Author: Sun, Z.
Paper Title Page
SUPFDV009 Thermal Annealing of Sputtered Nb3Sn and V3Si Thin Films for Superconducting RF Cavities 82
 
  • K. Howard, M. Liepe, Z. Sun
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
 
  Funding: U.S. National Science Foundation under Award PHY-1549132, the Center for Bright Beams and Cornell Center for Materials Research Shared Facilities supported through the NSF MRSEC program (DMR-1719875)
Nb3Sn and V3Si thin films are alternative material candidates for the next-generation of superconducting radio frequency (SRF) cavities. However, past sputtered films suffer from stoichiometry and strain issues during deposition and post annealing. As such, we aim to explore the structural and chemical effects of thermal annealing, both in-situ and post-sputtering, on DC-sputtered Nb3Sn and V3Si with varying thickness on Nb or Cu substrates. We successfully enabled recrystallization of 100 nm thin Nb3Sn films with stoichiometric and strain-free grains at 950 C annealing. For 2 um films, we observed removal of strain and slight increase in grain size with increasing temperature. A phase transformation from unstable to stable structure appeared on thick V3Si samples, while we observed significant Sn loss in thick Nb3Sn films at high temperature anneals. For films on Cu substrates, we observed similar Sn and Si loss during annealing likely due to Cu-Sn and Cu-Si phase generation and subsequent Sn and Si evaporation. These results encourage us to refine our process to obtain high quality films for SRF use.
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2021-SUPFDV009  
About • Received ※ 22 June 2021 — Revised ※ 06 July 2021 — Accepted ※ 12 August 2021 — Issue date ※ 17 March 2022
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SUPTEV007 Development of a System for Coating SRF Cavities Using Remote Plasma CVD 129
 
  • G. Gaitan, P. Bishop, A.T. Holic, G. Kulina, M. Liepe, J. Sears, Z. Sun
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
 
  Funding: This work was supported by the National Science Foundation under Grant No. PHY-1549132.
Next-generation, thin-film surfaces employing Nb3Sn, NbN, NbTiN, and other compound superconductors are destined to allow reaching superior RF performance levels in SRF cavities. Optimized, advanced deposition processes are required to enable high-quality films of such materials on large and complex-shaped cavities. For this purpose, Cornell University is developing a remote plasma-enhanced chemical vapor deposition (CVD) system that facilitates coating on complicated geometries with a high deposition rate. This system is based on a high-temperature tube furnace with a clean vacuum and furnace loading system. The use of plasma alongside reacting precursors will significantly reduce the required processing temperature and promote precursor decomposition. A vacuum quality monitor (VQM) is used to characterize the residual gases before coating. The CVD system has been designed and is currently under assembly and commissioning.
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2021-SUPTEV007  
About • Received ※ 09 July 2021 — Accepted ※ 21 August 2021 — Issue date; ※ 10 February 2022  
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SUPTEV008 CW Operation of Conduction-Cooled Nb3Sn SRF Cavity 133
 
  • N.A. Stilin, A.T. Holic, M. Liepe, R.D. Porter, J. Sears, Z. Sun
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
 
  Significant progress in the performance of SRF cavities coated with Nb3Sn films during the last few years has provided an energy efficient alternative to traditional Nb cavities, thereby initiating a fundamental shift in SRF technology. These Nb3Sn cavities can operate at significantly higher temperatures than Nb cavities while simultaneously requiring less cooling power. This allows for the use of new cryogenic cooling schemes based on conduction cooling with robust, commercialized turn-key style cryocoolers. Cornell University has developed and tested a 2.6 GHz Nb3Sn cavity assembly which utilizes such cooling methods. These tests have demonstrated stable RF operation at 10 MV/m with measured thermal dynamics which match numerical simulations. These results also serve as a foundation for designing a new standalone SRF cryomodule which will use a pair of cryocoolers to cool a 1.3 GHz Nb3Sn cavity with an input coupler capable of supporting high beam current operation.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2021-SUPTEV008  
About • Received ※ 22 June 2021 — Accepted ※ 13 August 2021 — Issue date; ※ 08 November 2021  
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TUOFDV07 Sample Test Systems for Next-Gen SRF Surfaces 357
 
  • T.E. Oseroff, M. Liepe, Z. Sun
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
 
  With the increasing worldwide focus on the development of new surfaces for SRF cavities, exploring alternative materials and multilayer structures, test systems that allow measuring the RF performance of simple sample geometries (e.g., flat samples) become increasingly essential. These systems provide RF performance results that are needed to guide the development of these surfaces. This contribution gives an overview of sample test systems currently available, including the improved Cornell sample host cavity. Recent advances in this important technology, performance specifications, and current limitations are discussed. In addition, an overview is given of interesting recent RF performance results on samples coated with non-niobium bulk and multilayer films.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2021-TUOFDV07  
About • Received ※ 08 July 2021 — Accepted ※ 21 August 2021 — Issue date; ※ 05 September 2021  
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TUPFDV010 New Recipes to Optimize the Niobium Oxide Surface From First-Principles Calculations 426
 
  • N. Sitaraman, T. Arias, Z. Baraissov, M.M. Kelley, D.A. Muller
    Cornell University, Ithaca, New York, USA
  • M. Liepe, R.D. Porter, Z. Sun
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
 
  Funding: This work was supported by the U.S. National Science Foundation under Award No. PHY-1549132, the Center for Bright Beams
The properties of niobium oxide are of critical importance for a wide range of topics, from the behavior of nitrogen during infusion treatments, to the nucleation of Nb3Sn, to the superconducting properties of the surface. However, the modeling of the oxide is often much simplified, ignoring the variety of niobium oxide phases and the extremely different properties of these phases in the presence of impurities and defects. We use density functional theory (DFT) to investigate how electrochemical treatments and gas infusion procedures change the properties of niobium oxide, and to investigate how these properties could be optimized for Nb3Sn nucleation and for niobium SRF performance.
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2021-TUPFDV010  
About • Received ※ 01 July 2021 — Accepted ※ 18 November 2021 — Issue date; ※ 22 February 2022  
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WEPTEV012 Characterization of Atomic-Layer-Deposited NbTiN and NbTiN/AlN Films for SIS Multilayer Structures 662
 
  • Z. Sun, M. Liepe, T.E. Oseroff
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
  • X. Deng
    University of Virginia, Charlottesville, Virginia, USA
 
  SIS (superconductor-insulator-superconductor) mul-tilayer structures are proposed designs to repel early flux penetration and ease the impact of defects in SRF cavities. The demonstration of such device physics is strongly affected by the film qualities ’ material struc-ture and composition. Here, we characterized 100 nm NbTiN / 2 nm AlN / bulk Nb SIS structures and investigated the effect of the presence of the AlN layer on the NbTiN film properties. We find that the hcp-structured AlN layer results in a Nb composition gra-dient as a function of film depth, whereas the Nb con-centration remains constant in the NbTiN/Nb samples, which suggests that interface mismatch could induce significant change in NbTiN composition. The surface composition variation further leads to different oxide structures, which might impact the superconducting performance. Our observations indicate that the choice of the insulating layer in SIS structures is critical, and that interface mismatch together with internal strain could deteriorate the superconducting film.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2021-WEPTEV012  
About • Received ※ 08 July 2021 — Revised ※ 06 August 2021 — Accepted ※ 22 November 2021 — Issue date ※ 02 January 2022
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WEOTEV03 Toward Stoichiometric and Low-Surface-Roughness Nb3Sn Thin Films via Direct Electrochemical Deposition 710
 
  • Z. Sun, G. Gaitan, M. Ge, K. Howard, M. Liepe, T.E. Oseroff, R.D. Porter
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
  • T. Arias, Z. Baraissov, M.M. Kelley, D.A. Muller, J.P. Sethna, N. Sitaraman
    Cornell University, Ithaca, New York, USA
  • K.D. Dobson
    University of Delaware, Newark, Delaware, USA
 
  Reducing surface roughness and attaining stoichiometry of Nb3Sn superconducting films are required to push their superheating field to the theoretical limit in SRF cavities. As such, we explore direct electrochemical processes that minimize involving foreign elements to deposit high-quality Sn, Nb, and NbxSn films on Nb and Cu surfaces. These films are then thermally annealed to Nb3Sn. We find that smooth Sn pre-depositions via electroplating on Nb surfaces significantly reduce the average roughness of resultant Nb3Sn to 65 nm, with a dramatic reduction in power intensity at medium special frequencies. Structural and superconducting properties demonstrate a Nb3Sn A15 phase with a stoichiometry of 25 at% Sn. This process is being scaled-up to a 3.9 GHz cavity. Moreover, preliminary results on electroplating on Cu surface show that Nb plating undergoes a slow growth rate while subsequent Sn plating on the plated Nb surface can be controlled with varied thickness. The Nb plating process is currently being optimized.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2021-WEOTEV03  
About • Received ※ 09 July 2021 — Revised ※ 09 August 2021 — Accepted ※ 21 August 2021 — Issue date ※ 16 January 2022
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THPTEV004 Surface Oxides on Nb and Nb3Sn Surfaces: Toward a Deeper Understanding 836
 
  • Z. Sun, M. Liepe, T.E. Oseroff, R.D. Porter
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
  • T. Arias, Z. Baraissov, D.A. Muller, N. Sitaraman
    Cornell University, Ithaca, New York, USA
  • C. Dukes
    University of Virginia, Charlottesville, Virginia, USA
  • D. Johnson-McDaniel, M. Salim
    CCMR, Ithaca, New York, USA
 
  Surface oxides on Nb and Nb3Sn SRF cavities, as a thin ’dirty’ layer, could be critical to their performance as suggested by recent theory. Although these oxides have been studied in the past, we intend here to provide a deeper understanding based on a systematic study on coupon samples that have been processed under the different conditions currently used in SRF cavity treatments. Our aim is to obtain a more complete picture of the oxide evolution. This then might help to explain the observed cavity performance variation, and might allow designing a process to achieve a designed, optimized surface with controlled oxides types and thickness. We find that the surface oxides are in amorphous phase that exhibits normal conducting behaviors, while the pentoxide further degrades with time. Also, we observed a thin hydroxide layer on the outermost surface and possibly Nb(OH)x motifs in the bulk. Moreover, distinctive oxide structures were found in Nb3Sn samples from vapor diffusion, electroplating, and sputtering. The semiconducting SnOx appeared through the oxide depth in vapor diffused Nb3Sn, while a ~1 nm SnOx layer merely exists at the outermost surface of electroplated Nb3Sn.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2021-THPTEV004  
About • Received ※ 09 July 2021 — Revised ※ 11 August 2021 — Accepted ※ 21 August 2021 — Issue date ※ 04 November 2021
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