9th International Workshop on Thin Films and New Ideas for Pushing the Limits of RF Superconductivity

Mar 15, 2021, 8:00 AM → Mar 18, 2021, 2:10 PM US/Eastern

Virtual Edition

Anne-Marie Valente-Feliciano (Jefferson Lab)

The 9th 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 virtually due to the Covid-19 pandemic. It supplements the next in-person edition to be held at Jefferson Lab, Newport News VA, USA and postponed to September 2022.

Abstract submission  deadline extended to March 1, 2021.

Registration deadline is March 8, 2021.

Monday, March 15

Workshop Welcome

Convener: Dr Anne-Marie Valente-Feliciano (Jefferson Lab)

1 Accelerator Associate Director Welcome

Speaker: Andrei Seryi (JLAB)

2 Welcome & Logictics

Speaker: Anne-Marie Valente-Feliciano (Jefferson Lab)

SRF thin film in international projects

Convener: Dr Cristian Pira (INFN Legnaro)

3 Main highlights of ARIES WP15 collaboration

Since May 2017, an international collaboration of 8 research teams from 7 countries: CEA (France), CERN (Switzerland), INFN/LNL (Italy), HZB and USI (Germany), IEE (Slovakia), RTU (Latvia) and STFC/DL (UK), are working together on better understanding of how to improve the properties of superconducting thin films for RF cavities. The collaboration has been formed as WP15 (Thin Film for Superconducting RF) in the H2020 ARIES project funded by EC.
The systematic study of superconducting thin films covers:

• Cu substrate preparation: polishing with 5 different techniques (EP, SUBU, EP+SUBU, tumbling, laser) in 3 labs (CERN, INFN, RTU),
• Nb, NbN, Nb3Sn and SIS thin film deposition and characterisation in 3
  labs (INFN, USI, STFC),
• Laser post deposition treatments at RTU,
• DC magnetisation characterisation at IEE, DC magnetic field penetration measurements
  at STFC,
• and, finally, application of all obtained knowledge on polishing,
  deposition and characterisation to the QPR samples for testing the
  films at RF conditions (HZB).

The preparation, deposition and characterisation of each sample involves 3-5 partners enhancing the capability of each other and resulting in a more complete analysis of each film.

The talk will give an overview of the collaborative research and will be an introduction to the detailed talks and posters given by the team members.

Speaker: Oleg Malyshev (ASTeC, STFC Daresbury Laboratory)

Oleg Malyshev - ARIES WP15 15-03-2021 audio.mp4

Oleg Malyshev - ARIES WP15 15-03-2021 audio.pptx

4 FCC-ee SRF Work package

Speakers: Dr Guillaume Rosaz (CERN), Dr Olivier Brunner (CERN)

FCC_workpackage_slides.pdf

5 TTC TF Working Group

Speakers: Marc Wenskat (DESY), Sebastian Keckert (Helmholtz-Zentrum Berlin)

2021-03-15 TTC TF working group.pdf

6 Status of the ALPI Upgrade for the SPES project facilities in INFN LNL

The SPES project is based at INFN LNL and covers basic research in nuclear physics and astrophysics, radionuclide production, materials science research, nuclear technology, and medicine. ALPI is a linear accelerator, equipped with superconducting quarter-wave resonators (QWRs) and it will be used also to accelerates SPES radioactive beams. The ALPI enhancement is critical with respect to the SPES project facilities. The ALPI upgrade is based on the implementation of two additional cryostats in the high-β section. The technology is well established, and it also confirmed from the fact that cavities installed in 1999 on accelerator were recently re-measured and they demonstrate the same performance after 22 years of activity on accelerator. The production technology of Nb/Cu QWRs should be adjusted for high-β cavities for increasing the performance of resonators and production 8 cavities for the ALPI upgrade. In the framework of the upgrade two vacuum systems (for biased sputtering and for cryogenic measurements) were refurbished. Optimal parameters of the biased sputtering processes of copper QWR cavities and plates were defined. The process of mechanical, chemical, and electrochemical preparation, sputtering and cryogenic measurement of the high-β Nb/Cu QWR cavities was adjusted. The best result of cryogenic measurements of produced QWR is Q0 = 2.0 · 109; Q7W = 2,76 · 108 ; Eacc (at 7W) = 5,54 MV/m. Currently, the production of the Nb/Cu QWR cavities and plates is ongoing.

Speakers: Mr Andrii Tsymbaliuk (INFN LNL), Mr Giorgio Keppel (INFN LNL)

Status of the ALPI Upgrade for the SPES project facility in INFN LNL.pptx

Theoretical Modeling RF Behavior

Convener: Prof. Alexander Gurevich (ODU)

7 Nonlinear RF losses of a trapped vortex in different pinning landscapes under strong RF field in a superconducting film

Trapped vortices can contribute significantly to a residual surface resistance $R_i(H)$ of SRF cavities, but the behavior of an elastic vortex line driven by strong RF Meissner currents through a random pinning potential remains poorly understood. Here we report extensive numerical simulations of large-amplitude oscillations of a trapped vortex under a strong RF magnetic field parallel to a superconducting film with different pinning landscapes, including bulk, surface, and cluster pinning. The RF power dissipated by an oscillating vortex was calculated, taking into account the vortex line tension, vortex mass, the linear Bardeen-Stephen and a nonlinear Larkin-Ovchinnikov viscous drag coefficients. We show that the global surface resistance $\bar{R}_i(H)$ averaged over statistical realizations of a random pinning potential exhibits a linear increase at small fields. However, $R_i(H)$ at strong RF field becomes a nonmonotonic function of $H$ and decreases with $H$ at higher frequencies, which can contribute to the negative $Q(H)$ slope observed on SRF cavities. Overheating can mask the descending field dependence of $R_i(H)$ as frequency increases. Our simulations also show that the strong RF field can cause a jumpwise increase in the RF power.

Speaker: Mr Manula Pathirana (Old Dominion University)

Thin film workshop 2021_manula.pptx

Break

Nb/Cu Technology: Advanced Substrates

Convener: Dr Mingqi Ge (Cornell University)

8 Plasma electrolytic polishing technology as a modern, powerful, and green way to prepare surfaces for SRF

It is known that any field requires Its own preparation protocol for a successful result. Same for particle accelerator industry, that are working in nuclear and high-energy physics research. It is important to have more and more efficient superconducting radio frequency (SRF) cavities, which performance is highly depended on substrate preparation. Niobium cavities became a standard choice for the SRF field approaching a theoretical limit of accelerating field, other solutions are also known, such as a magnetron sputtering of the superconducting layer onto copper cavities. All those solutions require similar protocol in cleaning, and different treatment chemistry. Currently the best results are only achieved with electropolishing (EP) technique. Harsh and non-environmentally friendly solutions are typically used: HF and H2SO4 mixture for Nb, and H3PO4 with n-Butanol mixtures for EP of Cu.
Our research is dedicated to an application of recent and promising technology – plasma electrolytic polishing (PEP) on cavity for SRF. PEP is a technology, that is similar to EP, it uses low-concentrated solutions, higher voltages, and provide higher removing rates. It is also claimed to achieve lower roughness. It can substitute several steps, such as mechanical and/or (electro)chemical polishing, degreasing in a one single treatment. In the study it was developed and studied various solutions for Cu and Nb polishing and was conducted attempts to polish 6 GHz cavities.

Speaker: Dr Eduard Chyhyrynets (LNL - INFN; University of Padua)

TFSRF21_Chyhyrynets.pptx

9 Electrodeposition of copper applied to the manufacture of seamless SRF cavities

Copper, niobium thin film coated, superconducting radiofrequency (SRF) elliptical cavities have demonstrated for several applications a strong potential as an alternative to bulk niobium ones. The present manufacturing techniques include electron-beam welding in order to join the half-cells and the cut-offs together. The weld lines can carry geometrical defects and induce porosities, which can in turn act as nucleation sites for defects in the subsequently grown Nb layer. Seamless methods are under development to avoid welding steps and improve the quality of the films, with the side effect of reducing the manufacturing costs. This study concerns a new alternative route where a cavity is built by electrodeposition of copper around a sacrificial aluminium mandrel. For this process no welding joints are necessary and also the flanges can be directly integrated in the electroforming step. For this purpose, two different electroforming procedures using either direct or pulsed plating have been investigated, leading to a high quality material exhibiting similar mechanical robustness, cryogenic properties and purity as the oxygen-free copper. Electron Backscatter Diffraction (EBSD) was used to study Geometrically Necessary Dislocation (GND) densities showing very low dislocation densities for samples extracted from a plated cavity. Finally, the fabrication process was validated with the production of the first 1.3 GHz electroformed cavity.

Speaker: Lucia Lain Amador (CERN)

TFSRF_Lucia Lain Amador.pdf

10 First results of the RF performance of 1.3 GHz Nb coated cavities on high quality copper substrates

A degradation of the RF performance at high fields has been historically observed in SRF thin film coated cavities. It is known that the presence of defects in the superconducting thin film causes a local increase of the surface resistance, which is correlated to power being dissipated in the surroundings of the defect. This leads to the observed Q drop which becomes more important when testing at high fields. Thus, it is imperative to avoid defects in order to mitigate this phenomena. Efforts have been made not only to improve the deposition techniques, but also to produce high quality substrates, as defects are propagated to the thin film deposited on top. To this end, seamless copper substrates have been produced by two different methods: chemical electroplating and machining of a bulk Cu billet. Two 1.3 GHz cavities have been manufactured, each one with the aforementioned methods, followed by a Nb film deposition via HiPIMS technique. In this work we will present the preliminary results of the RF performance of those cavities, which have been measured down to 1.85 K using a mobile coupler to ensure a good matching between the input coupler and the cavity, hence reducing the measurement uncertainty.

Speaker: Dr Lorena Vega Cid (CERN)

20210315_TFSRF21_First_results.pptx

Discussion

Convener: Dr Walter Venturini Delsolaro (CERN)

Tuesday, March 16

Nb thin film technology

Convener: Dr Teng Tan (Institute of Modern Physics, Chinese Academy of Science)

11 Current status of 6 GHz DCMS thick film deposition at Legnaro

At LNL the R&D on Niobium on Copper cavities coating film technology is of great importance since the development of Quarter Wave Resonators of the ALPI accelerator. The 6 GHz elliptical cavities represent a low-cost research that is a key step to go through from the prototypes into the real cavities in the framework of the accelerator technology. Thick films deposited in long pulse DCMS deposition mode onto 6 GHz cooper cavities have demonstrated the mitigation of the Q-slope at low accelerating fields. Nb thick films (~40 microns) show the possibility reproduce the bulk niobium superconducting properties. In this talk, the current status of the thick films onto 6 GHz cavities at LNL will be shown, including RF tests, morphological characterizations and the future studies.

Speaker: Vanessa Garcia Diaz (LNL-INFN)

Garcia - ThickFilm - ThinFilmWorkshop.pdf

12 Tuning of thin film thickness profile in SRF cavities

Coating low beta accelerating structures has been shown [1,2,3] to be challenging, first because the standard coating technique that is Direct Current Magnetron Sputtering leads to more porous layers as the cavities’ beta factor decreases. A second issue is the non-uniform thickness distribution resulting in a film that can be six times thicker at the iris than at the equator. This can lead to local peel-off and requires a time-consuming coating process due to the minimum thickness required at the equator to properly screen the electromagnetic field.
To address both problems we first discuss the possibility to tune the metallic ions spatial distribution by acting on the magnetic profile leading eventually to a tuning of the coating profile.
In a second time, we present the technical solution to tune the magnetic configuration in a cylindrical geometry supported by numerical simulation of both the magnetic assemblies and plasma discharge.
Finally, we report the effect of magnetic balance degree, substrate bias and total magnetic strength on the niobium thickness profile obtained in low beta accelerating cavities.
[1]: D. Tonini, C. Greggio, G. Keppel, F. Laviano, M. Musiani, G. Torzo and V. Palmieri,11th Workshop on RF superconductivity, Lübeck/Travemünder, 2003.
[2]: S. Calatroni, Physica C: Superconductivity, vol. 441, pp. 95-101, 2006.
[5]: C. Benvenuti, D. Boussard, S. Calatroni, E. Chiaveri and J. Tuckmantel, 8th Workshop on RF superconductivity, Abano Terme, 1997.

Speaker: Dr Guillaume Rosaz (CERN)

Thickness_Profile_ROSAZ.pdf

13 Sputtering as a line-of-sight process – a computational case study

Especially in neutral-dominant, low-power DCMS discharges, sputtering is treated often implicitly as a line-of-sight process: The relative growth-rate of a film at a given location is dominated by the position and orientation with respect to the closest sputtering surface. However, complications can arise due to the complex geometry of a substrate, particularly with multiple sputtering targets. One such substrate is the Wide Open Waveguide Crab Cavity (WOWCC), investigated in the Future Circular Collider study at CERN [1]. With niobium sputtering profiles extracted from a Particle-In-Cell Monte-Carlo (PIC-MC) plasma simulation [2], different tools are compared to investigate the ensuing transport processes.

Here, we present results from Molflow [3], an open-source particle tracing code originally designed for modelling high-vacuum systems. As a ray-tracing MC simulation, it only models collisions on boundaries. Volume collisions with the process gas are ignored in favour of a drastic reduction in computational effort. These purely line-of-sight based results are benchmarked against an established collisional code [2]. While their overall agreement is remarkable, the impact of volume collisions is still locally appreciable. Discrepancies are quantified and explained e.g. through the influence of distant targets and shadowing. Rules-of-thumb for general settings are outlined. Further comparisons against SiMTra [4] are foreseen, but hampered due to 3D modelling limitations.

Speaker: Dr Fabian Manke (CERN)

TFSRF21_fmanke_sputtering_los.pdf

TFSRF21_fmanke_sputtering_los.pptx

14 Bipolar HiPIMS: Correlating plasma parameters to thin film properties

Retarding Field Analyzer (RFA) and Langmuir probe measurements were carried out in a $3$ inch magnetron system with a niobium target operating in argon to determine how positive polarity pulses in the HiPIMS voltage waveform can change the dynamics of the plasma and influence thin film properties.

The positive voltage pulses were introduced in the early afterglow with heights $U_{pos}$ between $10$ and $100\,V$ and a variation of durations between $10$ and $100$ $\mu s$. Operating argon pressures were varied from $5$ to $24\,mTorr$. The plasma parameters were determined with a time-resolution of $10\,\mu s$ for the RFA at a typical substrate position $10$ $cm$ away from the target. The time-resolution for Langmuir probe measurements was $1\,\mu s$ at different position along the centre line of the magnetron and above the racetrack.

RFA measurements show a clear increase in ion energy when a positive polarity pulse is applied. In contrary the ion flux is identical for both cases. Since the ion flux is the product of ion density and ion energy a faster decay in ion density is required, which was verified by Langmuir probe measurements. To correlate the effects a bipolar HiPIMS pulse has on the crystal structure and the superconductivity of thin films, Niobium films were analysed by SEM, XRD and Squid measurements.

Speaker: Felix Walk

presentation_Felix_Walk.pdf

15 Improvement of Mechanical and Magnetic Properties of Nb/Cu Structure by Laser Radiation for SRF Applications

Nb on Cu structure was proposed instead of the bulk Nd for SRF applications because it is chipper and possesses better thermal properties. For such structure is very important to have good mechanical, electrical, and magnetic properties. An adhesion of Nb on Cu substrate depends on both Nb and Cu crystallinity and Nb deposition technology and thickness of Nb film. The DC entry field Hen and critical temperature were studied in the Nb films deposited by magnetron sputtering on Cu substrate. The Nb/Cu structure was further treated by Nd:YAG laser at four energy doses. The superconducting properties and other characteristics obtained by AFM, SEM, and XRD were compared before and after laser treatment. The laser treatment increased the field of first flux entry by up to 65% compared to a non-irradiated sample. The laser irradiation also led to reduction of surface roughness, improvement of the surface morphology, reduction in crystallites size, and increase of adhesion on 40%. Optical, electrical, and magnetic properties of QPR before and after laser processing were studded.

Acknowledgment
This project has received funding from the European Union’s Horizon 2020 Research and Innovation programme under Grant Agreement No 730871. The authors would also like to thank the ARIES WP15 collaboration team for useful discussions and suggestions.

Speaker: Prof. Arturs Medvids (Riga Technical University)

2. ARIES Medvids 2021. Oleg4.pptx

16 SRF Thin Film Activities at JLab

Speaker: Anne-Marie Valente-Feliciano (Jefferson Lab)

TF@JLab-AMVF-TFSRF21.pdf

Break

Nb thin film technology: Discussion

Convener: Dr Guillaume Rosaz (CERN)

Alternate material based SRF thin films: Part 1

Convener: Dr Grigory Eremeev (Fermilab)

17 Electrochemical and thermal synthesis of Nb3Sn coatings on Nb substrates

Coating superconducting Nb3Sn thin film on the inner surface of a superconducting RF cavity is one of the most promising approaches to improve the performance of the accelerating cavity. Compared with traditional tin deposition and sputtering processes, electrochemical coating has the advantages on process simplicity, low cost and mass production. In this paper, Electrochemical and thermal synthesis of superconducting Nb3Sn films were studied. The Nb3Sn films were obtained by 700 °C ex-situ vacuum annealing of electrochemically deposited bronze layer on Nb substrates. Precursors were prepared via electrochemical deposition with aqueous electrolyte at room temperature. Samples were then precisely electropolished to remove the a-few- micrometres of remnant bronze phase on top of the surface and then characterized. Focused Ion Beam Scanning Electron Microscope (FIB-SEM) measured the thickness of the Nb3Sn layer to be 1.2 μm. X-ray diffraction (XRD) patterns confirmed the existence of a cubic Nb3Sn phase (A15 structure). The highest Tc observed on these samples was 17.6 K.
Key words: SRF, electrochemical coating, thin film

Speakers: Ming Lu (The Institute of Modern Physics), Teng Tan (Institute of Modern Physics, Chinese Academy of Science)

TFSRF-MingLu-IMP.pdf

18 Bronze route inspired Nb3Sn thin film coatings for SRF cavities

Bronze route Nb3Sn growth is a well mastered technique used in conductor development which guarantees phase pure Nb3Sn at low temperatures, 600 – 750 °C. Use of bronze route will allow development of Nb3Sn coated Cu or bronze cavities which is much cheaper alternative to Nb3Sn coated Nb cavities. In addition, high thermal conductivity of Cu will facilitate heat removal from the superconducting coating efficiently. Nb3Sn films were grown on bronze substrates using two approaches and resultant films showed slightly reduced Tc, ~ 14 – 15 K due to the high strain in films induced by CTE mismatch between substrate and the Nb3Sn film. Nb films deposited on bronze substrates at 200 °C followed by a high temperature post-reaction (conventional bronze process) produced Nb3Sn films with ~24% Sn content and small grains. Nb3Sn films produced by sputtering Nb onto hot bronze, ~ 700 °C produced high quality Nb3Sn films with Sn content ~26% and columnar grains similar to structure zone 2 in Thornton structure zones. XRD measurements showed slightly large strain in the hot bronze samples compared to the post-reacted samples. This indicates high intrinsic stress in the Nb3Sn films produced with hot bronze approach compared to the post-reacted samples. In addition, we are investigating several different approaches such as evaporation of high Sn bronze films on Nb and use of multilayers on Cu substrate with Ta diffusion barriers to synthesize Nb3Sn coatings on Nb and Cu substrates.

Speaker: Wenura Withanage (Applied Superconductivity Center, NHMFL-FSU)

Bronze_route_Nb3Sn_TFSRF2021.pdf

19 Nb3Sn films via liquid tin diffusion for SRF application

In the present work, an alternative route to grow Nb3Sn film for SRF application is presented. Liquid tin diffusion is an established technology for wire fabrication, already explored in the past at LNL for SRF application.
The deposition process has been completely revised, adopting sample pre-anodization to enhance the Nb3Sn nucleation from the tin vapor diffusion and introducing a double annealing process. The new deposition procedure solves the tin droplet problem that normally afflicts liquid tin diffusion. XRD shows a single Nb3Sn phase, without the presence of spurious phases as in the old process. Evaluation of SRF properties in 6 GHz cavities are ongoing. Moreover, the process has been adapted to axion cavities and behavior of Nb3Sn in static high magnetic field, for dark matter application, will be tested as well.

Speaker: Cristian Pira (INFN LNL)

Pira - Nb3Sn Thinfilms Workshop 2021.pdf

Wednesday, March 17

Alternate material based SRF thin films: Part 2

Convener: Dr Thomas Proslier (IRFU, CEA Saclay)

20 First Cavity Performance Test for Nb3Sn Superconducting Cavity by Vapor Diffusion Process at KEK

A tin vapor diffusion system was constructed for Nb3Sn cavity R&D at KEK.
After construction of the vapor diffusion system, we have performed Nb3Sn coating test for several samples.
Several samples are observed and we optimized the coating parameter for cavity coating.
After coating Nb3Sn for the single-cell cavity, flux expulsion and cavity performance were measured.
In this presentation, sample coating results and cavity performance results are reported.

Speaker: Kotaro Takahashi (SOKENDAI / KEK)

KotaroTakahashiTFSRF21.pdf

21 Nb3Sn growth by multilayer sequential sputtering for SRF application

Nb3Sn is considered as a potential candidate for superconducting radiofrequency (SRF) cavities for particle acceleration due to its higher critical temperature of 18.3 K and higher superheating field of 400 mT. We fabricated Nb3Sn films by multilayer sputtering and the film properties were optimized by varying the fabrication conditions (multilayer thickness, substrate temperature, annealing temperature, and annealing time). The films were characterized by scanning electron microscopy, X-ray diffraction, atomic force microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy. The DC superconducting properties of the films were characterized by a four-point probe technique down to cryogenic temperature. RF surface resistance of the films were characterized by using the surface impedance characterization system at Jefferson Lab. The films had a DC superconducting critical temperature up to 17.93 K and an RF superconducting transition at 17.2 K. To extend this process to SRF cavities, a cylindrical magnetron sputtering system is designed to fabricate Nb3Sn by multilayer sputtering inside a 2.6 GHz single-cell SRF cavity.

Speaker: Md Nizam Sayeed (Old Dominion University)

TFSRF-Nizam.pdf

22 Study of alternative materials for next generation SRF cavities at Cornell University

Alternative materials are required in the SRF field to push the cavity performance limits. At Cornell, we investigate various A15 superconductor options that include Nb3Sn, V3Si, MgB2, and NbTiN thin films. We also explore different deposition approaches and post processes to optimize the material properties. In the workshop, we provide an overview of the ongoing progress at Cornell. Specifically, we discuss the electroplating and sputtering approaches to generate high quality Nb3Sn thin films via direct deposition or taking advantage of post annealing. We also report a passivation study on the MgB2 films. Lastly, we discuss the structural characterization on NbTiN films for S-I-S multilayer structures.

Speaker: Zeming Sun (CLASSE)

Zeming Sun_Thin film workshop slide.pdf

23 Using HiPIMS for V3Si Superconducting Thin Films

Nearly all superconducting RF (SRF) cavities today use superconducting niobium: whether it be bulk niobium or thin niobium films on copper. However, Nb/Cu cavities must be operated in very strict thermal conditions so as to operate in the residual resistive regime. To overcome this limitation, new thin film materials must be investigated and A-15 superconductors are a promising avenue of investigation. V3Si is an A-15 superconductor, shown to have high critical temperature (17.1 K) and RRR value up to 80, making it a promising candidate for SRF-cavities[1].

The RRR and critical temperature of V3Si is closely linked to the Stoichiometry, which in turns depends on substrate and deposition temperature [2] . Previous experiments have relied on dual magnetron sputtering or reactive sputtering, which have had problems achieving the correct stoichiometry.

The magnetron sputtering technique HiPIMS has been shown to ion bombarding films during deposition. This ion bombardment has been shown to have a similar effect to sample heating allowing greater control of stoichiometry [3]. This combined with new heating techniques to improve the existing benchmarks of V3Si films.

[1] S. M. Deambrosis, et al, Phys. C Supercond. its Appl., vol. 441, no. 1–2, pp. 108–113, 2006.
[2] V. Palmieri, “New Materials for Superconducting Cavities,” Proc. SRF 2001, no. 6, pp. 162–169, 2001.
[3] A. Anders, Thin Solid Films, vol. 518, no. 15, pp. 4087–4090, 2010.

Speaker: Francis Lockwood Estrin (University of Liverpool)

Presentation.pptx

24 Synthesis of Alternative Superconducting Film to Nb for SRF Cavity

It is widely accepted that for further improvement in cavity RF performance, innovation is needed and one may have to turn to other forms of Nb and to other superconducting materials. The potential benefits of using materials other than Nb would be a higher Tc, a potentially higher critical field Hc, leading to potentially significant cryogenics cost reduction if the cavity operation temperature is 4.2 K or higher. Achieving higher accelerating gradient will reduce the overall length of the accelerator and hence shorter tunnel. In Superconducting Cavity made of Niobium only the very few microns of the internal surface layer is important and the rest of the bulk is used as the support. Copper is a much better heat conductor than Nb, so using copper as the support and having several layers of Nb or multilayer of Superconductor/insulator/Superconductor will bring the cost down considerably. Other materials known as A15, such as Nb3Sn or V3Si and B1 such as NbTiN and NbN are much easier to synthesise in thin films rather than being made as bulk cavity.
We report on optimised deposition parameters for successfully synthesising the alternative superconducting thin film with high superconducting properties (Tc and Hsh) on flat substrates samples in single and multi-layer. The dependence of superconducting properties of the total structure on deposition parameters is been determined. The films have been characterized via SEM, EDS, TEM, XRD, RBS,and SQUID magnetometer.

Speaker: Reza Valizadeh (STFC)

TF SRF workshop JLAB 17th 03 2021 .pdf

TF SRF workshop JLAB 17th 03 2021.pptx

Break

SIS structures for SRF

Convener: Dr Michael Vogel (University of Siegen)

25 The Development of HiPIMS Multilayer SIS film coatings on Copper for SRF Applications

In recent years, the use of alternatives to bulk Nb in the fabrication of SRF cavities, including novel materials and fabrication techniques, have been extensively explored by the SRF community. One of these new methodologies is the use of a superconductor-insulator-superconductor (SIS) multilayer film coating. Typically, these have been envisaged for use with bulk Nb cavities. However, it is possible to combine the benefits of SIS coatings with the benefits of coated Cu cavities. It is also clear that the use of so-called energetic deposition techniques, such as HiPIMS, provide significant benefits over typical DC MS coatings, in terms of SRF performance.
In light of this, experiments have been completed in the pursuit of HiPIMS-deposited, multilayer SIS film coatings on copper samples. The SIS system studied here was Nb-AlN-NbN. This research culminated in the deposition of two QPR samples, using the final optimized coating process. This contribution details the development of these coatings and the required optimization of the coating parameters of the separate material systems, through the use of multiple material and superconducting characterization techniques.

Speaker: Stewart Leith

TSRF 2021_Leith_new_final.pdf

26 Material engineering of ALD- deposited multilayer to improve the superconducting performances of RF cavities under intense RF 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].
As a first step for the multilayer, we aim at replacing the deleterious niobium native oxide by a clean interface between an insulator synthesized by ALD (Al2O3, Y2O3 and MgO) and the Niobium metal. To that end, I will present the results obtained on both flat niobium samples and 1.3 GHz elliptical cavities. Our study shows that ALD deposited films are in fact a good diffusion barrier, resist to thermal treatments and reduce significantly the presence of the niobium native oxide on the surface. Low SEY material such as TiN was also deposited on top of the insulator film to reduce multipacting phenomena. RF test on ALD coated cavities shows already a slight improvement of the superconducting performances.
In parallel we started synthesizing superconducting NbTiN alloys by ALD. I will present preliminary results on the superconducting properties of NbTiN films grown on AlN by ALD with various compositions on Nb substrates.
[1] A.Gurevich,”Enhancement of RF breakdown field of SC by multilayer coating”.Appl. Phys.Lett, 2006.

Speaker: Yasmine KALBOUSSI (IRFU/DACM CEA Saclay)

27 ALD-Based NbTiN studies for SIS R&D

Superconductor-Insulator-Superconductor multilayers improve the performance of SRF cavities providing magnetic screening of the bulk cavity and lower surface resistance.

In this framework NbTiN mixtures stand as a potential material of interest combining the excellent superconducting properties of NbN with the good metallic and structural properties of TiN. One method which enables fine tuning of the stoichiometry and precise thickness control in sub-nm range is atomic layer deposition (ALD). ALD bases on a sequence of self-limiting gas-solid surface reactions and allows for uniform coating of complex geometries.

In this talk, we report about $Nb_{0.66}Ti_{0.33}N$ thin films deposited by plasma-enhanced ALD on insulating AlN buffer layer, which has been previously deposited in situ without vacuum break with the same technique. The deposition process has been optimized by studying the superconducting electrical properties of the films. Post-deposition thermal annealing studies with varying temperatures, annealing times, and gas atmospheres have been performed to further improve the thin film quality and the superconducting properties. Our experimental studies show an increase in $T_{c}$ by 87.5% after thermal annealing and a maximum $T_{c}$ of 13.9 K has been achieved for $Nb_{0.66}Ti_{0.33}N$ of 23 nm thickness. Future steps include lattice characterization, using XRR/XRD/EBSD/PALS, and SRF measurements to obtain $H_{c1}$ and the superconducting gap $\Delta$.

Speaker: Isabel González Díaz-Palacio

ALD-Based NbTiN studies for SIS R&D_TFWorkshop _IGDP.pptx

28 Development and characterization of Nb3Sn/Al2O3 superconducting multilayers for high-performance radio-frequency applications

We report the growth and multi-technique characterization of stoichiometric Nb3Sn/Al2O3 multilayers with good superconducting and RF properties. We developed an adsorption-controlled growth process by co-sputtering Nb and Sn at high temperatures with a high overpressure of Sn. SIS multilayers with up to 4 Nb3Sn layers of thickness 50-60 nm, separated by 5-6 nm thick Al2O3 interlayers have been grown on 2” sapphire wafers. The cross-sectional scanning electron transmission microscope images show no interdiffusion between Al2O3 and Nb3Sn and STM measurements reveled a superconducting gap corresponding to a stoichiometric Nb3Sn. Low-field RF measurements have shown that our Nb3Sn trilayer has quality factor comparable with cavity-grade Nb at 4.2 K. These results provide a materials platform for the development and optimization of high-performance SIS multilayers for SRF cavity applications.

Speaker: Christopher Sundahl

Development and characterization of Nb3SnAl2O3 superconducting multilayers for high-performance radio-frequency applications - TFSRF 2021.pdf

29 NbN Thin Film-Based Multilayer (S-(I)-S) Structures for SRF Cavities

A.Ö. Sezgin1, M. Vogel1, B.R. Lakki Reddy Venkata1, A. Akter1, X. Jiang1, I. Gonzales Diaz-Palacio2, R. Zierold2

1 University of Siegen, Institute of Materials Engineering, Siegen, Germany
2 University of Hamburg, Center for Hybrid Nanostructures (CHyN), Hamburg, Germany

Keywords: SRF cavities, superconducting NbN thin films, multilayer, SIS, magnetron sputtering

In order to obtain more sustainable next generation compact particle accelerators, overcoming the limitations set by the existing bulk niobium SRF cavities is crucial. One of the promising candidates to push the limits of the bulk niobium cavities are thin film-based multilayer structures in the form superconductor-insulator-superconductor (SIS).
On the path towards formulating optimized coating recipes, the research focuses on innovating the best performing multilayer structures, consisting of alternating NbN superconducting thin films with metal oxide (e.g., AlxOy) as well as AlN insulating layers produced by means of (PE)-ALD and magnetron sputtering techniques with industrial upscaling potential. This contribution shows first results of the above-mentioned multilayer systems on flat samples characterized mainly by SEM, EDX and XRD.

Acknowledgements
This research is funded by the Federal Ministry of Education and Research of Germany in the framework of SMART (project number 05K19PSA).
Part of this work was performed at the Micro- and Nanoanalytics Facility (MNaF) of the University of Siegen.

Speaker: Ms A. Özdem Sezgin (Universität Siegen)

9th_TFSRF21_Presentation_AOS_20210317-6.pdf

SRF films other than Nb: Discussion

Convener: Dr Claire Antoine (CEA)

T_2021-03- intro group disc TFSRF.pptx

Thursday, March 18

RF and material characterization for SRF films: Part 1

Convener: Dr Reza Valizadeh (STFC Daresbury)

30 Overview of thin-film studies at KEK and Kyoto University.

We have a system for the third harmonic measurement of thin-film sample at KEK, and NbN thin-film samples were measured. We have another system for the third harmonic measurement of thin-film sample at Kyoto University and NbN and NbTiN thin-film samples were measured. The NbN thin-film samples were created by ULVAC, Inc. in collaboration with KEK. The NbTiN thin-film sample was created by JLab. These measurement systems are cooled down with liquid helium and various setups can be inserted into the cryostats for measurements of samples, and in particular, we are going to measure 3.0-GHz cavities by the cryostat at KEK to focus on the thin-film experiment. 3.0-GHz caivities were fabricated in collaboration with JLab. This article presents the details of the activities and the measurements of NbN and NbTiN samples by these systems at KEK and Kyoto Univiersity.

Speaker: Takayuki Saeki (KEK)

2021TFSRF-WS-KEK-Overview-saeki-v1.pdf

31 Development of 3 Ghz cavities at KEK

Construction of test system for 3.0-GHz single-cell cavity at KEK.
Measurement system for lambda of thin-film sample at Kyoto University

Speaker: Takayuki Saeki (KEK)

2021TFSRF-WS-3GHzCavity-saeki-v1.pdf

32 A low power test facility for SRF thin film testing with high sample throughput rate

A low-power SRF test facility is currently being developed at Daresbury Laboratory as part of the superconducting thin film testing programme. The facility consists of a bulk niobium choked test cavity operating at 7.8 GHz surrounded by three RF chokes housed within a dry, liquid helium free cryostat. The cavity is able to test thin film planar samples of up to 10 cm in diameter at temperatures down to 4 K with a maximum RF power of 1 W. Having the cavity surrounded by RF chokes allows it to be physically and thermally isolated from the sample, thus reducing the need for a complicated and time consuming sample mounting procedure, while at the same time minimising the electromagnetic field leakage out of the cavity. Heaters connected to the sample allow for direct measurements of the surface resistance of samples using a DC-RF calorimetry method. Due to the simple sample mounting procedure, we are able to test samples with a fast turnaround time of two to three days per sample. This is the main advantage of this facility compared with other test facilities. When fully operational, results from RF tests will be compared alongside the thin film deposition parameters, deposited film characterisation with SEM, XPS, XRD and other techniques, and DC/AC measurements to fully analyse the performance of different thin film samples. Details of this facility will be presented at this workshop.

Speaker: Daniel Seal (Cockcroft Institute/Lancaster University)

Thin Films Workshop March 2021.pptx

33 Upgrades to the Cornell sample host cavity

The Cornell sample host cavity was designed to measure the surface resistance of 5” diameter disks at 4 GHz as a function of temperature and RF field strength. Major changes to the system have been completed, including the addition of a transmitted power probe and a complete redesign of the fundamental power coupler. Calibration results demonstrating the improved performance are presented and conclusions about measurement reliability are considered.

Speaker: Thomas Oseroff (Cornell University)

Thinfilms2021 (19).pdf

34 A New Quadrupole Resonator for SRF R&D

As superconducting radio-frequency (SRF) cavities are now approaching the theoretical limits of the material, a variety of different surface treatments have been developed to further improve their performance; although no fully understood theory is yet available. Small superconducting samples are studied to characterize their material properties and their evolution under different surface treatments. To study the RF properties of such samples under realistic SRF conditions at low temperatures, a test cavity called quadrupole resonator (QPR) is currently being fabricated. In this work we report the status of the QPR at Universität Hamburg in collaboration with DESY. Our device is based on the QPRs operated at CERN and at HZB and its design will allow for testing samples under cavity-like conditions, i.e., at temperatures between 2 K and 8 K, under magnetic fields up to 120 mT and with operating frequencies of 433 MHz, 866 MHz and 1300 MHz. Fabrication tolerance studies on the electromagnetic field distributions and simulations of the static detuning of the device, together with a status report on the current manufacturing process, will be presented.

Speaker: Marc Wenskat (DESY)

TF_QPR_Wenskat.pdf

Break

RF and material characterization for SRF films: Part 2

Convener: Dr Marc Wenskat (DESY)

35 INVESTIGATION OF SIS MULTILAYER FILMS AT HZB

The systematic study of multilayer SIS films (Superconductor-Insulator-Superconductor) is being conducted in Helmholtz-Zentrum Berlin. Such films theoretically should boost the performance of superconducting cavities, and reduce some problems related to bulk Nb such as magnetic flux trapping. Up to now such films have been presented in theory, but the RF performance of those structures have not been widely studied. In this contribution we present the results of the latest tests of AlN-NbN films, deposited on micrometers-thick Nb layers on copper. It has, also, been shown previously at HZB that such SIS films may show some unexpected behavior in surface resistance versus temperature parameter space. In this contribution we continue to investigate those effects with the variation of different parameters of films (such as insulator thickness) and production recipes.

Speaker: Dmitry Tikhonov (Helmholtz Zentrum Berlin)

Pres HZB templete.pptx

36 Investigation of Local Nonlinear Microwave Response of Nb3Sn and Nb-on-Cu in the Superconducting State

The microscopic origins of Superconducting Radio Frequency (SRF) cavity breakdown by surface defects are still not completely understood. To locally study the electrodynamics of superconductors, a near-field magnetic microwave microscope was built. We study the 3rd harmonic response as a function of rf field amplitude and temperature. In previous experiments on Nb, two different types of nonlinearity were observed, which we call Low-field and Periodic. The Low-field response can be explained by the intrinsic response of the sample due to vortex semiloops created by the magnetic writer probe. As for the Periodic case, the response can be linked to the Josephson effect at or near the surface. We present new results for Nb3Sn films (from Cornell) and Nb films on Cu (from CERN). The Nb3Sn shows evidence for multiple superconducting transitions, probably because there are different Sn concentrations in the sample. In particular, there is a distinct phase with a transition temperature of about 5.5 K. The most prominent nonlinear response appears to be due to the intrinsic Low-field mechanism. The Nb film on Cu shows evidence for superconducting transitions below 9 K, perhaps due to oxides of Nb. For both of the samples, 3rd harmonic response measurement reveals the existence of lower-Tc regions.

Acknowledgement:
This work is funded by US Department of Energy / High Energy Physics through grant # DE-SC0017931 and the Maryland Quantum Materials Center.

Speaker: Chung-Yang Wang

ThinFilmSRF 2021 slide (version 3).pptx

37 DC magnetic field penetration results for Nb thin films with superconducting radiofrequency applications

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, Happ, such that the thicker superconducting layer will see a lower H than on the surface. The screening allows Happ 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 drawbacks, such as early flux penetration due to edge effects as the Happ area is larger than the sample, and H 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 field is applied from one side of the sample using a C-shaped dipole magnet. Hall probes measure Happ and the penetrated field. The largest sample tested is 50×35 mm. Samples are tested at a range of temperatures from as low as 2.8K. So far 3µm and 10µm of Nb on Cu undergoing treatments such as EP, tumbling and SUBU from CERN and INFN have been tested. Various thickness’ of Nb have also been tested. The sample operates in a cryogen free environment.

Speaker: Daniel Turner (Cockcroft Institute/Lancaster University)

TFSRF_D_Turner.pptx

RF and material characterization for SRF films: Discussion

Convener: Tobias Junginger (University of Victoria and TRIUMF)

Worskshop Closeout

Convener: Anne-Marie Valente-Feliciano (Jefferson Lab)