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# 10th International Workshop on Thin Films and New Ideas for Pushing the Limits of RF Superconductivity

US/Eastern
Jefferson Lab

#### Jefferson Lab

Description

The 10th 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 predominately 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 at Jefferson Lab, Newport News VA, USA.

Abstract submission  deadline  September 8, 2022.

Registration deadline is September 8, 2022.

Registration
Registration
Participants
• A. Ozdem Sezgin
• Akira Yamamoto
• ALEJANDRO CASTILLA
• Alexander Gurevich
• Andre Juliao
• Andres Salas
• Anne-Marie Valente-Feliciano
• Antonio Bianchi
• Bektur Abdisatarov
• Carlota Pereira
• Carrie Baxley
• Charles Reece
• Claire Antoine
• Cristian Pira
• Daniel Seal
• Daniel Turner
• David Beverstock
• David Longuevergne
• Davide Ford
• Didi Luo
• dmitry tikhonov
• Eduard Chyhyrynets
• Eric Lechner
• Felix Walk
• Francis Lockwood Estrin
• Getnet Kacha Deyu
• Gianluigi Ciovati
• Grigory Eremeev
• Harshani Senevirathne
• Isabel González Díaz-Palacio
• Jayendrika Tiskumara
• Jiyuan Zhai
• Johannes Bernardi
• Liam Smith
• Lorena Vega
• Marc Wenskat
• Maria Iavarone
• Massimiliano Bonesso
• Md Sharifuzzaman Shakel
• Michael Kelley
• Michael Vogel
• MIchele Bertucci
• Mingqi Ge
• Nicola Pompeo
• Oleg Malyshev
• Oleksandr Hryhorenko
• Pablo Vidal Garcia
• Paul Plattner
• Ping He
• SARRA BIRA
• Sebastian Keckert
• Shreyas Balachandran
• Stewart Leith
• Taaj Sian
• Teng Tan
• Thomas Proslier
• Tobias Junginger
• Tsuyoshi Tajima
• Uttar Pudasaini
• Valerie Bookwalter
• Yasmine KALBOUSSI
Conference Services
• Monday, 19 September
• 08:30 09:00
Registration
• 09:00 09:30
Welcome Auditorium

### Auditorium

Convener: Anne-Marie Valente-Feliciano (Jefferson Lab)
• 09:30 10:30
Perspective of SRF thin film in international projects Auditorium

### Auditorium

#### Jefferson Lab

Convener: Cristian Pira (INFN LNL)
• 09:30
International Context for SRF Thin Film Technology: Snowmass & European Strategies 30m
Speaker: Anne-Marie Valente-Feliciano (Jefferson Lab)
• 10:00
The thin film activities in the IFAST program 30m
Speaker: Claire Antoine (CEA)
• 10:30 10:50
Coffee Break 20m
• 10:50 12:40
Theoretical approach for SRF Thin Films & Structures Auditorium

### Auditorium

Convener: Uttar Pudasaini (Jefferson Lab)
• 10:50
Model for heat dissipation by constrictions of electric field in granular thin film Niobium 30m

We propose an alternative model to explain power dissipation leading to the formation of hot spots in the inner walls of niobium porous thin film superconducting rf cavities. The physical mechanism that we explore is related to the constriction of surface electrical current flow at grain interface boundaries. These constrictions create an additional electrical contact resistance which induces localized punctual heat dissipation. The model is compared to experimental results and is able to very well fit the Q-slope data

Speaker: Claire Antoine (CEA)
• 11:20
Modelisation of Oxygen profile tailoring in bulk and thin film Nb 20m
Speaker: Eric Lechner (Jefferson Lab)
• 11:40
Nonlinear Meissner effect in Nb3Sn thin film coplanar resonators 30m

We investigated the nonlinear Meissner effect (NLME) in Nb3Sn thin film coplanar resonators by measuring the resonance frequency as a function of a dc magnetic field applied parallel to the film surface at different temperatures. We used low rf power probing in films thinner than the London penetration depth λ(B) to significantly increase the field onset of vortex penetration and measure the NLME under equilibrium conditions. Contrary to the quadratic increase of λ(B), expected in s-wave superconductors, we observed a nearly linear increase of the penetration depth with B. Theoretical analysis of our experimental data has shown that that the observed behavior of λ(B) does not result from the conventional dc current pairbreaking effects but results from weak linked grain boundaries which significantly increase the kinetic inductance of our polycrystalline Nb3Sn films.

Speaker: Alexander Gurevich (ODU)
• 12:10
Thermal-conductivity-related New Strategy for Breaking the Eacc Limit 30m

In this talk, we started with the cavity degradations' appearance and found two strategies to increase the cavity's thermal conductivity--- the outer-wall groove structure (OGS) and the inner-wall thermal conducting film (ITCF). We performed COMSOL simulations and found them effective. We hope these two structures can improve the thermal conductivity, thus increasing the cavity's Eacc limit.

Speaker: Didi Luo (Institute of Modern Physics, Chinese Academy of Sciences (Remote))
• 12:40 14:10
Lunch 1h 30m
• 14:10 15:40
Nb thin film technology: 1 Auditorium

### Auditorium

Convener: Anne-Marie Valente-Feliciano (Jefferson Lab)
• 14:10
Study of the influence of the manufacturing process and thermal cycling on the RF performance of 1.3 GHz Nb/Cu SRF cavities 30m

Nb/Cu coated SRF cavities present several advantages with respect to bulk Nb cavities. However, a systematic degradation of the performance at high accelerating gradients is observed.

At CERN, a vast R&D program has been conceived to find an optimized recipe for manufacturing Nb/Cu coated cavities, from the production and treatments of the copper substrates to the thin film deposition. In the context of this research campaign, 19 1.3 GHz single cell elliptical cavities have been tested since 2021.

On the one hand, the research has been focused on pushing the performance of the cavities at 4.2K to use this technology in the FCC accelerator. The goal is to minimize the BCS resistance so that the operational requirements of the FCC SRF system are met. Besides, in view of the large scale of the FCC project, the copper substrates have been manufactured with different techniques in order to select the most suitable one in terms of reliable quality and economy of scale. The results at this temperature are encouraging, showing repeatable and optimized RF performance.

On the other hand, the research has been directed towards a more fundamental understanding of the mechanisms behind the Q slope. This would allow to mitigate this phenomenon and ultimately to extend the application of this technology to high energy, high gradient accelerators. For that, RF tests have been done at 1.85 K as well, as it is not the BCS resistance that dominates at this temperature, but the residual resistance, and the Q-slope appears more clearly. Emphasis has been made to study of the influence of the thermal cycles on the performance, equipping the cavities with flux gates to correlate the trapped flux and their performance. A systematic improvement has been observed of both the Q slope and the residual resistance with slow thermal cycles. The results of these studies are presented.

Speaker: Lorena Vega (CERN)
• 14:40
Ion energy analysis of a bipolar HiPIMS discharge using a retarding field energy analyser 30m

The time evolution of the positive ion energy distribution functions (IEDF's) at the substrate position in an asymmetric bipolar high-power impulse magnetron (HiPIMS) system was determined using a gridded energy analyser. This was done for a range of operating conditions, namely the positive voltage Urev and “on-time” negative pulse duration neg. The magnetron sputtering discharge was equipped with a Nb target. Based on the knowledge of the IEDF's, the bombarding ion flux density $\Gamma_i$ and energy flux density $Q_i$ to a grounded surface were calculated. Time-resolved IEDF measurements showed that ions with energies approaching the equivalent of the positive pulse voltage Urev were generated as the reverse positive voltage phase developed.

On time-average, we observed that increasing the set $U_{rev}$ value (from 0 to 100 V), resulted in a marginal decrease in the ion flux density $\Gamma_i$ to the analyser. However, this is accompanied by a 5-fold increase in the ion energy flux density $Q_i$ compared to the unipolar, $U_{rev}$ = 0 V case. Reducing the negative HiPIMS pulse duration neg (from 130 to 40 µs) at a constant discharge power leads to a modest increase in $\Gamma_i$, but a 4-fold increase in $Q_i$. The results reveal the benefit of the bipolar HiPIMS technique, in which it is possible to control and enhance the power density of ions bombarding a grounded (or fixed bias) substrate, for potentially better tailoring of thin film properties.

Speaker: Felix Walk ((Remote))
• 15:10
SRF Thin Film Coating Development at IHEP 30m

This paper summarizes of SRF thin film coating facilities set-up and progress at the Institute of High Energy Physics (IHEP), and presents preliminary results of 1.3 GHz 1-cell cavities coated with Nb, Nb3Sn, etc. The characteristics of witness samples at different locations inside the cavity revealed the uniform quality of the coating.

Speaker: Dr Ping He (IHEP)
• 15:40 16:05
Coffee Break 25m
• 16:05 17:20
Nb thin film technology: 2
Convener: Anne-Marie Valente-Feliciano (Jefferson Lab)
• 16:05
Characterization of TESLA-shaped single-cell Nb thin-film cavity with varying RRR values at low temperatures 30m

Niobium thin films are used in macroscopic SRF cavities for particle accelerators which are under study for microscopic superconducting qubits for quantum computing. The superconducting properties of niobium in microwave fields vary significantly with lattice defects and impurity content, where sub-at.% impurity level can reduce or increase microwave surface resistance by an order of magnitude. In this study, we investigated the microwave properties of Nb films deposited by different physical vapor deposition (PVD) techniques, correlating microwave properties at dilution fridge temperatures with material properties characterized by surface analytical techniques. Nb thin films were grown on single crystal Nb substrates and, on the inside surface of 1.3 GHz TESLA-shaped single-cell Nb SRF cavities. We also studied the microstructure, surface morphology, and superconducting properties of the Nb thin film samples. The SRF cavity performance was tested in Fermilab's VTS and dilution fridge systems. The intrinsic Q0 as a function of the accelerating gradient was measured at 2 and 1.5 K in the liquid helium dewar. Then the cavity was assembled into the dilution fridge without breaking the cavity vacuum and was tested from 20 mK to 40 K. The frequency and quality factor dependence as a function of temperature at low fields was investigated. The mid-T baking treatment was applied at 340 ͦ C degrees for 1 hour to improve the performance of the cavity. After mid-T baking cavity performance was tested again and compared with previous results.

Speaker: Bektur Abdisatarov
• 16:35
Nb film Technology - Discussion 45m
• 17:20 17:40
Group Photo 20m
• 18:00 20:00
Welcome Networking Reception CEBAF Center Atrium

### CEBAF Center Atrium

• Tuesday, 20 September
• 08:30 09:00
Registration
• 09:00 10:40
Beyond Nb: Alternate materials and mulilayer structures: 1 CEBAF Center Auditorium

### CEBAF Center Auditorium

Convener: Grigory Eremeev (Fermilab)
• 09:00
Nb3Sn Coating of a 2.6 GHz SRF cavity using a cylindrical magnetron sputtering system 25m

Due to the higher superconducting critical temperature (Tc ~18.3 K) and superheating field (Hsh ~400 mT) compared to Nb (Tc ~9.25 K, Hsh ~200 mT), Nb3Sn is a promising material as a surface coating on Nb superconductive radio frequency (SRF) cavities. Nb3Sn coated Nb cavities can deliver a higher quality factor and acceleration gradient when operated at 4 K replacing the bulk Nb SRF cavity operated at 2 K, hence reducing the operation cost significantly. A cylindrical magnetron sputtering system was commissioned at Old Dominion University and used to coat Nb3Sn into a 2.6 GHz Nb SRF cavity. Using two identical cylindrical magnetrons, Nb3Sn was fabricated on Nb substrates mounted on the equivalent positions of the beam tubes and the equator of the SRF cavity. Microanalysis of the films confirms the growth of single phase Nb3Sn. The Nb3Sn films from all three positions showed good superconducting properties (Tc = 17.61-17.76 K and Δ Tc = 0.06-0.1 K). We then coated ~1.2 μm Nb3Sn inside of the 2.6 GHz Nb SRF cavity. We will discuss the design and commissioning of the cylindrical magnetron sputtering system, first results from Nb3Sn fabrication on Nb flat substrates, and the Nb cavity coating by ~1.2 μm Nb3Sn and results on cavity testing.

Speaker: Md Sharifuzzaman Shakel (Old Dominion University)
• 09:25
Recent advances with bipolar HiPIMS-deposited Nb3Sn films 25m

Nb has been the material of choice for SRF cavities for many years now. However, because of its lower BCS resistance and increased critical temperature of 18.3 K, Nb3Sn has also been pushed as a potential candidate to allow the SRF community to surpass the performance of Nb. The feasibility of depositing Nb3Sn films onto Cu substrates using DC MS has previously been demonstrated at CERN. The resultant films displayed promising RF performance and a critical temperature up to 16 K on Cu substrates [1]. Given the superconducting performance improvements observed with HiPIMS-deposited Nb films [2], its use in the synthesis of Nb3Sn films was pursued. Initial results were promising, displaying a Tc in the region of 15.5 K.
This work focuses on the recent advances made with the elaboration of Nb3Sn films on Cu substrates using bipolar HiPIMS. In line with work completed on DC MS Nb3Sn films, significant differences were observed between high temperature coatings and those deposited without substrate heating, followed by a post-coating annealing. The differences between these approaches, in terms of film microstructure, crystallinity and superconducting performance, as well as future optimisation pathways will be presented.

[1] E. A. Ilyina et al., “Development of sputtered Nb 3 Sn films on copper substrates for superconducting radiofrequency applications,” Supercond. Sci. Technol., vol. 32, no. 3, p. 035002, Mar. 2019, doi: 10.1088/1361-6668/aaf61f.
[2] M. Arzeo et al., “Enhanced radio-frequency performance of niobium films on copper substrates deposited by high power impulse magnetron sputtering,” Supercond. Sci. Technol., vol. 35, no. 5, p. 054008, May 2022, doi: 10.1088/1361-6668/ac5646.

Speaker: Dr Stewart Leith (CERN (Remote))
• 09:50
Nb3Sn thin films on Cu base materials using bronze routes 25m

Nb3Sn has great potential to be the next generation superconducting material on the inside of Cu superconducting radiofrequency cavities (SRF) due to its relatively high critical temperature Tc of 18 K compared to superconducting metals such as Nb, Tc = 9.2 K. This allows access to superconducting radiofrequency (SRF) cavities at operating conditions not possible for Nb, as well as better efficiency for conduction-cooled cavities. Unfortunately, Nb3Sn is brittle, so it must be formed after the cavity shape is made. Here, we demonstrate multiple approaches to form Nb3Sn on a Cu substrate using the bronze route to facilitate Nb3Sn formation during reactions at ~700 °C. For all approaches, a Cu base plate was coated with Ti and Ta in sequence, Ti being a wetting layer and Ta being a diffusion barrier. Recipe 1 used a hot bronze method where bronze was thermally evaporated onto the substrate at ~200 °C and then Nb was deposited at ~700C in a second chamber. The high activity of Sn resulted in nearly instant reaction, achieving rapid growth of >20 nm / min. Recipes 2 and 3 both used a post-reaction method where Nb deposition and bronze evaporation were given different sequences, either bronze on Nb or Nb onto bronze, with temperature ~200 °C being too low to initiate Nb3Sn reaction. These structures were subsequently reacted at ~700 °C in a vacuum oven. Tc of coatings ranged from 14 to 16 K, being strongly affected by the thermal expansion coefficient mismatch between Nb3Sn and Cu, with some compensation due to the Ta diffusion layer.

This work was supported by the U.S. Department of Energy, Office of Science, Office of High Energy Physics under Award No. DE-SC 0018379. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-1644779 and the State of Florida.

Speaker: Andre Juliao (Applied Superconductivity Center, NHMFL, Florida State University)
• 10:15
Overview of Nb3Sn-cavity technology development for Jefferson Lab 25m

Nb3Sn promises better RF performance (higher Q and Eacc) than Nb at any given temperature because of superior superconducting properties. Since the Nb3Sn-coated cavities can operate at 4.3 K compared to 2 K for Nb, the demand for high-performing Nb3Sn-cavities is significantly growing for multiple accelerator applications. These cavities are produced routinely by growing a few microns thick Nb3Sn films inside Nb cavities via the tin vapor diffusion technique. Although continuous research and development in the last decade have notably enhanced the material's performance, there is still a long way to realize the material's full potential. Aiming at favorable surfaces that could enhance the RF performance, many coatings were produced and tested at JLab. The combined material and RF studies are in progress to better understand the interplay between coating growth parameters, surface characteristics, and performance of Nb3Sn. This talk will present the status of Nb3Sn-coated single-cell and multi-cell cavity development, including material and RF studies. This talk will also provide an overview of different projects progressing at JLab, to develop and use Nb3Sn for SRF accelerator application.

Speaker: Uttar Pudasaini (Jefferson Lab)
• 10:40 11:00
Coffee Break 20m
• 11:00 12:20
Beyond Nb: Alternate materials and mulilayer structures: 2 CEBAF Center Auditorium

### CEBAF Center Auditorium

Convener: Sebastian Keckert (Helmholtz-Zentrum Berlin)
• 11:00
EFFECT OF LOWER TEMPERATURE ANNEALING ON SURFACE PROPERTIES OF VAPOR DIFFUSION COATING OF Nb3Sn. 20m

Nb3Sn is a next-generation material for the SRF accelerator cavities to replace Nb, which promises superior performance and higher operating temperature than Nb because of higher critical temperature and superheating field, both twice compared to Nb. Accordingly, it promises significant cost reduction. The Sn vapor diffusion method is the most preferred and successful technique to coat niobium cavities with Nb3Sn so far. Although several post-coating techniques (chemical, electrochemical, mechanical) have been explored to improve the surface quality of the coated surface, an effective process has yet to be found. Since there are only a few studies available, we annealed Nb3Sn-coated samples at 850 ℃ - 950 ℃ for different periods to study the effect of heat treatments on surface properties, primarily aimed at removing surface Sn residues. Selected annealing parameters were applied to a coated cavity to assess the effect on the RF performance. This presentation discusses the systematic surface studies using samples and the effect of annealing on the RF performance of a coated single-cell cavity

Speaker: Jayendrika Tiskumara (Old Dominion University)
• 11:20
Nb3Sn on Cu Coating By Magnetron Sputtering From Target Synthesized via Liquid Tin Diffusion 20m

Nb3Sn on Nb thin films cavities by Tin Vapor Diffusion already show performance at 4.2 K comparable to Nb bulk cavities at 2 K, but a real breakthrough would be the use of copper as substrate, to enhance the thermal conductivity, opening up the possibility to cool down the cavity using cryocoolers instead of the more expensive helium bath.
Magnetron sputtering is the most studied technology for this purpose, however coating substrates with complex geometry (such as elliptical cavities) may require targets with non-planar shape, difficult to achieve with classic powder sintering techniques due to the brittleness of Nb3Sn.
One of the goal of the iFAST project is explored the possibility of using the Liquid Tin Diffusion (LTD) technique to produce sputtering targets for 6 GHz elliptical cavities. The LTD technique is a wire fabrication technology, already developed in the past at LNL for SRF applications, that allows the deposition of very thick and uniform coating on Nb substrates even with complex geometry.
This work reports the latest results and challenges encountered in target development via LTD and the study of the influence of deposition parameters via magnetron sputtering from single target on the properties of Nb3Sn films.

Speaker: Cristian Pira (INFN LNL)
• 11:40
V3Si thin film deposition using HiPIMS and DC 20m

V3Si is a types-II superconducting within the A-15 family of superconductors with a critical temperature (Tc) of 17K and a second critical field (HC2) of 24.2 T [1]. This means that an SRF cavities using V3Si rather than Nb would in theory be able to maintain a higher operating temperature and greater accelerator voltages. However, the material properties make bulk V3Si cavities impractical, so to gain the benefits of V3Si, it must be deposited as a thin film.

Previous studies showed SC properties of V3Si (as measured by RRR and Tc) are depends on deposition temperature via Stoichiometry, with temperatures of up to 800C needed to achieve satisfactory results [2]. HiPIMS is a deposition technique capable of performing ion bombardment during the deposition process and this ion bombardment has been shown to have similar effects to sample heating[3]. Theoretically, this would allow control of stoichiometry without the need for such high temperatures or post-annealing.

In this work, V3Si is deposited via both DC and from a range of HiPIMS supplies using a single target. Flat V3Si samples are produced on both copper and sapphire substrates, at temperatures up to 700C, however no post-annealing was employed. The samples are measured by VSM and have RRR taken to compare superconductive properties. SEM images are also taken to investigate surface properties. These were augmented with EDX for sociometry and EBSD. The best results showed a Tc of 15.1 K.

[1] Valente-Feliciano A-M, Supercond. Sci. Technol. 29 (2016) 113002
[2] ZhangY, China Institute of Atomic Energy Dissertation (2000)
[3] Anders A, Thin Solid Films 518 (2010) 4087–90

Speaker: Francis Lockwood Estrin (University of Liverpool (Remote))
• 12:00
Pushing the CW beam current limit of TESLA SRF Cavities with Nb3Sn and NbTiN Coating of HOM Antennas 20m

The Mainz Energy-Recovering Superconducting Accelerator (MESA), an
energy-recovering (ER) LINAC, is currently under construction at the Institute
for Nuclear physics at the Johannes Gutenberg-Universit ̈at Mainz, Germany. In
the ER mode continues wave (CW) beam is accelerated from 5 MeV up to 105
MeV. The energy gain of the beam is provided through 2 ELBE-type cryomod-
ules containing two 1.3 GHz 9-cell TESLA cavities each. By pushing the limits
of the beam current up to 10 mA, a quench can occur at the HOM Antennas.
This is caused by an extensive power deposition within the antenna. Calcula-
tions have shown, that a power transfer of 1 W has to be assumed. However
tests of the 1.5 GHz version of the TESLA HOM coupler has shown a quench
limit of 43 mW in CW. To prevent a quench of the HOM antennas by high
beam currents without mayor modification of the design of the HOM antenna
and F-part it is necessary to find suitable materials. Nb3Sn and NbTiN can be
applied as a coating to the HOM antennas and have higher critical parameters
than Nb which will lead to a higher power limit. The limit of the coated anten-
nas will be tested with the cavities of a cryomodule from the decommissioned
ALICE accelerator from STFC Daresbury.

Speaker: Paul Plattner (JGU Mainz Institut für Kernphysik)
• 12:20 13:30
Lunch 1h 10m
• 13:30 15:40
Beyond Nb: Alternate materials and mulilayer structures: 3 CEBAF Center Auditorium

### CEBAF Center Auditorium

Convener: Mingqi Ge (Jefferson Lab)
• 13:30
6 GHZ CAVITY DEPOSITION WITH B1 SUPERCONDUCTING THIN FILM AT ASTEC 25m

Nb3Sn, NbTiN and NbN are superconductors with a critical temperatures of 18.3, 12.6-17, 11.6-17.5 K, respectively, that are higher than that of Nb (9.3 K). Hence, at 4 K they have an RF resistance of an order of magni-tude lower than that of Nb, which leads to quality factors above those of Nb. In recent years, there has been an extensive effort converting Nb cavities into Nb3Sn by alloying the top inner layer of the cavity using Sn diffusion at a high temperature with some degree of success, however, the reproducibility remains a major hindering and limiting factor.
In this study, we report on PVD deposition of NbTiN inside 6 GHz cavity in an external magnetic coil configu-ration. The deposition is done at elevated temperature of about 650 C.
We report on the superconducting properties, film structure and its stoichiometry and surface chemical state. The films have been characterised with SEM, XRD, XPS, EDS and SQUID magnetometer.

• 13:55
AlN-NbTiN multilayers deposited by PEALD for SRF cavity studies 25m

Atomic layer deposition (ALD) is based on a sequence of self-limiting gas-solid surface reactions. It allows for conformal and smooth coating of highly structured, three-dimensional substrates without shadowing effect and with sub-nm thickness resolution, which makes it particularly interesting for coating the internal surface of SRF cavities. In particular, plasma-enhanced ALD (PEALD), enables the use of metalorganic precursor chemistry not usable in thermal ALD, may improve the quality of the deposited films, can lower deposition temperature.
As a preliminary study, we investigated AlN-NbTiN multilayers grown by PEALD in a supercycle approach. Different compositions and post-deposition thermal treatments have been investigated with respect to their superconducting properties, stoichiometry, and crystallinity and the results will be presented. The findings of our research might pave the way for coating superconductor-insulator-superconductor (SIS) multilayers in superconducting radio-frequency cavities.

Speaker: Isabel González Díaz-Palacio
• 14:20
Successful Al2O3 coating of superconducting niobium cavities by thermal ALD 25m

Al2O3 is one of the potential insulator materials in the superconductor-insulator-superconductor (SIS) multilayer coatings of superconducting radio-frequency (SRF) cavities for pushing their performance limits.
We report on the successful coating of two 1.3 GHz Tesla- shaped SRF cavities with 18 nm and 36 nm layers of Al2O3 deposited by thermal atomic layer deposition (ALD). The coating recipe was developed by thermal atomic layer deposition (ALD). The coating recipe was optimized with respect to different the applied process parameters such as exposure and purge times, substrate temperature and flow rates. After a proof-of-principle Al2O3 coating of a cavity, second the cavity maintained its maximum achievable accelerating field of more than 40 MV/m and no deterioration was observed. On the contrary, an improvement of the surface resistance above 10 MV/m has been observed, which is now further under investigation.

Speaker: Getnet Kacha Deyu (Universität Hamburg | DESY (Remote))
• 14:45
Material engineering of ALD- deposited multilayer to improve the superconducting performances of RF cavities under intense RF fields. 25m

Since its discovery at the beginning of the twentieth century, Superconductors have drown their paths into various fields from electromagnets with unprecedented magnetic fields to state of the art electronic circuits and radiofrequency cavities for particle accelerators. Despite the fact that they have enabled a real breakthrough in particular in the field of SRF cavities, Superconductors still witness performance limitations when exposed to intense magnetic fields.
We are exploring an original approach to improve the performance of bulk Niobium RF cavities through surface engineering with ALD superconducting multilayer capable of screening efficiently the magnetic fields and therefore inhibiting vortices penetration in Niobium SRF cavities [1]. This multilayer consists in a stack of superconducting films a few hundred nanometer thick separated by a 5 to 10 nm insulating layer. To our knowledge, Atomic layer deposition (ALD) is the only deposition technique able of providing high structural and chemical homogeneity over large surfaces with complex shapes.
To that end, we studied an ALD-deposited multilayer based on AlN and NbTiN superconducting films with critical temperature up to 16 K. Chemical and structural analysis will be presented as well as superconducting measurements such as critical temperatures and SQUID. We will also give updates on the development of our ALD-system dedicated to cavity coatings and we will share some of our preliminary results.

[1] A.Gurevich,”Enhancement of RF breakdown field of SC by multilayer coating”.Appl. Phys.Lett, 2006.

Speaker: Yasmine KALBOUSSI (IRFU/DACM CEA Saclay)
• 15:40 16:00
Coffee Break 20m
• 16:00 17:50
Beyond Nb: Alternate materials and mulilayer structures: 4 CEBAF Center Auditorium

### CEBAF Center Auditorium

Convener: Marc Wenskat (DESY)
• 16:00
THE INVESTIGATION OF SPUTTERED S(I)S STRUCTURES FOR SRF CAVITIES 20m

The proposed thin film-based multilayer structures in the form of superconductor-insulator-superconductor (SIS) may be the long-sought-after breakthrough for higher performance SRF cavities by enhancing both accelerating gradients and quality factors. In order to understand better the underlying mechanisms of SIS structures to be coated onto (S)RF cavities, we study sputtered S(I)S structures of Nb-(AlN)-NbN with different thicknesses which are designed to be coated mainly on OFHC copper (Cu) samples for more efficient SRF cavities. In this presentation, the results from both DC and AC magnetization characterizations of the aforementioned multilayer structures are going to be discussed along with their material characteristics in order to assess better the observed phenomena such as non-monotonic surface resistance (Rs) behavior of some SIS structures as well as the recently shown outperformance of the SS structure with respect to the SIS structure of the aforementioned thin films in terms of higher RF penetration field and lower Rs values, albeit having lower critical temperature (Tc).

Speaker: A. Özdem Sezgin (University of Siegen (Remote))
• 16:20
MgB2 Progress at Temple University 20m
Speaker: Dr Ke Chen
• 16:40
A new system for MgB2 coating R&D at LANL 20m

We have installed and commissioned a new furnace that can treat 1.3-GHz cavities for the studies on MgB2 coating. The coating method we will try first is based on the 2-step technique, i.e., coat B layer by flowing B2H6 gas in the first step, then react it with Mg vapor in the second step. We started to study the second step first, i.e., reaction of existing B samples from past projects attached on a 1.3-GHz single-cell cavity with Mg vapor. Since B2H6 gas is toxic, we plan to build a safe system to use it by the end of CY2022. The new facility and some first results on the B-Mg reaction tests will be shown.

Speaker: Tsuyoshi Tajima (Los Alamos National Laboratory)
• 17:00
Beyond Nb: discussion 30m
• Wednesday, 21 September
• 08:30 09:00
Registration
• 09:00 10:30

### Auditorium

Convener: Gianluigi Ciovati (JLab)
• 09:00
Recent advancements on Plasma Electrolytic Polishing technique 30m

A modern technique – Plasma Electrolytic Polishing (PEP), has been growing more attention in the various fields of industry for many applications including jewellery, aeronautics, automotive, medical and others. Surface cleaning, deburring, polishing are the key processes that can be done within the PEP. It is important to have more and more efficient superconducting radio frequency (SRF) cavities, which performance is highly depended on substrate preparation. The application of PEP into the SRF technology protocol may benefit on various fields: economical, ecological, scientific. The usage of greener solution is a sustainable choice in 2022, that allow avoiding the HF acid and other concentrated compounds usage, thus saving budgets, lower the security risks. The substitution of some mechanical treatments and cleaning steps can be a huge leap in the time production of any single cavity. Main economic advantage of PEP over EP is of course the performance of polishing, where it can achieve 10 times faster erosion rate. Additionally, the levelling effect take place faster in case of PEP than EP at the same removal thicknesses.
In this talk it will be presented recent advancements on the Plasma Electrolytic Polishing at LNL, a working solution for Nb PEP will be disclosed. The advantages and main obstacles of PEP application will be discussed, together with characterisation and comparison to the main Nb treatments available: Buffered Chemical Polishing and Electropolishing.

Speaker: Mr Eduard Chyhyrynets (LNL - INFN; University of Padua (Remote))
• 09:30
Additive Manufacturing for SRF copper cavities production and preliminary surface treatments on the printed prototypes 30m

In this work, Metal Additive Manufacturing (MAM) has been evaluated as alternative manufacturing technique to create seamless single-body 6 GHz pure copper prototypes. Preliminary printing tests have been performed to evaluate the printability since the absence of internal supports was mandatory.
The success of the creation of the first prototypes allowed us to proceed with further investigations: several surface treatments for smoothening the down-skin regions have been assessed, thanks to which the surface roughness (Ra) went down to less than 1 µm, starting from an initial value of 35 µm.
Other cavity prototypes have been printed, with different copper powders and different machines. Investigations about the quality of the printing have been conducted, involving tests like tomography, leak test, resonant frequency assessment, internal inspection etc.

Speakers: Massimiliano Bonesso (INFN - Padua Section; Dept. of Industrial Engineering (University of Padova) (Remote)), Cristian Pira (INFN - LNL)
• 10:00
• 10:30 10:50
Coffee Break 20m Cebaf Center Atrium

### Cebaf Center Atrium

• 10:50 12:30
SRF Thin Film Characterization: 1 CEBAF Center auditorium

### CEBAF Center auditorium

Convener: Lorena Vega (CERN)
• 10:50
Advanced characterization of SRF samples 30m

Measuring the surface resistance of samples is key for identifying and optimizing suitable materials and coated structures for SRF cavities with performances beyond the limits of niobium. The Helmholtz-Zentrum Berlin (HZB) routinely operates a Quadrupole Resonator (QPR) for the characterization of SRF samples. Over the past years, the setup has been continuously improved and now allows precision measurements of the surface resistance as a function of temperature, RF field and frequency. However, the effort for a single measurement run is high and limits the number of samples that can be tested in a given time.

Currently, a new sample test cavity is under development. Based on the simple geometry of a cylindrical cavity, tests with reduced resolution and restricted parameter space but with much higher throughput are possible, enabling pre-selection measurements for QPR runs.

Complementary to this, the technique of recording dynamic temperature and magnetic field maps is applied to coated samples to further investigate the issue of trapped flux in thin film systems.

Speaker: Sebastian Keckert (Helmholtz-Zentrum Berlin)
• 11:20
Optimization of Quadrupole Resonator geometry at JLab 30m

Nowadays, the most used superconducting cavities are generally made of bulk Niobium. The high quality of today’s processes uses an accelerating gradient very close to Niobium limit. New materials such as Nb3Sn, NbN and MgB2 that have higher critical temperature and magnetic field must be investigated to improve acceleration capabilities. As these materials could only be used as thin films, the tuning and optimization of the deposition processes require to be performed on small and flat samples. In that sense, it is necessary to perform RF tests, specifically measurement of surface resistance on flat samples. These tests have to be realized with high resolution measurements of surface resistance in a large range of magnetic field and operating temperature. In order to realize these measurements, a quadrupole resonator has carried out at CERN and HZB. These QPR are suffer from the Lorentz force detuning in their operating, more precisely, overlapping of the modes. In this paper, optimization of the QPR geometry is made to avoid these problems are discussed.

Speaker: SARRA BIRA
• 11:50
RF Characterisation of Bulk Niobium and Thin Film Coated Samples at 7.8 GHz 30m

A cost-effective facility for testing planar thin film samples under RF conditions has recently been commissioned at Daresbury Laboratory. This facility utilises a bulk Nb choked test cavity operating at 7.8 GHz, housed within a dry, liquid helium free cryostat. It is used to make low power surface resistance measurements of 10 cm diameter samples at temperatures down to 4 K and sample surface magnetic fields up to 1 mT. The main advantage of this system is the simple sample mounting procedure due to no physical welding between the sample and test cavity. This allows for testing of 2-3 samples per week. Given this high throughput rate, we are able to quickly identify which samples are performing well under RF conditions and should require further testing at higher gradient. After RF characterisation, the superconducting DC properties of these samples as well as surface analysis will be made using other facilities at Daresbury. Details of this facility as well as recent measurements of both bulk Nb and thin film coated samples will be presented.

Speaker: Daniel Seal (Cockcroft Institute, Lancaster University (Remote))
• 12:30 14:00
Lunch 1h 30m CEBAF Center Atrium

### CEBAF Center Atrium

• 14:00 16:00
CEBAF Tour
• 16:00 18:00
Networking Time
• 18:00 18:30
Bus Transportation to dinner Location 30m
• 18:30 21:30
Workshop Dinner
• Thursday, 22 September
• 09:00 10:30
SRF Thin Film Characterization: 2 CEBAF Center auditorium

### CEBAF Center auditorium

Convener: Eric Lechner (Jefferson Lab)
• 09:00
Temperature Mapping of Niobium-coated 1.3 GHz Copper Cavities 30m

CERN has pioneered development of thin film superconducting radio-frequency cavities for particle accelerators. This technology has been applied in LEP II, LHC and more recently in HIE-ISOLDE. Many efforts are put in place at CERN in view of its potential implementation in the FCC machines. However, niobium thin film cavities historically feature a progressive degradation of performance by increasing the accelerating field.
A temperature mapping system has been developed and is currently used to detect the mechanisms responsible of performance degradation. Unlike most of the temperature mapping systems in operation, this system is specially designed for copper coated cavities. Since the thermal diffusivity of copper is noticeably high at liquid helium temperatures, the detection of heat losses in copper coated cavities turns out to be extremely challenging in comparison to that in bulk niobium cavities. We will report how we overcome this limitation in order to cope with our requirements. Furthermore, the first temperature maps of a niobium-copper 1.3 GHz cavity, tested at CERN, will be shown.

Speaker: Antonio Bianchi (CERN)
• 09:30
SPLIT SRF THIN FILM 6 GHZ CAVITIES 30m

Many current accelerators use cavities that are manufactured as two half cells that are electron beam welded together, the weld is across the peak surface current of the cavity. This weld can lead to large increases in surface resistance and limit the performance of thin film coated cavities. Many problems with the coating process for thin film Superconducting Radio Frequency (SRF) cavities are also due to this weld. Thin film SRF cavities can perform as well as bulk niobium cavities if the cavity is manufactured seamlessly, without any weld, as they have a more uniform surface, however, they are much more difficult and expensive to manufacture. A cavity with a split longitudinally, parallel to the direction of the electric field, would not need to be welded. These seamless cavities are easier to manufacture and coat. This opens the possibilities to coat with new materials and multilayer coatings. These cavities may allow SRF cavities to operate at significantly better parameters (higher quality factor and maximum accelerating field) than current state of the art cavities. This work discusses development and testing of longitudinally split seamless cavities at Daresbury Laboratory (DL).

Speaker: Taaj Sian (Lancaster University (Remote))
• 10:00
Stress-induced omega (ω) phase transition in Nb thin films 30m

We analyzed omega (ω) phase transition in Nb thin film deposited by high power impulse magnetron sputtering (HiPIMS) using transmission electron microscopy (TEM) [1]. ~170 nm Nb thin film is deposited on Si (100) substrate and it showed a typical columnar structure with (110) texture on the surface. TEM analysis revealed that the Nb thin films contain ~1 vol.% of hexagonal structured omega (ω) phase Nb in bcc Nb and the size of the omega phase varies from 10-100 nm, which is comparable to the coherence length of Nb (~40 nm). The current finding indicates that Nb thin films are prone to structural change such as omega phase transition due to the internal stresses in the films owing to the low yield strength and critical resolved shear stresses of Nb. We conclude by discussing the superconducting properties of the ω-phase and their possible roles in the Nb thin film SRF cavities.

[1] J. Lee et al, arXiv 2207.12495 (2022)

Speaker: Jaeyel Lee (Fermilab)
• 10:30 10:50
Coffee Break 20m
• 10:50 12:25
SRF Thin Film Characterization: 3 CEBAF Center auditorium

### CEBAF Center auditorium

Convener: Tsuyoshi Tajima (Los Alamos National Laboratory)
• 10:50
A DC magnetic field penetration facility for the characterisation of planar multilayer structures for superconducting radiofrequency applications 30m

The material of choice for current SRF accelerators is bulk Nb which is reaching the theoretical limits in terms of maximum accelerating gradient, Eacc. One method to increase Eacc is to use superconductor-insulator-superconductor, SIS, structures, where the thin films on the surface are smaller than the London penetration depth to screen the applied field, Bapp, such that the thicker superconducting layer will see a lower B than on the surface. The screening allows Bapp to be increased resulting in a larger Eacc. Multilayer structures are challenging to fabricate and test. It is easier to deposit flat samples for multilayers than for 3D structures. Flat samples cannot be tested by commercial magnetometry such as a VSM without limitations, such as early flux penetration due to edge effects as the Bapp is larger than the sample, and B penetrating through the insulating layer so the screening effect will not be observed.
A field penetration facility has been designed, built, and commissioned at Daresbury laboratory. A DC magnetic field is applied from one side of the sample using a C-shaped dipole magnet. Hall probe sensors measure both Bapp and the penetrated field. Samples are mounted directly onto a cryocooler and can be tested at a range of temperatures from as low as 2.5K. The facility has been commissioned using a range of samples including Pb and Nb, which will be reported along with preliminary multilayer results.

Speaker: Daniel Turner (Lancaster University/Cockcroft Institute (Remote))
• 11:20
Development 3rd Harmonic Magnetometer at Jefferson Lab 30m

The development of specialized materials is required to surpass the material limits of bulk Nb superconducting radio frequency (SRF) cavities. Indeed, SRF is a surface phenomenon in superconducting materials with an RF penetration depth of hundreds of nanometers. At these thicknesses, thin films can be tuned to achieve specific superconducting and RF properties. Layering thin films provide potentially even greater improvement. Theoretical analysis [1] of multilayer structures of superconductor/insulator/superconductor (SIS) materials may slow magnetic flux penetration in accelerator cavities, allowing acceleration gradients that are currently not achievable in SRF cavities
The fabrication of small-scale samples allows cost-effective tests of the parameters predicted by this theory. One such essential parameter is the penetration field of magnetic vortices. Measurements with a superconducting quantum interference device (SQUID) is a trusted technique for these parameters, but with thin films, multiple issues arise. By changing the geometry of the applied magnetic field, a 3rd harmonic magnetometer can solve some of these problems [2] allowing the measurement of the magnitude of the first flux penetration (Hfp). In addition, this measurement will help establish the optimum thickness to enhance the Hfp in SIS structures.
The 3rd harmonic magnetometer consists of three interworking systems, thermal control, signal analysis, and a Cu coil to generate the probe magnetic field and measure the 3rd harmonic signal. The thermal system allows active temperature control of the sample and lessens sample heating from the coil, which can measure fields up to 130 mT. The signal analysis system analyzes the noise floor and fits the 3rd harmonic response to extract Hc1 and Tc. This presentation examines the current 3rd harmonic development efforts at Jlab, including interactions of the three interlinked systems and plans for the use of the system on multilayer SIS structures.

Speaker: David Beverstock (Jefferson Lab)
• 11:50
DC MAGNETIC HALL PROBE TECHNIQUE TO CHARACTERIZE THE MATERIALS FOR ACCELERATING CAVITIES 30m

The maximum accelerating gradient of niobium radio frequency cavities are currently reaching their theoretical limits. It can be enhanced by increasing the cavity’s peak surface magnetic field at which the magnetic field penetrating into the superconductor in the form of vortices. To delay the penetrating of vortices into the bulk Nb, SIS structure which is superconductor (S) and insulator (I) layers alternatively coated on bulk niobium has been proposed. Magnetic field at full flux penetration, Bp through a superconductor is a useful characteristic to test the high field performance of SIS structure. DC magnetic Hall probe technique has been developed at Jefferson Lab, which can measure Bp of a superconducting sample placed under parallel DC magnetic field. Using a field much smaller than the sample allows limitations such as edge effects to be significantly reduced. Bulk Pb and Nb samples have been tested for varying thickness to determine how the thickness affect to Bp measurements. The multilayers based NbTiN and AlN deposited on bulk Nb were used to test the proposed field enhancement.

Speaker: Harshani Senevirathne (Old Dominion University)
• 12:25 13:55
Lunch 1h 30m
• 13:55 15:40
SRF Thin Film Characterization: 4 CEBAF Center auditorium

### CEBAF Center auditorium

Convener: Claire Antoine (CEA)
• 13:55
Experimental evidence for counter current flow in superconductor-superconductor bilayers 30m

We report evidence for counter current flow in superconductor-superconductor (SS) bilayers from depth-resolved measurements of their Meissner screening profiles using the low energy muon spin rotation (LE-$\mu$SR) technique. In these experiments, the implantation depth of the muons can be tuned/adjusted/controlled between ${\sim 10}$ nm and $\sim 150$ nm, below the surface, wherein their spin-precession reveals the field distribution inside the material (communicated via their radioactive decay products). Here we studied two prototypical SS bilayers [Nb-Ti-N(50 nm)/Nb and Nb-Ti-N(80 nm)/Nb] and compared the Meissner screening profiles obtained from LE-$\mu$SR against Kubo's counter current flow model, as well a naive (bi)exponential model. From fits to Kubo's model, we obtain a magnetic penetration depth for the thin Nb-Ti-N layers of $\lambda_\mathrm{Nb-Ti-N} = ({201 \pm 4})$ nm, in good agreement with literature values. In contrast, a naive exponential model overestimates the $\lambda_\mathrm{Nb-Ti-N}$ value by a factor of two, suggesting that it is inappropriate for quantifying $\lambda_\mathrm{Nb-Ti-N}$ in the SS bilayer. This also indicates that the surface current is suppressed by the counter current flow in the bottom superconductor. Our result suggests that SS bilayers are a viable means of overcoming the theoretical field limit of Nb.

• 14:25
STM for SRF Thin Films 45m
Speaker: Prof. Maria Iavarone (Temple University (Remote))
• 15:10
SRF Thin Films: Characterization: Discussion 30m
• 15:40 16:00
Coffee Break 20m
• 16:00 17:00
Beyond SRF: quantum & devices CEBAF Center Auditorium

### CEBAF Center Auditorium

Convener: Cristian Pira (INFN LNL)
• 16:00
Metamaterials based on NbTiN 20m
Speaker: Anne-Marie Valente-Feliciano (Jefferson Lab)
• 16:20
Impact of Superconductors’ Properties on the Measurement Sensitivity of Resonant-Based Axion Detectors 35m

Axions, candidates for Dark Matter to be found in the galactic halos, could be detected, in presence of a static magnetic field B, by their conversion to microwave photons collected in resonant cavities. The expected signal power is ∝ B²Qν, where Q is the cavity quality factor and ν the photon frequency. The latter is proportional to the foreseen axion masses which translate to microwave frequencies. Hence the need for high Q microwave cavities to be operated in intense (~tesla) fields.
In zero magnetic field, superconductors outperform copper, the best normal conductor, because of their very low surface impedance.
When a magnetic field drives the superconductor in the mixed state, the additional dissipation due to vortex motion makes the comparison more complex. Indeed, a NbTi coated cavity has provided higher B²Q than bulk Cu at ~15 GHz and 4.2 K up to the maximum ~6 T field reached in the measurements [1]. On the other hand, the actual gain in performance mainly depends on the vortex dynamics response, with its interplay between flux flow dissipation, vortex pinning effects and vortex thermal creep, and the dependence of these mechanisms on the field B, temperature T and frequency ν. By exploiting the knowledge on the high frequency vortex dynamics attained through measurements made on various superconductors (NbTi, Nb$_3$Sn, YBa$_2$Cu$_3$O$_{7−δ}$ and FeSeTe), we present here a thorough comparative study [2] on the performance that can be attained in haloscopes. YBCO and FeSeTe can improve the measurement sensitivity by one order of magnitude. Nb$_3$Sn proves to be able to deliver intermediate improvements which, along with the promising advances in cavity coatings, makes it a very interesting candidate. Finally, NbTi results suitable mainly for the lower frequency region.
This work was partially supported by the national project SAMARA.

[1] D. Di Gioacchino et al, IEEE Trans. Appl. Supercond. 29 3500605 2019
[2] A. Alimenti et al, Instruments 6 1 2022

Speaker: Prof. Nicola Pompeo (Università Roma Tre, DIIEM and INFN, Sezione Roma Tre - Italy)
• 17:00 17:30
Workshop Close-out CEBAF Center Auditorium

### CEBAF Center Auditorium

• Friday, 23 September
• 09:00 13:00
iFast Meeting CEBAF CENTER F2224/25

### CEBAF CENTER F2224/25

Convener: Claire Antoine (CEA)
• 10:40 11:00
Coffee Break 20m