SRF2019 - List of Authors (Liepe, M.)

Paper	Title	Page
MOP010	Ab Initio Calculations on the Growth and Superconducting Properties of Nb₃Sn	39
SUSP023	use link to see paper's listing under its alternate paper code
	N. Sitaraman, T. Arias, P. Cueva, M.M. Kelley, D.A. Muller Cornell University, Ithaca, New York, USA J.M. Carlson, A.R. Pack, M.K. Transtrum Brigham Young University, Provo, 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 research was funded by the Center for Bright Beams. In this work, we employ theoretical ab initio techniques to solve mysteries and gain new insights in Nb₃Sn SRF physics. We determine the temperature dependence of Nb₃Sn antisite defect formation energies, and discuss the implications of these results for defect segregation. We calculate the phonon spectral function for Nb₃Sn cells with different combinations of antisite defects and use these results to determine Tc as a function of stoichiometry. These results allow for the first-ever determination of Tc in the tin-rich regime, where experimental measurements are unavailable and which is critical to understanding the impact of tin-rich grain boundaries on superconducting cavity performance. Finally, we propose a theory for the growth mechanism of Nb₃Sn growth on a thick oxide, explaining the puzzling disappearing droplet behavior of Sn on Nb oxide and suggesting how in general an oxide layer reacts with Sn to produce a uniform Nb₃Sn layer.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP010
About •	paper received ※ 02 July 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019
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MOP011	High Frequency Nb₃Sn Cavities	44
SUSP020	use link to see paper's listing under its alternate paper code
	R.D. Porter, M. Liepe, J.T. Maniscalco Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Niobium-3 Tin (Nb₃Sn) is an alternative material to Nb for SRF cavities. This material is capable of higher temperature operation and has high theoretical maximum accelerating gradients. Cornell University is a leader in the development of this material for SRF applications, and current Nb₃Sn 1.3 GHz single cells produced at Cornell achieve quality factors above 10^zEhNZeHn at 4.2 K at medium fields, far above what can be reached with niobium. Most of the recent Nb₃Sn cavity development has been done at 1.3 GHz. In this paper, we present new results from Nb₃Sn cavities at 2.6 GHz and 3.9 GHz. We compare relative cavity performance and flux trapping sensitivities, and extract frequency dependencies. Results show that the frequency can be increased without degrading the performance of the cavities, opening the path towards a new generation of compact and efficient SRF cavities for a wide range of future applications.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP011
About •	paper received ※ 05 July 2019 paper accepted ※ 12 July 2019 issue date ※ 14 August 2019
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MOP013	Reducing Surface Roughness of Nb₃Sn Through Chemical Polishing Treatments	48
	H. Hu, M. Liepe, R.D. Porter Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Niobium-3 tin (Nb₃Sn) is a promising alternative material for SRF cavities, with theoretical limits for critical temperatures and superheating fields reaching twice that of conventional Nb cavities. However, currently achievable accelerating gradients in Nb₃Sn cavities are much lower than their theoretical limit. One limitation to the maximum accelerating gradient is surface magnetic field enhancement caused by the surface roughness of Nb₃Sn. However, there are currently no standard techniques used to reduce Nb₃Sn surface roughness. Since Nb₃Sn is only 2-3 microns thick, it is difficult to selectively polish Nb₃Sn without removing the entire layer. Here, we investigate reducing the surface roughness of Nb₃Sn through applying chemical polishing treatments, including modified versions of standard techniques such as Buffered Chemical Polishing (BCP) and Electropolishing (EP). Through data acquired from Atomic Force Microscope (AFM) scans, SEM scans, and SEM-EDS analysis, we show the effects of these chemical treatments in reducing surface roughness and consider the changes in the chemical composition of Nb₃Sn that may occur through the etching process. We find that BCP with a 1:1:8 solution is ineffective while EP halves the surface roughness of Nb₃Sn.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP013
About •	paper received ※ 01 July 2019 paper accepted ※ 04 July 2019 issue date ※ 14 August 2019
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MOP014	Electroplating of Sn Film on Nb Substrate for Generating Nb₃Sn Thin Films and Post Laser Annealing	51
SUSP036	use link to see paper's listing under its alternate paper code
	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, A.B. Connolly, J.M. Scholtz, N. Sitaraman, M.O. Thompson Cornell University, Ithaca, USA X. Deng University of Virginia, Charlottesville, Virginia, USA K.D. Dobson University of Delaware, Newark, Delaware, USA
	Controlling film quality of Nb₃Sn is critical to its SRF cavity performance. The state-of-the-art vapor diffusion approach for Nb₃Sn deposition observed surface roughness, thin grain regions, and misfit dislocations which negatively affect the RF performance. The Sn deficiency and non-uniformity at the nucleation stage of vapor deposition is believed to be the fundamental reason to cause these roughness and defects issues. Thus, we propose to pre-deposit a uniform Sn film on the Nb substrate, which is able to provide sufficient Sn source during the following heat treatment for Nb₃Sn nucleation and growth. Here, we demonstrated successful electrodeposition of a low-roughness, dendrite-free, excellent-adhesion Sn film on the Nb substrate. More importantly, we further achieved a uniform, low-roughness (Ra = 66 nm), pure-stoichiometric Nb₃Sn film through thermal treatment of this electroplated Sn film in the furnace. Additionally, we provide preliminary results of laser annealing as a post treatment for epitaxial grain growth and roughness reduction.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP014
About •	paper received ※ 22 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019
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MOP032	Effect of Low Temperature Infusion Heat Treatments and "2/0" Doping on Superconducting Cavity Performance	118
	P.N. Koufalis, M. Ge, M. Liepe, J.T. Maniscalco Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Under specific circumstances, low temperature infusion heat treatments of niobium cavities have resulted in the ubiquitous "Q-rise". This is an increase in quality factor with increasing field strength or equivalently a decrease in the temperature-dependent component of the surface resistance. We investigate the results of various infusion conditions with infusion bake time as a free parameter. To study the very near surface effects of infusion, we employ HF rinsing, light VEP, and oxypolishing to remove several or tens of nm at a time. We present results from RF performance tests of low temperature infusion heat treated niobium cavities, and correlate these with SIMS impurity depth profiles obtained from witness samples. We also present results of a cavity doped at 800 C with the "2/0" recipe.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP032
About •	paper received ※ 26 June 2019 paper accepted ※ 02 July 2019 issue date ※ 14 August 2019
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MOP045	The LCLS-II HE High Q and Gradient R&D Program	154
	D. Gonnella, S. Aderhold, A. Burrill, G.R. Hays, T.O. Raubenheimer, M.C. Ross SLAC, Menlo Park, California, USA D. Bafia, M. Checchin, A. Grassellino, M. Martinello, A.S. Romanenko Fermilab, Batavia, Illinois, USA M. Ge, M. Liepe, S. Posen Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA A.D. Palczewski, C.E. Reece JLab, Newport News, Virginia, USA
	Funding: US DOE and the LCLS-II HE Project The LCLS-II HE project is a high energy upgrade to the superconducting LCLS-II linac. It consists of adding twenty additional 1.3 GHz cryomodules to the linac, with cavities operating at a gradient of 20.8 MV/m with a Q₀ of 2.7·10¹⁰. Performance of LCLS-II cryomodules has suggested that operations at this high of a gradient will not be achievable with the existing cavity recipe employed. Therefore a research program was developed between SLAC, Fermilab, Thomas Jefferson National Accelerator Facility, and Cornell University in order to improve the cavity processing method of the SRF cavities and reach the HE goals. This program explores the doping regime beyond what was done for LCLS-II and also has looked to further developed nitrogen-infusion. Here we will summarize the results from this R\&D program, showing significant improvement on both single-cell and 9-cell cavities compared with the original LCLS-II cavity recipe.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP045
About •	paper received ※ 25 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019
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TUFUA1	The Field-Dependent Surface Resistance of Doped Niobium: New Experimental and Theoretical Results	340
	J.T. Maniscalco, M. Ge, P.N. Koufalis, M. Liepe Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA T. Arias, D. Liarte, J.P. Sethna, N. Sitaraman Cornell University, Ithaca, New York, USA
	We present systematic work investigating how different doping and post-doping treatments affect the BCS surface resistance at 1.3~GHz and higher frequencies. We examine the field-dependent BCS resistance at many temperatures as well as the field-dependent residual resistance and use the results to reveal how impurity species and concentration levels affect the field-dependent RF properties. We further demonstrate the importance of thermal effects and their direct dependence on doping level. We use the tools of Density Functional Theory to work towards an {\em ab initio} model of electron overheating to theoretically confirm the impact of doping, create a full model that includes thermal effects to predict the field dependent resistance, and show that the predictions of the model agree with results from doped and non-doped cavities ({\em e.g.} the strength of the anti-Q-slope and the high-field Q slope). Finally, we use our experimental results to systematically assess and compare theories of the field-dependent BCS resistance, showing that the current theory on smearing of the density of states is incomplete.
	Slides TUFUA1 [6.780 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUFUA1
About •	paper received ※ 01 July 2019 paper accepted ※ 02 July 2019 issue date ※ 14 August 2019
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TUFUB8	CVD Coated Copper Substrate SRF Cavity Research at Cornell University	381
	M. Ge, T. Gruber, J.J. Kaufman, M. Liepe, J.T. Maniscalco, T.E. Oseroff, R.D. Porter, Z. Sun Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA V.M. Arrieta, S.R. McNeal Ultramet, Pacoima, California, USA
	Chemical vapor deposition (CVD) is a promising alternative to conventional sputter techniques for coating copper substrate cavities with high-quality superconducting films. Through multiple SRF-related DOE SBIR projects, Ultramet has developed CVD processes and CVD reactor designs for SRF cavities, and Cornell University has conducted extensive RF testing of CVD coated surfaces. Here we report results from thin-film CVD Nb₃Sn coated copper test plates, and for thick-film CVD niobium on copper including full-scale single cell 1.3 GHz copper substrate cavities. Detailed optical inspection and surface characterization show high-quality and well-adhered coatings. No copper contamination is found. The Nb₃Sn coated plates have a uniform Nb₃Sn coating with a slightly low tin concentration (19 -22%), but a BCS resistance well in agreement with predictions. The CVD Nb coatings on copper plates demonstrate excellent adhesion characteristics and exceeded surface fields of 50 mT without showing signs of a strong Q-slope that is frequently observed in sputtered Nb cavities. Multiple single-cell 1.3 GHz copper cavities have been coated to date at Ultramet, and results from RF testing of these are presented and discussed.
	Slides TUFUB8 [12.488 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUFUB8
About •	paper received ※ 01 July 2019 paper accepted ※ 05 July 2019 issue date ※ 14 August 2019
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TUP045	Ab Initio Calculations on Impurity Doped Niobium and Niobium Surfaces	523
	N. Sitaraman, T. Arias Cornell University, Ithaca, New York, USA R.G. Farber, S.J. Sibener, R.D. Veit The University of Chicago, Chicago, Illinois, USA M. Liepe, J.T. Maniscalco Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: This work was funded by the Center for Bright Beams We develop and apply new tools to understand Nb surface chemistry and fundamental electronic processes using theoretical ab initio methods. We study the thermodynamics of impurities and hydrides in the near-surface region as well as their effect on the surface band gap. This makes it possible for experimentalists to relate changes in STM dI/dV measurements resulting from different preparations to changes in subsurface structure. We also calculate matrix elements for electron-impurity scattering in Nb for common impurities O, N, C, and H. By transforming these matrix elements into a Wannier function basis, we calculate lifetimes for a dense set of states on the Fermi surface and determine the mean free path as a function of impurity density. This technique can be generalized to calculate other scattering amplitudes and timescales relevant to SRF theory.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP045
About •	paper received ※ 02 July 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019
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TUP051	Progress Towards Commissioning the Cornell DC Field Dependence Cavity	543
SUSP014	use link to see paper's listing under its alternate paper code
	J.T. Maniscalco, T. Gruber, A.T. Holic, M. Liepe Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	The Cornell DC Field Dependence Cavity is a new coaxial test resonator designed to study the impact of strong (up to 200 mT or more) DC surface magnetic fields on the superconducting surface resistance, providing physical insight into the root of the ‘‘anti-Q-slope’’ and probing critical fields. In this report we report progress in the commissioning of this new apparatus, including finalized design elements and results of prototype tests.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP051
About •	paper received ※ 25 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019
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WETEB6	Active Suppression of Microphonics Detuning in High QL Cavities	776
	N. Banerjee, G.H. Hoffstaetter, M. Liepe, P. Quigley Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: This work was performed through the support of New York State Energy Research and Development Agency (NYSERDA). SRF 2019 provided travel support in the form of a student grant. Accelerators operating with low beam loading such as Energy Recovery Linacs (ERL) greatly benefit from using SRF cavities operated at high loaded quality factors, since it leads to lower RF power requirements. However, large microphonics detuning several times the operating bandwidth of the cavities severely limit the maximum accelerating fields which can be sustained in a stable manner. In this talk, I will describe an active microphonics control technique based on the narrow band Active noise Control (ANC) algorithm which we have used in CBETA, a multi-turn SRF ERL being commissioned at Cornell University. I will describe its stability and performance during beam operations of CBETA with consistent reduction of peak detuning by almost a factor of 2 on multiple cavities.
	Slides WETEB6 [10.296 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-WETEB6
About •	paper received ※ 23 June 2019 paper accepted ※ 29 June 2019 issue date ※ 14 August 2019
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THP023	RF Commissioning of the CBETA Main Linac Cryomodule	881
SUSP017	use link to see paper's listing under its alternate paper code
	N. Banerjee, J. Dobbins, G.H. Hoffstaetter, R.P.K. Kaplan, M. Liepe, C.W. Miller, P. Quigley, E.N. Smith, V. Veshcherevich Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: This work was performed through the support of New York State Energy Research and Development Agency (NYSERDA). The Cornell BNL ERL Test Accelerator (CBETA) employs a superconducting Main Linac Cryomodule in order to perform multi-turn energy recovery operation. Optimizing the field stability of the low bandwidth SRF cavities in the presence of microphonics with limited available RF power is a challenging task. Despite of this, the Main Linac Cryomodule has been successfully used in CBETA to impart a maximum energy gain of 54 MeV, well above the energy gain requirement of CBETA. In this paper, we present an overview of our RF commissioning procedure including automatic coarse tuning, measurement of DAC and phase offsets. We further detail our microphonics measurements from our most recent run period.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP023
About •	paper received ※ 23 June 2019 paper accepted ※ 29 June 2019 issue date ※ 14 August 2019
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THP044	RF Characterization of Novel Superconducting Materials and Multilayers	950
SUSP021	use link to see paper's listing under its alternate paper code
	T.E. Oseroff, M. Liepe, Z. Sun Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA B. Moeckly STI, Santa Barbara, California, USA M.J. Sowa Veeco-CNT, Medford, USA
	Cutting edge SRF technology is likely approaching the fundamental limitations of niobium cavities operating in the Meissner state. This combined with the obvious advantages of using higher critical temperature superconductors and thin film depositions leads to interest in the RF characterization of such materials. A TE mode niobium sample host cavity was used to characterize the RF performance of 5" (12.7 cm) diameter sample plates as a function of field and temperature at 4 GHz. Materials studied include MgB₂ and thin film atomic layer deposition (ALD) NbN and NbTiN on Nb substrates. These higher critical temperature superconductors all having coherence lengths on the order of a few nm. It is therefore likely that defects on the order of the coherence lengths will cause early flux penetration well before the theorized superheating field of an ideal superconducting surface. Superconductor-insulator-superconductor (SIS) multilayers have been proposed as a mechanism of arresting these early penetration flux avalanches and are therefore studied here as well, using the same NbN and NbTiN films, but over thin layers of insulating AlN on Nb substrates.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP044
About •	paper received ※ 02 July 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019
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THP045	Improvements to the Cornell Sample Host System	956
	T.E. Oseroff, M. Liepe Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	RF characterization of arbitrary superconducting samples has been of interest for many years but, due to the experimental complexities, has never been achieved to its full potential. A TE mode niobium sample host cavity has been used at Cornell to characterize the RF performance of 5" (12.7 cm) diameter sample plates. It was designed and built in 2012 – 2013 and since then has encountered a range of problems. The focus of this work is to highlight these and to present solutions to assist future researchers hoping to design novel RF characterization instruments. Topics covered include coupler design, cryostat support structure, sample preparation, and a discussion of potential systematic errors introduced by the data extraction and calibration methods applied to this device.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP045
About •	paper received ※ 01 July 2019 paper accepted ※ 01 July 2019 issue date ※ 14 August 2019
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THP046	Magnetic Field Mapping System for Cornell Sample Host Cavity	961
	S.N. Lobo, M. Liepe, T.E. Oseroff Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: Cornell Laboratory for Accelerator-Based Sciences and Education Dissipation due to flux trapping is a persistent problem experienced in SRF cavity testing and cryomodule operation. This work addresses accurately and cheaply measuring magnetic fields in a cryostat without using delicate and expensive fluxgate magnetometers. Anisotropic Magnetoresistive (AMR) magnetic field sensors were investigated for the detection of small fields in a cryogenic environment. Initial development of instrumentation using 16 AMR sensors is presented for the purpose of measuring magnetic fields perpendicular the normal of a 5" diameter sample plate on the Cornell sample host cavity.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP046
About •	paper received ※ 29 June 2019 paper accepted ※ 01 July 2019 issue date ※ 14 August 2019
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Paper

Title

Page

MOP010

Ab Initio Calculations on the Growth and Superconducting Properties of Nb₃Sn

39

SUSP023

use link to see paper's listing under its alternate paper code

N. Sitaraman, T. Arias, P. Cueva, M.M. Kelley, D.A. Muller
Cornell University, Ithaca, New York, USA
J.M. Carlson, A.R. Pack, M.K. Transtrum
Brigham Young University, Provo, 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 research was funded by the Center for Bright Beams.
In this work, we employ theoretical ab initio techniques to solve mysteries and gain new insights in Nb₃Sn SRF physics. We determine the temperature dependence of Nb₃Sn antisite defect formation energies, and discuss the implications of these results for defect segregation. We calculate the phonon spectral function for Nb₃Sn cells with different combinations of antisite defects and use these results to determine Tc as a function of stoichiometry. These results allow for the first-ever determination of Tc in the tin-rich regime, where experimental measurements are unavailable and which is critical to understanding the impact of tin-rich grain boundaries on superconducting cavity performance. Finally, we propose a theory for the growth mechanism of Nb₃Sn growth on a thick oxide, explaining the puzzling disappearing droplet behavior of Sn on Nb oxide and suggesting how in general an oxide layer reacts with Sn to produce a uniform Nb₃Sn layer.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP010

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paper received ※ 02 July 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019

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MOP011

High Frequency Nb₃Sn Cavities

44

SUSP020

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R.D. Porter, M. Liepe, J.T. Maniscalco
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Niobium-3 Tin (Nb₃Sn) is an alternative material to Nb for SRF cavities. This material is capable of higher temperature operation and has high theoretical maximum accelerating gradients. Cornell University is a leader in the development of this material for SRF applications, and current Nb₃Sn 1.3 GHz single cells produced at Cornell achieve quality factors above 10^zEhNZeHn at 4.2 K at medium fields, far above what can be reached with niobium. Most of the recent Nb₃Sn cavity development has been done at 1.3 GHz. In this paper, we present new results from Nb₃Sn cavities at 2.6 GHz and 3.9 GHz. We compare relative cavity performance and flux trapping sensitivities, and extract frequency dependencies. Results show that the frequency can be increased without degrading the performance of the cavities, opening the path towards a new generation of compact and efficient SRF cavities for a wide range of future applications.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP011

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paper received ※ 05 July 2019 paper accepted ※ 12 July 2019 issue date ※ 14 August 2019

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MOP013

Reducing Surface Roughness of Nb₃Sn Through Chemical Polishing Treatments

48

H. Hu, M. Liepe, R.D. Porter
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Niobium-3 tin (Nb₃Sn) is a promising alternative material for SRF cavities, with theoretical limits for critical temperatures and superheating fields reaching twice that of conventional Nb cavities. However, currently achievable accelerating gradients in Nb₃Sn cavities are much lower than their theoretical limit. One limitation to the maximum accelerating gradient is surface magnetic field enhancement caused by the surface roughness of Nb₃Sn. However, there are currently no standard techniques used to reduce Nb₃Sn surface roughness. Since Nb₃Sn is only 2-3 microns thick, it is difficult to selectively polish Nb₃Sn without removing the entire layer. Here, we investigate reducing the surface roughness of Nb₃Sn through applying chemical polishing treatments, including modified versions of standard techniques such as Buffered Chemical Polishing (BCP) and Electropolishing (EP). Through data acquired from Atomic Force Microscope (AFM) scans, SEM scans, and SEM-EDS analysis, we show the effects of these chemical treatments in reducing surface roughness and consider the changes in the chemical composition of Nb₃Sn that may occur through the etching process. We find that BCP with a 1:1:8 solution is ineffective while EP halves the surface roughness of Nb₃Sn.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP013

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paper received ※ 01 July 2019 paper accepted ※ 04 July 2019 issue date ※ 14 August 2019

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MOP014

Electroplating of Sn Film on Nb Substrate for Generating Nb₃Sn Thin Films and Post Laser Annealing

51

SUSP036

use link to see paper's listing under its alternate paper code

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, A.B. Connolly, J.M. Scholtz, N. Sitaraman, M.O. Thompson
Cornell University, Ithaca, USA
X. Deng
University of Virginia, Charlottesville, Virginia, USA
K.D. Dobson
University of Delaware, Newark, Delaware, USA

Controlling film quality of Nb₃Sn is critical to its SRF cavity performance. The state-of-the-art vapor diffusion approach for Nb₃Sn deposition observed surface roughness, thin grain regions, and misfit dislocations which negatively affect the RF performance. The Sn deficiency and non-uniformity at the nucleation stage of vapor deposition is believed to be the fundamental reason to cause these roughness and defects issues. Thus, we propose to pre-deposit a uniform Sn film on the Nb substrate, which is able to provide sufficient Sn source during the following heat treatment for Nb₃Sn nucleation and growth. Here, we demonstrated successful electrodeposition of a low-roughness, dendrite-free, excellent-adhesion Sn film on the Nb substrate. More importantly, we further achieved a uniform, low-roughness (Ra = 66 nm), pure-stoichiometric Nb₃Sn film through thermal treatment of this electroplated Sn film in the furnace. Additionally, we provide preliminary results of laser annealing as a post treatment for epitaxial grain growth and roughness reduction.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP014

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paper received ※ 22 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019

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MOP032

Effect of Low Temperature Infusion Heat Treatments and "2/0" Doping on Superconducting Cavity Performance

118

P.N. Koufalis, M. Ge, M. Liepe, J.T. Maniscalco
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Under specific circumstances, low temperature infusion heat treatments of niobium cavities have resulted in the ubiquitous "Q-rise". This is an increase in quality factor with increasing field strength or equivalently a decrease in the temperature-dependent component of the surface resistance. We investigate the results of various infusion conditions with infusion bake time as a free parameter. To study the very near surface effects of infusion, we employ HF rinsing, light VEP, and oxypolishing to remove several or tens of nm at a time. We present results from RF performance tests of low temperature infusion heat treated niobium cavities, and correlate these with SIMS impurity depth profiles obtained from witness samples. We also present results of a cavity doped at 800 C with the "2/0" recipe.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP032

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paper received ※ 26 June 2019 paper accepted ※ 02 July 2019 issue date ※ 14 August 2019

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MOP045

The LCLS-II HE High Q and Gradient R&D Program

154

D. Gonnella, S. Aderhold, A. Burrill, G.R. Hays, T.O. Raubenheimer, M.C. Ross
SLAC, Menlo Park, California, USA
D. Bafia, M. Checchin, A. Grassellino, M. Martinello, A.S. Romanenko
Fermilab, Batavia, Illinois, USA
M. Ge, M. Liepe, S. Posen
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
A.D. Palczewski, C.E. Reece
JLab, Newport News, Virginia, USA

Funding: US DOE and the LCLS-II HE Project
The LCLS-II HE project is a high energy upgrade to the superconducting LCLS-II linac. It consists of adding twenty additional 1.3 GHz cryomodules to the linac, with cavities operating at a gradient of 20.8 MV/m with a Q₀ of 2.7·10¹⁰. Performance of LCLS-II cryomodules has suggested that operations at this high of a gradient will not be achievable with the existing cavity recipe employed. Therefore a research program was developed between SLAC, Fermilab, Thomas Jefferson National Accelerator Facility, and Cornell University in order to improve the cavity processing method of the SRF cavities and reach the HE goals. This program explores the doping regime beyond what was done for LCLS-II and also has looked to further developed nitrogen-infusion. Here we will summarize the results from this R\&D program, showing significant improvement on both single-cell and 9-cell cavities compared with the original LCLS-II cavity recipe.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP045

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paper received ※ 25 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019

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TUFUA1

The Field-Dependent Surface Resistance of Doped Niobium: New Experimental and Theoretical Results

340

J.T. Maniscalco, M. Ge, P.N. Koufalis, M. Liepe
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
T. Arias, D. Liarte, J.P. Sethna, N. Sitaraman
Cornell University, Ithaca, New York, USA

We present systematic work investigating how different doping and post-doping treatments affect the BCS surface resistance at 1.3~GHz and higher frequencies. We examine the field-dependent BCS resistance at many temperatures as well as the field-dependent residual resistance and use the results to reveal how impurity species and concentration levels affect the field-dependent RF properties. We further demonstrate the importance of thermal effects and their direct dependence on doping level. We use the tools of Density Functional Theory to work towards an {\em ab initio} model of electron overheating to theoretically confirm the impact of doping, create a full model that includes thermal effects to predict the field dependent resistance, and show that the predictions of the model agree with results from doped and non-doped cavities ({\em e.g.} the strength of the anti-Q-slope and the high-field Q slope). Finally, we use our experimental results to systematically assess and compare theories of the field-dependent BCS resistance, showing that the current theory on smearing of the density of states is incomplete.

Slides TUFUA1 [6.780 MB]

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUFUA1

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paper received ※ 01 July 2019 paper accepted ※ 02 July 2019 issue date ※ 14 August 2019

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TUFUB8

CVD Coated Copper Substrate SRF Cavity Research at Cornell University

381

M. Ge, T. Gruber, J.J. Kaufman, M. Liepe, J.T. Maniscalco, T.E. Oseroff, R.D. Porter, Z. Sun
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
V.M. Arrieta, S.R. McNeal
Ultramet, Pacoima, California, USA

Chemical vapor deposition (CVD) is a promising alternative to conventional sputter techniques for coating copper substrate cavities with high-quality superconducting films. Through multiple SRF-related DOE SBIR projects, Ultramet has developed CVD processes and CVD reactor designs for SRF cavities, and Cornell University has conducted extensive RF testing of CVD coated surfaces. Here we report results from thin-film CVD Nb₃Sn coated copper test plates, and for thick-film CVD niobium on copper including full-scale single cell 1.3 GHz copper substrate cavities. Detailed optical inspection and surface characterization show high-quality and well-adhered coatings. No copper contamination is found. The Nb₃Sn coated plates have a uniform Nb₃Sn coating with a slightly low tin concentration (19 -22%), but a BCS resistance well in agreement with predictions. The CVD Nb coatings on copper plates demonstrate excellent adhesion characteristics and exceeded surface fields of 50 mT without showing signs of a strong Q-slope that is frequently observed in sputtered Nb cavities. Multiple single-cell 1.3 GHz copper cavities have been coated to date at Ultramet, and results from RF testing of these are presented and discussed.

Slides TUFUB8 [12.488 MB]

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUFUB8

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paper received ※ 01 July 2019 paper accepted ※ 05 July 2019 issue date ※ 14 August 2019

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TUP045

Ab Initio Calculations on Impurity Doped Niobium and Niobium Surfaces

523

N. Sitaraman, T. Arias
Cornell University, Ithaca, New York, USA
R.G. Farber, S.J. Sibener, R.D. Veit
The University of Chicago, Chicago, Illinois, USA
M. Liepe, J.T. Maniscalco
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: This work was funded by the Center for Bright Beams
We develop and apply new tools to understand Nb surface chemistry and fundamental electronic processes using theoretical ab initio methods. We study the thermodynamics of impurities and hydrides in the near-surface region as well as their effect on the surface band gap. This makes it possible for experimentalists to relate changes in STM dI/dV measurements resulting from different preparations to changes in subsurface structure. We also calculate matrix elements for electron-impurity scattering in Nb for common impurities O, N, C, and H. By transforming these matrix elements into a Wannier function basis, we calculate lifetimes for a dense set of states on the Fermi surface and determine the mean free path as a function of impurity density. This technique can be generalized to calculate other scattering amplitudes and timescales relevant to SRF theory.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP045

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paper received ※ 02 July 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019

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TUP051

Progress Towards Commissioning the Cornell DC Field Dependence Cavity

543

SUSP014

use link to see paper's listing under its alternate paper code

J.T. Maniscalco, T. Gruber, A.T. Holic, M. Liepe
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

The Cornell DC Field Dependence Cavity is a new coaxial test resonator designed to study the impact of strong (up to 200 mT or more) DC surface magnetic fields on the superconducting surface resistance, providing physical insight into the root of the ‘‘anti-Q-slope’’ and probing critical fields. In this report we report progress in the commissioning of this new apparatus, including finalized design elements and results of prototype tests.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP051

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paper received ※ 25 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019

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WETEB6

Active Suppression of Microphonics Detuning in High QL Cavities

776

N. Banerjee, G.H. Hoffstaetter, M. Liepe, P. Quigley
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: This work was performed through the support of New York State Energy Research and Development Agency (NYSERDA). SRF 2019 provided travel support in the form of a student grant.
Accelerators operating with low beam loading such as Energy Recovery Linacs (ERL) greatly benefit from using SRF cavities operated at high loaded quality factors, since it leads to lower RF power requirements. However, large microphonics detuning several times the operating bandwidth of the cavities severely limit the maximum accelerating fields which can be sustained in a stable manner. In this talk, I will describe an active microphonics control technique based on the narrow band Active noise Control (ANC) algorithm which we have used in CBETA, a multi-turn SRF ERL being commissioned at Cornell University. I will describe its stability and performance during beam operations of CBETA with consistent reduction of peak detuning by almost a factor of 2 on multiple cavities.

Slides WETEB6 [10.296 MB]

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-WETEB6

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paper received ※ 23 June 2019 paper accepted ※ 29 June 2019 issue date ※ 14 August 2019

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THP023

RF Commissioning of the CBETA Main Linac Cryomodule

881

SUSP017

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N. Banerjee, J. Dobbins, G.H. Hoffstaetter, R.P.K. Kaplan, M. Liepe, C.W. Miller, P. Quigley, E.N. Smith, V. Veshcherevich
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: This work was performed through the support of New York State Energy Research and Development Agency (NYSERDA).
The Cornell BNL ERL Test Accelerator (CBETA) employs a superconducting Main Linac Cryomodule in order to perform multi-turn energy recovery operation. Optimizing the field stability of the low bandwidth SRF cavities in the presence of microphonics with limited available RF power is a challenging task. Despite of this, the Main Linac Cryomodule has been successfully used in CBETA to impart a maximum energy gain of 54 MeV, well above the energy gain requirement of CBETA. In this paper, we present an overview of our RF commissioning procedure including automatic coarse tuning, measurement of DAC and phase offsets. We further detail our microphonics measurements from our most recent run period.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP023

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paper received ※ 23 June 2019 paper accepted ※ 29 June 2019 issue date ※ 14 August 2019

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THP044

RF Characterization of Novel Superconducting Materials and Multilayers

950

SUSP021

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T.E. Oseroff, M. Liepe, Z. Sun
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
B. Moeckly
STI, Santa Barbara, California, USA
M.J. Sowa
Veeco-CNT, Medford, USA

Cutting edge SRF technology is likely approaching the fundamental limitations of niobium cavities operating in the Meissner state. This combined with the obvious advantages of using higher critical temperature superconductors and thin film depositions leads to interest in the RF characterization of such materials. A TE mode niobium sample host cavity was used to characterize the RF performance of 5" (12.7 cm) diameter sample plates as a function of field and temperature at 4 GHz. Materials studied include MgB₂ and thin film atomic layer deposition (ALD) NbN and NbTiN on Nb substrates. These higher critical temperature superconductors all having coherence lengths on the order of a few nm. It is therefore likely that defects on the order of the coherence lengths will cause early flux penetration well before the theorized superheating field of an ideal superconducting surface. Superconductor-insulator-superconductor (SIS) multilayers have been proposed as a mechanism of arresting these early penetration flux avalanches and are therefore studied here as well, using the same NbN and NbTiN films, but over thin layers of insulating AlN on Nb substrates.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP044

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paper received ※ 02 July 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019

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THP045

Improvements to the Cornell Sample Host System

956

T.E. Oseroff, M. Liepe
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

RF characterization of arbitrary superconducting samples has been of interest for many years but, due to the experimental complexities, has never been achieved to its full potential. A TE mode niobium sample host cavity has been used at Cornell to characterize the RF performance of 5" (12.7 cm) diameter sample plates. It was designed and built in 2012 – 2013 and since then has encountered a range of problems. The focus of this work is to highlight these and to present solutions to assist future researchers hoping to design novel RF characterization instruments. Topics covered include coupler design, cryostat support structure, sample preparation, and a discussion of potential systematic errors introduced by the data extraction and calibration methods applied to this device.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP045

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paper received ※ 01 July 2019 paper accepted ※ 01 July 2019 issue date ※ 14 August 2019

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THP046

Magnetic Field Mapping System for Cornell Sample Host Cavity

961

S.N. Lobo, M. Liepe, T.E. Oseroff
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: Cornell Laboratory for Accelerator-Based Sciences and Education
Dissipation due to flux trapping is a persistent problem experienced in SRF cavity testing and cryomodule operation. This work addresses accurately and cheaply measuring magnetic fields in a cryostat without using delicate and expensive fluxgate magnetometers. Anisotropic Magnetoresistive (AMR) magnetic field sensors were investigated for the detection of small fields in a cryogenic environment. Initial development of instrumentation using 16 AMR sensors is presented for the purpose of measuring magnetic fields perpendicular the normal of a 5" diameter sample plate on the Cornell sample host cavity.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP046

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paper received ※ 29 June 2019 paper accepted ※ 01 July 2019 issue date ※ 14 August 2019

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