. These studies provide a basis for identifying the substrate, interface, and processing conditions that govern Nb$_3$Sn/Cu performance in SRF applications.
Speaker: Caroline Hain (CERN) -
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Nb₃Sn thin film development on 1.3 GHz cavities 25m
Nb₃Sn thin films on copper represent one of the most promising routes for superconducting radio-frequency accelerating cavities for future colliders. At INFN – Legnaro National Laboratories, Nb₃Sn thin films have been successfully deposited on a QPR sample via DC magnetron sputtering, achieving a surface resistance Rs < 9 nΩ. This work presents the design and development of a dedicated system to scale this deposition recipe from small samples to a full-size 1.3 GHz copper cavity. First results on a mock-up cavity are presented, together with the ongoing adaptation of the established recipe to the new experimental configuration through dedicated sample studies. The samples have been characterized by SEM, EDS and XRD, and their critical temperature has been measured.
Speaker: Dr Matteo Lazzari (INFN LNL) -
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Calibration and deposition of 1.3 GHz Nb cavity coated with Nb3Sn thin film by PVD planar magnetron sputtering. 25m
Nb3Sn thin film cavities are new generation of superconducting cavities, which have the potential to replace traditional pure niobium cavities owing to their superior theoretical radio frequency (RF) performance. Higher theoretical acceleration gradient and quality factor give Nb3Sn cavities more possibilities in the future. There have been relatively high success in producing such cavities by Sn diffusion method. The production of such cavities through thin film deposition on copper cavity foresees lower cost of material and at the same time profiting from higher thermal conductance of copper .
At Daresbury Laboratory, we have commissioned a new Nb3Sn deposition facility for depositing a Nb cavity with Nb3Sn. The system is based on planar magnetron where the optimised parameters for Nb3Sn on flat surfaces is stablished inside the segmented Nb cavity.
The Cavity was deposited at 650 °C using two special designed 2 inch magnetrons travelling inside cavity simultaneously from each end of the cavity. The deposition was done at optimum deposition parameter in DC power.
We further report on the RF performance of the cavity at 4 K in liquid He, demonstrating quality factor Q0 of 3x1010 comparable to Nb cavity at 2 K and extending to accelerating gradient of up to 16 MV/m.Speaker: Reza Valizadeh (STFC)
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Coffee Break 30m
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Nb3Sn Technology: Recent Technical Advances & Scaling HorizonsConvener: Uttar Pudasaini (Jefferson Lab)
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Suppressed thermoelectric currents in Nb3Sn-on-Cu structures 25m
We present trapped magnetic flux data of Nb3Sn films on copper substrates. The samples are prepared via DC magnetron sputtering and bronze-route. Additionally, we test samples with an insulating layer between substrate and superconductor, which was deposited via atomic layer deposition. By cooling down the sample with various temperature gradients and measuring the resulting trapped magnetic flux we can characterize how susceptible samples are to thermoelectric currents. Our results show that for the same material combination the measured trapped flux can vary by over an order of magnitude. This highlights the impact of the film’s morphologic properties on trapped flux due to thermoelectric currents. They also show the potential of an insulating layer to suppress the thermoelectric currents in the bi-metallic structures.
Speaker: Felix Kramer (Helmholtz-Zentrum Berlin) -
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Recent Progress on Bronze-Route Nb₃Sn Thin Films at HZB 25m
At Helmholtz-Zentrum Berlin (HZB), the Bronze Route is being developed as an alternative synthesis route for Nb₃Sn thin films on both Nb and Cu substrates. Recent work has focused on optimizing precursor composition, electroplating parameters, substrate preparation, reaction conditions, and post-processing to improve phase purity, microstructure, and superconducting performance. Particular emphasis is placed on understanding the influence of substrate quality, Nb diffusion barriers on Cu substrates, Sn concentration, and chemical surface treatments on Nb₃Sn formation.
The coatings are characterized by SEM, EDX and XRD to establish process–structure relationships, while their superconducting performance is evaluated by RF surface resistance measurements using HZB’s Quadrupole Resonator. In addition, the degree of flux-trapping, including thermo-current generated flux, is investigated using the recently commissioned CRAFT facility. The combination of these tools allows us to correlate microstructural features with RF losses, including those caused by trapped flux. The presented results demonstrate the progress of the Bronze Route towards a reproducible coating technology for Nb₃Sn thin films and provide new insights into the relationship between processing, microstructure, and superconducting performance.
Speaker: Oliver Kugeler (Helmholtz Zentrum Berlin) -
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Study on the synergistic optimization of material and thickness of diffusion barrier layer based on copper-based Nb3Sn film by bronze method 25m
To address the low superconducting transition temperature (Tc) of Cu-based Nb₃Sn films caused by Cu/Sn interdiffusion and thermal mismatch, Ta or Cr barrier layers (0.5/1 μm) together with Nb, Cu, and Sn were deposited on Cu substrates by magnetron sputtering using the bronze method, followed by three-step annealing. All films achieved Tc>17 K (max 17.42 K). The 0.5 μm Cr layer gave the narrowest transition width and higher Tc than the same?thickness Ta layer. XRD revealed that Cr promotes (210) preferred orientation, possibly due to interfacial Cr₂Nb formation. Cross?section EDS confirmed that Cr more effectively suppresses Sn back?diffusion to Cu. Overall, the 0.5 μm Cr barrier exhibits comprehensive advantages in superconductivity, diffusion inhibition, and texture control, offering a promising route for high?performance Cu?based Nb₃Sn superconducting cavities.
Speaker: BoHao Zhang (中国科学院近代物理研究所)
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Nb3Sn Technology: Recent Technical Advances & Scaling HorizonsConvener: Cristian Pira (INFN LNL)
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Understanding and Improving the Tin Vapor Diffusion Process for Nb₃Sn-Coated SRF Cavities at JLab 20m
Nb₃Sn is a promising alternative to bulk niobium for superconducting radio-frequency cavities, offering efficient operation near 4 K with performance comparable to conventional niobium cavities operated at 2 K. At Jefferson Lab, single-cell cavity coating experiments were conducted using the tin vapor diffusion technique while varying key process parameters, including the temperature profile, tin-source configuration, and coating setup. The coated Nb cavities and associated witness samples were evaluated through RF testing and materials characterization. The results are used to examine correlations among coating conditions, film uniformity, material properties, and cavity performance. These studies improve understanding of the coating process and support the development of more reproducible, high-performance Nb₃Sn-coated SRF cavities.
Speaker: Uttar Pudasaini (Jefferson Lab) -
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Role of surface oxide and anodization in Nb3Sn nucleation 20m
The performance of Nb3Sn as the next-generation material for SRF cavities is limited by the ability to achieve uniform, dense nucleation during the vapor diffusion growth process. In this work, we study how the Nb surface oxide evolves under UHV annealing, following the temperature profile used in Nb3Sn vapor diffusion growth. By comparing native and anodized oxide surfaces, we investigate how the different surface chemistries and morphologies influence nucleation, providing insight into strategies for improving Nb3Sn film quality and SRF cavity performance.
Speaker: Liana Shpani -
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Development of Nb3Sn coatings on Novel Cavity Geometries Using Vapor-Diffusion Technique 20m
In this contribution, we present recent progress in the development of Nb₃Sn-coated superconducting radio-frequency (SRF) cavities for continuous-wave, high-gradient applications using the vapor-diffusion coating technique. Several cavities with different geometries and sizes have been successfully coated with Nb₃Sn, including one that achieved an unprecedented peak magnetic field of about 105 mT. The coated cavities support a variety of applications, including the development of a low-beta Nb3Sn cavity for ion acceleration at ATLAS and initiatives aimed at the industrialization of SRF cavity fabrication at IARC. The lessons learned and technical challenges during the coating process will be discussed.
Speaker: Nikki Tagdulang (Fermilab) -
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Towards Industrial Nb3Sn-based Turn-Key SRF Systems 20m
Over the past 5 years, R&D activities aimed at the improvement of Nb3Sn coating technologies for SRF systems have gained increasing support from the European Union within projects like iFAST, iSAS and EPITA. The ultimate goal is the development of high-Q Nb3Sn-coated cavities for conduction-cooled cryomodules and further technology commercialization. This requires a significant effort for bridging the gap between research and industry. RI Research Instruments GmbH (RI) has a long-term interest in advancement of SRF technologies and a broad collaboration with research institutes world-wide. As a world-leading company in production of SRF cavities and turn-key cryomodules, RI possesses extensive and in-depth expertise required to advance toward industrialization of Nb3Sn-based conduction-cooled SRF systems for wide-range applications.
Speaker: Vitalii Shcherbinin (RI)
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Coffee Break 30m
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Nb3Sn Technology: Recent Technical Advances & Scaling HorizonsConvener: Cristian Pira (INFN LNL)
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Gray Enid I: Record Beam Acceleration with a Nb₃Sn SRF Cryomodule at JLab 30m
Nb₃Sn-coated cavities offer a promising route toward energy-efficient superconducting accelerators operating near 4 K with reduced cryogenic requirements. Gray Enid I, a cryomodule containing two five-cell, 1.5 GHz Nb₃Sn-coated niobium cavities, has completed two beam-acceleration campaigns at Jefferson Lab’s Upgraded Injector Test Facility. During the latest campaign, the cryomodule accelerated a continuous-wave electron beam from 200 keV to 7.1 MeV at 4 K, establishing a record beam energy for Nb₃Sn SRF technology. One cavity achieved an accelerating gradient of 11.8 MV/m with an intrinsic quality factor of approximately 4 × 10⁹ at 4.3 K. The accelerated beam was also delivered for a user application, providing an end-to-end demonstration of Nb₃Sn technology from coated multicell cavities and cryomodule integration to stable accelerator operation and practical beam use.
Speaker: Uttar Pudasaini (Jefferson Lab) -
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Nb3Sn Technology: Recent Technical Advances & Scaling Horizons- Discussion 1h
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Beyond Nb & Nb3Sn: Alternate materials, multilayer SIS structuresConvener: Marc Wenskat (DESY/Universität Hamburg)
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Advancements in MgB2-Coated Cu Superconducting RF Cavities 25m
Superconducting RF cavities fabricated using MgB$_2$-coated Cu possess the potential to operate at higher temperatures and magnetic fields than SRF cavities made from Nb and Nb compounds. We report the ongoing efforts to develop MgB$_2$-coated Cu Superconducting RF cavities through the Hybrid Physical-Chemical Vapor Deposition technique. Microns-thick MgB$_2$ films coated on mockup 1.3 GHz TESLA-type Cu cavities with small witness samples attached on the inner wall exhibiting sharp superconducting transitions at around 37 K uniformly as measured by AC susceptibility. Rutherford backscattering spectrometry and Energy-dispersive X-ray spectroscopy were employed to analyze the depth profile of the elementary constituents in the MgB$_2$ films. RF properties of the MgB$_2$ films on Cu were investigated using the dielectric resonator technique, showing the RF surface resistance comparable to Nb at 10.8 GHz and 6 K. We are nearing the completion of coating and testing a prototype Cu SRF cavity.
Speaker: Ke Chen (Temple University) -
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In-depth material studies of SIS multilayers for application using the new cavity-coating system 25m
Operating gradients more than twice as high as those in bulk Nb cavities as well as significantly higher Q-values can be achieved by using nm-thin SIS (superconducting-insulating-superconducting) multilayers on the inner surface of a cavity to delay the penetration of vortices. Based on this theory, the University of Hamburg focuses on Atomic Layer Deposition (ALD) for thin film deposition as the most promising technique to coat SIS multilayers. A versatile material study provides insight into the interplay between the insulator film and the superconductor film under the influence of post-deposition thermal annealing. The results encompass depth profiles of as-deposited and annealed multilayer samples deduced from cross-sectional energy-dispersive X-ray spectroscopy (EDX) illustrating high-temperature annealing-induced changes in thin films. Moreover, the energy gap as well as the Dynes parameter are determined by measuring the complex optical conductivity at THz frequencies. These studies aim to use the newly developed HADES system (Hamburg's Atomic Layer Deposition Instrument for Enhanced Superconductors) to coat single-cell niobium cavities. The system's current commissioning is being compared with previously conducted simulations of its operation.
Speaker: Lea Preece (Universität Hamburg) -
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Design and Preliminary Testing of a DC Magnetron Sputtering Cathode for Nb–Sn Precursor Deposition inside a 3 GHz SRF Cavity: Toward an Nb3Sn–I– Nb Multilayer Structure 25m
The maximum accelerating gradient, Eacc,max, of a superconducting radio- frequency (SRF) cavity is limited by the surface magnetic field at which a vortex avalanche occurs on the inner cavity surface. Theoretical studies predict that a superconducting–insulator–superconducting (S′–I–S) multilayer structure can effectively enhance the threshold field for vortex avalanche. Furthermore, an optimal combination of superconducting- and insulating-layer thicknesses is expected to maximize the threshold field.
We aim to fabricate an Nb3Sn–I–Nb structure on the inner surface of a 3 GHz elliptical bulk-niobium SRF cavity and thereby increase Eacc,max by a factor of two or more, as theoretically predicted. Nb3Sn thin films are generally synthesized by heat-treating precursor materials containing Nb and Sn at 900–1100 °C for several hours. We therefore are developing a specialized cathode for depositing an Nb–Sn precursor layer on the inner surface of the SRF cavity by DC magnetron sputtering method. The cathode is designed to achieve the Nb-to-Sn atomic ratio of 3:1.
In this presentation, we report progress in the following areas: (1) particle- in-cell/Monte Carlo simulations used to design a DC magnetron-sputtering system capable of producing a uniform film-thickness distribution and the desired Nb-to-Sn atomic ratio inside the cavity, and (2) sputtering tests using a prototype cathode designed for the simultaneous deposition of two different metallic targets onto the inner surface of a coupon cavity.
Speaker: ryo katayama (High Energy Accelerator Research Organization) -
10:15
Zr–Nb Alloy Coatings for SRF Cavities: Surface Characterization, Deposition Method Development, and Initial RF Performance 20m
Zr-Nb alloys are a promising candidate for SRF cavities due to their reduced sensitivity to stoichiometric deviations, intrinsically smoother interfaces rising from their continuous phase with niobium, and large estimated coherence length. We present surface characterization of this material, introduce a method to coat SRF cavities with this alloy that overcomes the electrochemical instability of zirconium, and report early RF performance results to demonstrate the feasibility of this alloy for SRF applications.
Speaker: Alexis Grassl (Cornell University)
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Coffee Break 30m
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Beyond Nb & Nb3Sn: Alternate materials, multilayer SIS structuresConvener: Marc Wenskat (DESY/Universität Hamburg)
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Nb3Al: A Second Viable Material for High‑Gradient 4.2 K SRF Operation 20m
The focus of next-generation SRF cavities has increasingly shifted towards materials capable of sustaining high-gradient continuous-wave operation at 4.2K. Material candidates capable of this feature need lower BCS resistance than niobium and must be free of defects that cause thermal instability and/or quench. Among these candidates, Nb3Sn has emerged as the leading alternative, while others have failed to reach practical acceleration fields. We report RF performance and surface characterization results for Nb3Al, which is now only the second material to demonstrate stable operation at practical fields. Results indicate that Nb3Al is less defect-sensitive than Nb3Sn and, while recipe optimization has only recently begun, shows the potential to have improved RF performance relative to niobium.
Speaker: Alexis Grassl (Cornell University) -
11:25
Zr Inclusion as a New Route to Improve SRF Cavity Materials 20m
Here, we evaluate two co-sputtered compounds: Nb–Zr and Nb–Sn–Zr, as prospective superconducting materials for future RF cavity fabrication. We have studied how the crystal lattice structures, surface and bulk morphologies, oxidation states, and superconducting properties modify with variable Zr fraction, to determine the most useful range of Zr inclusions in these compounds for SRF applications.
In the first study, a variable content of Zr was introduced along with Nb and Sn during co-sputtering process, which resulted in Nb₃Sn matrix consisting of phase-separated ZrO₂ precipitates of 20 to 100 nm after annealing. Increasing the Zr concentration substantially reduced both the density and average dimensions of surface and bulk voids in Nb₃Sn thin film. We also observed that increasing the Zr content up to an optimal level can improve the superconducting transition temperature (T$_c$) and upper critical magnetic field (H$_{c2}$). In addition, higher Zr concentrations appeared to reduce oxygen diffusivity in the films, resulting in the formation of a thinner primary surface oxide layer.
In case of the Nb–Zr system, room-temperature co-sputtering directly produced single-phase bcc Nb-Zr alloys without the requirement of any post-annealing process. The resulting films also exhibited a preferred out-of-plane crystallographic orientation along the Nb (110) plane. Both the T$_c$ and H$_{c2}$ values increased monotonically with increasing Zr content up to approximately 15%; beyond this concentration, however, both properties began to degrade.
Our results suggest that Zr inclusion in sputtered Nb and Nb₃Sn coatings is promising for SRF applications, particularly for cavities operating in presence of high magnetic fields of several Tesla.Speaker: Malvika Tripathi (Fermi National Accelerator Laboratory) -
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Superconducting properties of Nb-Zr thin films for particle accelerators and quantum applications 20m
We investigated the superconducting properties of 200 nm thick Nb-Zr thin films with various Zr concentrations ranging from 0 to 22 at.%. 200 nm thick Nb-Zr thin films were deposited on c-plane sapphire substrates by magnetron sputtering at RT, and the microstructures and superconducting properties of the films were systematically analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM), and physical property measurement systems (PPMS). XRD and TEM analyses show that the bcc Nb-Zr films are typical (110) textured polycrystalline films with 20-30 nm grain diameter columnar grains. We find that critical temperature (Tc) increases from 8.5 K to 9.9 K with increasing Zr concentration in Nb-Zr thin film from 0 to 15-22 at.% and upper critical field (Hc2) values increase from ~3 T to above 10 T. Current results demonstrate that Nb-Zr thin films have improved Tc and Hc2, which is beneficial for potential application to thin film SRF cavities and quantum device applications.
Speaker: Dr Malvika Tripathi (Fermi National Accelerator Laboratory) -
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Beyond Nb & Nb3Sn - Discussion 20m
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Tour
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Functional layers for SRF & BeyondConvener: Yasmine Kalboussi (CEA)
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Atomic Layer Deposition of Functional Oxide Interfaces for Mitigation of Two-Level-System Losses in Niobium SRF Cavities 30m
Niobium-based superconducting radio-frequency (SRF) cavities have demonstrated exceptionally high quality factors, making them an attractive platform for investigating fundamental loss mechanisms in superconducting quantum systems. To further reduce surface-related dissipation, we investigate a surface-engineering approach based on thermal atomic layer deposition (ALD) of functional oxide thin films designed to suppress the formation of amorphous native niobium oxides, which are widely considered a major source of two-level-system (TLS) losses. Engineered ALD layers, including ZrO₂ and Ta₂O₅, serve as functional barrier and interface layers that tailor the superconductor–dielectric interface, stabilize the niobium surface, and mitigate dielectric losses.
The effectiveness of this approach relies on achieving a sharp, well-controlled interface between the deposited oxide and the underlying niobium. Post-deposition thermal treatments play a critical role in optimizing the interfacial structure, suppressing native oxide evolution, and enhancing cavity performance in both the low-field quantum regime and the high-field accelerator regime. Extensive materials characterization has been performed to correlate thin-film composition, interface evolution, and processing conditions with RF performance, providing insight into the mechanisms governing surface-related dissipation.
To evaluate the effectiveness of these engineered interfaces over the full operating range of SRF cavities, new RF measurements have been performed in the SQMS dilution refrigerator at millikelvin temperatures and down to the single-photon regime, enabling direct investigation of TLS-dominated losses and their saturation behavior. Complementary measurements at the Fermilab Vertical Test Stand (VTS) characterize cavity performance under accelerator-relevant operating conditions, extending the study across a broad range of RF fields. Together, these measurements establish a comprehensive framework for assessing the impact of ALD-engineered functional oxide interfaces from the quantum regime to accelerator operation.
This presentation will describe the ALD processing methodology, interface engineering strategy, materials characterization, and new RF measurements obtained in both the dilution refrigerator and the Vertical Test Stand. Particular emphasis will be placed on the role of functional oxide interfaces and optimized thermal processing in mitigating TLS-related losses, improving the ambient stability of niobium surfaces, and enabling next-generation SRF cavities for quantum and accelerator applications.Speaker: Laura Grassellino (Fermilab) -
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Surface passivation of cavities for reduction of two-level systems at low gradients 30m
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. The main goal is to reduce these TLS losses by introducing a passivating layer over the niobium which reduces oxygen diffusion into the substrate. When the cavity is coated with a passivating layer like Al2O3 and then subjected to the Mid-T Bake, this subsequent reoxidation of the niobium is inhibited. Here, we report the tests of several cavities tested at low fields, after Mid-T baking, with and without passivating coatings, and discuss these results in the framework of the TLS model.
Speaker: Rakshith Venugopal (Deutsches Elektronen-Synchrotron DESY) -
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Tuning the secondary electron yield with nanoheterostructures 30m
Secondary electron emission (SEE), arising from the interaction of energetic electrons with material surfaces, can induce deleterious effects in vacuum radio-frequency (RF) systems, notably multipacting, thereby degrading performance and threatening device integrity. Despite extensive efforts, effective and widely adopted mitigation strategies remain limited. Here, we report an approach to simultaneously tailor SEE and electrical conductivity using thin, multi-element films deposited by atomic layer deposition (ALD). The design and fabrication of these heterostructures are presented, and a range of films is systematically characterized to establish correlations between emission properties, electrical conductivity, and chemical and structural features. We demonstrate that such heterostructures enable combinations of total electron emission yield (TEEY) and conductivity unattainable in single-phase materials, including regimes combining low TEEY with relatively high conductivity. These results open new avenues for the rational design of functional surfaces for vacuum electronic applications.
Speaker: Thomas Proslier (CEA)
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Coffee Break 30m
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Tailored Substrates for Advanced Film GrowthConvener: Caroline Hain (CERN)
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Plasma Electrolytic Polishing of 1.3 GHz Copper Cavities for Thin-Film SRF Applications 25m
The performance of thin-film superconducting RF cavities is strongly dependent on the quality of the underlying copper substrate, whose surface morphology directly influences film growth, adhesion, and final RF behaviour. In the framework of thin-film R&D for future accelerators such as FCC-ee, achieving smooth, defect-free copper surfaces on full elliptical geometries is an essential prerequisite.
Plasma Electrolytic Polishing (PEP) offers an attractive alternative to conventional electropolishing, providing effective surface smoothing with environmentally friendlier electrolytes while avoiding aggressive acids. At INFN-LNL, PEP has been developed and applied to 1.3 GHz copper cavities for superconducting coatings — initially Nb, with the aim of transitioning to Nb₃Sn once the deposition process is mature.
In this contribution, we first present the principal challenges encountered in scaling PEP to a full elliptical cavity and the strategies adopted to make the treatment feasible. We then report on three cavities of different origin and fabrication history — bulk and hydroformed, with differing pre-treatments — all PEP-treated at INFN, Nb-coated at CERN, and RF-tested at CERN and KEK, reflecting a joint effort across the three laboratories. Finally, we outline challenges and mitigation strategies, including an electrolyte agitation approach.
Speaker: Eduard Chyhyrynets (INFN - LNL) -
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Scaling Up Plasma Electrolytic Polishing of Bulk Niobium for SRF Applications 25m
In previous editions of this workshop, we demonstrated that Plasma Electrolytic Polishing (PEP) works well as a surface-finishing route for small niobium coupons, offering effective smoothing with environmentally benign electrolytes while avoiding aggressive HF-based acids. Scaling the process to larger surfaces, however, proved far less straightforward: results were inconsistent and difficult to reproduce, with recurring issues such as surface oxidation and non-uniform oxide staining that ultimately made the treated surfaces unusable.
In this contribution, we show how these limitations have been overcome through a revised electrolyte formulation, voltage tuning, and mixing protocol developed at INFN-LNL, which suppresses oxide formation and restores stable, reproducible polishing on larger workpieces.
The improved process delivers smooth, reflective surfaces with average roughness reduced from above 3 µm down to below 100 nm Sa. We present results obtained on medium-sized samples up to approximately 10 × 5 cm and on QPR niobium samples, possibly including RF measurements if timing allows, and we show that the process is equally applicable to 3D-printed niobium samples.
Finally, we report on the first steps toward the internal surfaces polishing of elliptical cavities, with a preliminary test on dummy samples.
Speaker: Tochukwu Emmanuel Ezeaba (University of Padua) -
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Hybrid Wire Laser Additive Manufacturing and CNC machining for novel SRF cavity fabrication 20m
The fabrication of Superconducting Radio Frequency (SRF) cavities traditionally relies on forming and welding high-purity metal sheets, resulting in a local surface discontinuity that degrades the final SRF performance. In this work, we propose and explore a novel hybrid approach combining Wire Laser Additive Manufacturing (WLAM), with in situ CNC machining for the fabrication of mono-cell 1.3 GHz SRF cavity. This technique enables the layer-by-layer deposition of high-purity metals with precise dimensional control, while simultaneously integrating subtractive steps to maintain tolerances and surface quality crucial for RF performance. The hybrid WLAM and CNC machining stands as a candidate for next-generation SRF cavity production minimizing material waste, eliminating the need for electron beam welding, through the direct creation of complex geometries, and enhancing the surface finishing in the as-built condition. Results on stainless steel and CuCrZr 1.3 GHz prototypes are presented.
Speaker: Cristian Pira (INFN LNL) -
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Tailored Substrates for Advanced Film Growth - Discussion 20mSpeaker: Caroline Hain (CERN)
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Emerging Frontlines in Characterizion methods: Advanced Imaging & Novel ProbesConvener: Oliver Kugeler (Helmholtz Zentrum Berlin)
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Tunneling spectroscopy for SRF applications 30m
Point Contact Tunnel Spectroscopy (PCTS) is a powerful technique for probing the local density of states (DOS) of superconductors. Tunneling spectra provide direct insight into key superconducting properties, including the energy gap, inelastic scattering processes, and the presence of defects such as normal-conducting phases and magnetic impurities.
In contrast to conventional DC transport measurements, PCTS offers a local probe of superconducting properties with high sensitivity to spatial inhomogeneities. This makes it particularly well suited for superconducting radio-frequency (SRF) applications, where localized defects can give rise to RF losses and ultimately limit cavity performance.
In this presentation, I will show how PCTS can be used as a diagnostic tool to identify loss mechanisms relevant to RF performance. Results from materials of current interest to the SRF community will be discussed, such as Nb and Nb₃Sn thin-film technologies for next-generation SRF cavities.Speaker: Ivana Curci (CEA-Saclay) -
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Commissioning and First RF Results from a LHe-Free Thin Film SRF Cavity Test Facility 30m
To support the development of thin film SRF technology, STFC Daresbury Laboratory has commissioned a dedicated conduction-cooled SRF test facility for the rapid qualification of thin film coated cavities.
The facility is based on a Cryomech pulse tube cryocooler providing 2.7 W of cooling power at 4.2 K and supports RF measurements of both closed and split cavity geometries from 1.3 to 6 GHz. Since its initial design, the cryogenic facility and low level RF system have been fully commissioned. Cavity cooldown times of approximately 14 hours demonstrate the potential to test up to two cavities per week. Initial RF measurements using a bulk Nb 1.3 GHz single-cell TESLA cavity have demonstrated measurements of $Q_0(T)$ above 3.2 K, $Q_0(E_\rm{acc})$ measurements at fixed temperature and frequency shift measurements as a function of temperature through the superconducting transition. In addition, the first RF tests of thin film coated cavities have begun, providing valuable feedback for the optimisation of deposition parameters.
This talk will present the commissioning of the facility together with its first results obtained on bulk and thin film SRF cavities. Ultimately, the facility is intended to accelerate the development of high-performance Nb$_3$Sn-coated cavities by providing rapid RF qualification and feedback to guide coating optimisation prior to high-power liquid helium testing.
Speaker: Daniel Seal (STFC) -
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A new addition to the Vertical Test Facility (VTF) at STFC Daresbury Laboratory for testing Thin Films SRF cavities 30m
UKRI-STFC Daresbury Laboratory has a large vertical cryostat capable of testing 3 large jacketed HB704 SRF cavities for ESS or a single 650 MHz jacketed cavity for PIP-II. As a part of the development of SRF Thin Films Cavities a smaller 300 litres mini-cryostat has been developed and added to the vertical test facility (VTF) enabling testing of a range of bare cavities at operating temperatures between 2K and 4.2K. The 400 mm ID bath cryostat has a modular cavity support insert fully equipped with all the necessary cryogenic and RF instrumentation. The top plate of the cryostat is designed in such a way that the Cryogenic, Vacuum, RF and other control interfaces are compatible with the existing VTF infrastructure for easy switch over between the different cavity tests runs. In this Paper I will give an overview of the SURFlab Vertical Test facilities.
Speaker: Mr Shrikant Pattalwar (UKRI- STFC Daresbury Laboratory) -
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Split cavity progress at Daresbury Laboratory 30m
To investigate new and novel materials, two split cavities have been designed and built at Daresbury laboratory,
operating at 1.3 GHz and 6 GHz. Both cavities are split in half longitudinally to ensure no electrical contact
resistance. Split cavities are beneficial to be used as research cavities as they allow inspection of the radio-frequency
surface at all stages of coating, and after testing. Furthermore, a split cavity is not constrained to depositions using
a cylindrical target, and allow planar targets to also be used. This is beneficial for materials such as Nb3Sn, where
cylindrical targets do not exist.
The design of both split cavities will be presented. Initial results on the 6 GHz cavity will be shown for niobium,
and for thick (micrometers) film of Nb3Sn with a niobium buffer layer, and also a multilayer cavity consisting of a
niobium substrate and a Nb3Sn and thin (nanometer) film on the surface.
1Speaker: Daniel Turner (-)
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Discussion
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Functional layers for SRF & Beyond - Discussion 30mSpeaker: Yasmine Kalboussi (CEA)
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Emerging Frontlines in Characterizion methods: Advanced Imaging & Novel Probes - Discussion 30mSpeaker: Oliver Kugeler (Helmholtz Zentrum Berlin)
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Emerging Frontiers & Synergies in SRF Thin Films: From Quantum, Sensing to AI-Driven MaterialsConvener: Thomas Proslier (CEA)
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Advancements in microwave properties of low-Tc and high-Tc superconducting films for haloscopes and other large physics experiments 20m
Large experiments in fundamental physics, from axion haloscopes [1] to future accelerator technologies like the CERN FCC [2], can benefit enormously from superconductors with low surface impedance in high magnetic fields. In these regimes, vortex motion is the main source of rf dissipation. The achievable performance is thus governed not only by the intrinsic superconducting properties, but also by the material defects which at the same time should provide efficient vortex pinning centers without significantly impacting on the degree of cleanness of the vortex cores, where dissipation due to quasi-particles mainly occurs. Understanding and controlling the interplay between microstructure and overall vortex dissipation is mandatory to optimize the superconducting coatings.
In this work we focus first on Nb3Sn, extending our previous investigations on DC magnetron sputtered films, examining the role of the Sn-rich islands recently observed on those DCMS samples [3] and their impact on the pinning landscape. A comparison is carried out on newly prepared Nb3Sn coatings grown on sapphire substrates, 500 nm thick, where the reduced thickness should strongly suppress the formation of islands and provide a cleaner reference system. We present dual frequency microwave (ν=8 and 27 GHz) measurements in perpendicular fields up to 1.2 T whence the vortex dynamics parameters are extracted. We compare the pinning strength (measured by Labusch parameter kp), the flux-flow dissipation (probed through the flux flow resistivity ρff), and thermal creep, aiming at identifying the effect of mesoscale structural inhomogeneities on the Nb3Sn high frequency dissipation.
We further broaden the investigation to coated-conductors (CCs) REBCO tapes, mechanically peeled to expose the superconducting film from the CC tape by two independent laboratories, ICMAB and ENEA, to assess their potential for axion haloscopes and particle accelerators beam-screen applications. Indeed, REBCO provides very high upper critical field with mature tape technology, but its practical performance in rf resonant structures significantly depends on the preparation stage and on the resulting vortex response after delamination. We find that CCs exhibit very strong (beneficial) vortex pinning, with flux-flow resistivity and quasiparticle states close to single crystals, thus offering an excellent mix for applications. Overall, the study provides a comparative picture of Nb3Sn and REBCO as candidate materials for haloscopes [4] and other high-field applications.Acknowledgments
Work partially supported by INFN CSN5 project SUPERMAD and FCC collaboration under MoU Addendum FCC-GOV-CC-0218 (KE5084/ATS).
We acknowledge sample preparation and useful discussions with S. Calatroni, G. Celentano, A. Masi, C. Pira, S. Posen, T. Puig, A. Vannozzi.References
1. D Alesini et al, Phys. Rev. D 99, 101101(R) (2019)
2. S Calatroni, IEEE Trans. Appl. Supercond. 26, 3500204 (2016)
3. D. Fonnesu et al., Sci Rep. 16, 3539 (2026)
4. A. Alimenti et al, Instruments 6, 1 (2022)Speaker: Prof. Nicola Pompeo (University Roma Tre & INFN Roma Tre Section) -
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ALD: A pathway to mitigate two-Level System Defects in Niobium 3D Resonator for quantum applications 20m
Superconducting quantum bits (qubits) are regarded as the key technological building blocks for future quantum computers and sensors. One of the primary obstacles to extending qubit performance and in particular coherence times is the presence of photon-absorbing defects, commonly modeled as two-level systems (TLS). Microscopic sources of TLS—such as oxygen vacancies, hydroxyl groups, and amorphized structures—have been identified in dielectric surfaces, interfaces, and Josephson junctions. Thanks to its atomic-scale control of composition and thickness, atomic layer deposition (ALD) offers a powerful approach to address this challenge and mitigate some of these defect mechanisms. ALD is also fully compatible with standard microelectronic fabrication processes and can be readily integrated into the production of two-dimensional superconducting films.
Niobium (Nb) superconducting cavities, widely used in particle accelerators, provide a simpler type of resonator compared to qubits, with the advantage of involving only a single interface—Nb and vacuum. This makes them ideal platforms for investigating how the structure, chemical composition, and thickness of various oxide capping layers affect resonator performance and TLS behavior.
In this work, I will present performance measurements from Nb superconducting resonators coated via ALD with amorphous as well as crystalline films with thicknesses between 2 and 10 nm. Following thermal treatments, these coatings were found to enhance quality factors and coherence times relative to bare niobium with its native oxide. TLS-model fits of RF measurements, combined with surface characterization techniques such as XPS and TEM, enable the extraction of TLS-related properties—including dielectric losses and defect concentrations—in the various capping layers.
These findings offer valuable insight for future technological developments of superconducting resonators operating in the quantum regime, including qubit architecturesSpeaker: Dr Yasmine kalboussi (CEA) -
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Advances in Superconducting Digital Logic 20mSpeaker: Anne-Marie Valente-Feliciano (Jefferson Lab)
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Heterodyne Gravitational Wave Detection ? 20mSpeaker: Marc Wenskat (DESY/Universität Hamburg)
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Coffee Break 30m
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Summaries & Working SessionConvener: Anne-Marie Valente-Feliciano (Jefferson Lab)
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Pushing SRF Thin Films Limits: Theoretical Advances, Deposition Modeling & DFT - Summary 10m
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Latest Advances in Nb Thin Film Technology - Summary 10m
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Nb3Sn Technology: Recent Technical Advances & Scaling Horizons- Summary 15m
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Beyond Nb & Nb3Sn - Summary 10m
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Tailored Substrates for Advanced Film Growth - Summary 10m
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Emerging Frontlines in Characterizion methods - Summary 10m
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Opportunities & Strategies for SRF Thin Films Advancement - Discussion 55m
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Workshop Close-out
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