LDRD Template

Thomas jefferson National Accelerator facility

Thomas jefferson National Accelerator facility

Proposal for FY2021 Laboratory Directed Research and Development Funds

Title: QUANTIFICATION OF TUNING LIMITATIONS OF NB3SN COATED CAVITIES

Topic: Addressing R&D issues relevant for new research directions using existing JLab facilities

Lead Scientist or Engineer:

Uttar Pudasaini

Phone:

X7445

Email:

uttar@jlab.org

Date:

05/20/2020

Department/Division:

SRFR&D/Accelerator

Other Personnel:

Charles Reece

Gary Chang

Frank Marhauser

Proposal Term:

From: 10/2020

Through: 09/2021

If continuation, indicate year (2nd/3rd): No

Division Budget Analyst

Annie Ungaro

Phone:

atran@jlab.org

Email:

X5752

ii SLAC Annual Laboratory Plan

This document and the material and data contained herein were developed under the sponsorship of the United States Government. Neither the United States nor the Department of Energy, nor the Thomas Jefferson National Accelerator Facility, nor their employees, makes any warranty, express or implied, or assumes any liability or responsibility for accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use will not infringe privately owned rights. Mention of any product, its manufacturer, or suppliers shall not, nor it is intended to imply approval, disapproval, or fitness for any particular use. A royalty-free, non-exclusive right to use and disseminate same for any purpose whatsoever, is expressly reserved to the United States and the Thomas Jefferson National Accelerator Facility.

2 of 13JLab LDRD Proposal

Abstract

Nb3Sn cavities are capable of operating at 4 K, delivering a similar performance to that of Nb cavities at 2 K, which can significantly simplify cryogenic infrastructure and power requirements. Jefferson Lab has recently fabricated a pair of Nb3Sn-coated CEBAF five-cell cavities with useful gradients for accelerator application, which are aiming to be installed into a cryomodule for the first-ever beam test. It is essential to look into technical challenges toward the practical deployment of this technology for such and potential other small-scale accelerator applications in the future. One such known challenge is the limited tunability of cavities due to the brittle character of Nb3Sn. While this phenomenon is qualitatively well recognized, it has not yet been quantitatively characterized, so specific implications for the useful tuning range of cavities are poorly parameterized. The mechanical vulnerability must be well understood and controlled for the Nb3Sn cavity technology to be useful. The proposed research project aims to be the first-ever quantitative characterization of the tuning limitations of Nb3Sn coated cavities at cryogenic temperatures and a validated FEA model in ways that will be generally useful for the design of future Nb3Sn cavities for accelerator applications.

Summary of ProposalDescription of Project

The proposed program aims to characterize the mechanical vulnerability of Nb3Sn film while tuning a Nb3Sn-coated cavity at operational cryogenic temperatures. Two new single-cell Nb cavities will be acquired for the project. The cavities will then be coated with Nb3Sn film using the vapor diffusion technique that has been in practice at Jefferson Lab. The RF measurements of the cavities will be carried out at 4.2 K with a setup that allows creating incremental axial stress conditions mocking a tuning action, with known loadings suspended to the cavity. The RF characteristics (frequency and Q vs. Eacc curve) are measured at different stresses, while the strain conditions at different locations of the cavity are monitored with cryogenic strain gauges, up to the state at which RF performance severely degrades. Ultimately, this project will use these experimental "mock-tuning" data to calibrate a finite element analysis (FEA) model of the system to quantify strain conditions leading to performance failure. The outcome will benefit future cavity designs and accelerator applications looking forward to using the Nb3Sn coating technology. The proposed research is well-aligned with the FY21 focus area of technology development for the application of particle beams.

Expected Results

The significant portion of the work is expected to provide an essential dataset consisting of RF performance of Nb3Sn coated cavities mocking different tuning ranges, up to the point where the quality of the thin film is harmed to degrade the performance severely. The next potential outcome will be a calibrated FEA model of the system that quantifies material strain conditions interpreting the frequency detuning at which rf-affecting failure occurs. The combination of measurement data and the model should well characterize the suitable tuning range and future design of Nb3Sn cavities for accelerator applications.

Proposal Narrative

Nb3Sn promises better performance and significant reduction of production and operational cost of SRF cavities because both the critical temperature and superheating field of Nb3Sn, Tc ~18.2 K and Hsh ~400 mT, are nearly twice that of niobium [1]. Presently, it is the front running alternative material to replace niobium in SRF accelerator cavities. The potential operation of Nb3Sn SRF cavities at 4 K, delivering a similar performance to that of Nb cavities at 2 K, has excited the accelerator community. These cavities may be operated with atmospheric liquid helium, simplifying and reducing the cost of the cryogenic facilities. The successful deployment of Nb3Sn cavities will be a transformational technology that will bring enormous benefit to numerous SRF accelerator applications. Several research institutions are currently pursuing research on the material to identify and mitigate current limitations on the performance.

Jefferson Lab is one of the leaders pursuing research and development of Nb3Sn cavities since 2012 [2]. It should be noted that the application of Nb3Sn is restricted to a thin-film/coating form because of its brittleness and lower thermal conductivity. The current coating facility at JLab allows the coating of accelerating structures up to the size of the original CEBAF five-cell cavities via the vapor diffusion technique [3]. The continuous R&D program over the past several years focused on producing quality cavity coatings systematically guided by material and RF studies [4-9]. Although several single-cell cavities were initially limited by a precipitous Q-slope, also known as "Wuppertal-slope," the continuous improvement in the coating process has resulted in the disappearance of such slopes [10]. Figure 1 [left] compares the performance of two single-cell cavities, RDT7 and RDT10, between the first and the latest coating. RDT7 had a Q0 of ≥ 2×1010 at 4 K and > 4×1010 at 2 K before quench at ≥ 15 MV/m. The measured value of low field Q0 was 3×1010 at 4 K and 1×1011 at 2 K without any significant Q-slope. We also have produced Nb3Sn-coated CEBAF 5-cell cavities with accelerating gradients in excess of 14 MV/m [7], suitable to be installed in an accelerator cryomodule, see Figure 1 [right]. Since the performance of these cavities is already suitable for some accelerator applications, several projects in different labs are considering Nb3Sn-coated cavities for small scale industrial or environmental accelerators [11-12]. Two CEBAF "C75-shaped" five-cell cavities, Figure 1 [right], are progressing toward a CEBAF type quarter cryomodule for the first-ever beam test in an accelerator environment, utilizing the upgraded injector test facility (UITF) at Jefferson Lab [13].

Figure 1: RF performance of single-cell cavities, RDT7 and RDT10, from the latest coating compared with those from their first coatings, is shown on the left. Test results from two CEBAF five-cell cavities, C75-RI-NbSn1 and C75-RI-NbSn2, are shown on the right.

While recent progress on the development of Nb3Sn cavities is significant, the future of the material can be offset by the relative difficulty in producing a practical accelerating structure, and the limitations set by the brittleness of the material. If the elastic limit of the Nb3Sn film is exceeded, fracture results, compromising the SRF performance. Superconducting properties of the material are known to degrade severely with strain [14-15]. Several Nb3Sn-coated multi-cell cavities were subjected to bench tuning at room temperature. Cold RF measurements before and after tuning indicate performance degradation following the tuning process [16-17]. While this phenomenon is qualitatively well recognized, it has not yet been quantitatively characterized, so specific implications for the useful tuning range of cavities are poorly parameterized. Although several labs have recently demonstrated the feasibility of the fabrication of practical single-cell and multi-cell cavities [7, 18-19], the readiness of the technology for accelerator application needs an adequate understanding of the mechanical sensitivity of the material during the tuning process. The proposed research project aims to characterize the tuning limitations of Nb3Sn coated cavities at cryogenic temperatures in ways that will be generally useful for the design of future Nb3Sn cavities and their functional applications.

Purpose/Goals

The typical tuning-actions from a bench tuner at room temperature are more likely to be localized in particular areas of the cavities. The deterioration of the performance qualitatively demonstrated the mechanical vulnerability to the usual tuning process. The best approach for the future application appears to be careful pre-tuning the cavities before the coating, and "fine-tuning" them at operational cryogenic temperatures. The tuning action from a cold tuner (e.g., standard CEBAF tuner that is fixed in the end cells of a cavity) is expected to be maximally distributed along the whole cavity and provide a limited elastic tuning range with no detrimental effect to the coating.

The purpose of the project is to quantitatively parameterize the tuning limitation of Nb3Sn-coated cavities at operational cryogenic temperature and develop a calibrated FEA model that may directly benefit cavity designs looking forward to adopting Nb3Sn coating technology. The first goal of the proposed research is to determine the useful tuning range for Nb3Sn-coated cavities by producing mock tuning conditions with known loadings at cryogenic temperatures. The second goal is to use those measurements to calibrate an FEA model that quantifies material strain conditions interpreting the frequency detuning at which RF-affecting coating failure occurs. The model that interprets the experimental data linking mechanical deformation distribution, corresponding frequency detuning, and the change in RF performance will be valuable for future cavity designs to enable a robust Nb3Sn cavity technology.

Approach/Methods

The primary goal of the proposed research is to characterize the mechanical vulnerability of typical Nb3Sn-coated cavities during the operational tuning process. The main objectives to achieve this goal are:

· Fabricate and qualify two single-cell Nb3Sn-coated Nb cavities

· Measure tuning limitation by applying incremental known axial loadings while under test at 4 K to the point of destruction – twice for each cavity.

· Develop an FEA model that interprets and predicts strain conditions at different parts of the Nb3Sn coated cavities during the tuning process, and calibrate the FEA model with experimental data and RF simulations to quantify material strain conditions at which failure occurs

While multi-cell cavities like CEBAF-shaped five-cell cavities would ultimately be more realistic for the planned research, we propose to use two 1.3 GHz niobium single-cell cavities to reduce the cost and experimental complexities. These cavities can be produced in house at Jefferson Lab or purchased from outside vendors. Each newly fabricated cavity will receive usual treatments (200 µm EP, 800 °C × 3 h heat treatment, 30 µm final EP, and HPR) before the baseline RF testing. Prior to the Nb3Sn film deposition, the cavities will reach >25 MV/m at 2 K.

Each cavity will be coated using the vapor diffusion technique in the Jefferson Lab Nb3Sn coating deposition system [3]. The coating protocol that resulted in the best performing single-cell cavity will be used [7]. We propose three coatings for each cavity. The first coating will be used to assess the quality of the coating and to refine the process design if necessary. Material analysis of witness samples and optical examination of coated cavities will also characterize the quality of coating prior to RF testing. Two coatings of each cavity will be subjected to RF testing under different dynamic tuning conditions.

The RF measurements of the cavities will be carried out at 4.2 K with a setup that allows creating incremental axial stress conditions with known loadings applied to the cavity. The axial tension produced by the load will mock an operational tuning effect. The RF characteristics (frequency and Q vs. Eacc curve) will be measured at different stresses up to the state at which RF performance severely degrades. The strain condition will be monitored outside the cavity using cryogenic strain gauges fixed at strategic locations of the cavity. Although minor cracks in the coating may not be visible, each cavity will be optically inspected before and after the tuning test for major surface damage. The coatings will then be stripped off the cavity to renormalize the cavity surface, and subjected to a fresh Nb3Sn coating again. The tuning direction will be opposite to that of the cavity encountered in the first test. The experimental data will be a valuable input to calibrate the FEA model describing material strain conditions correlated to specific RF frequency shifts.

An existing VTA top plate will be modified to accommodate a setup to apply tension and compression to coated single-cell cavities during a cold RF test cycle. A mechanical system will be developed to attach and vary loadings at the bottom of the cavity. The system will provide variable axial stress based on the known load suspended to the cavity for a particular measurement, and mock a cavity tuner to probe the dynamic tuning range. The mechanical system consists of a frame capable of tuning the test cavity, a pair of thrust tubes connecting the cavity and frame to the top plate to apply force, and a drive mechanism comprised of a stepper motor with appropriate gear reduction and a drive screw that applies force to the system. The force applied to the cavity will be measured using a commercially available load cell integrated into the thrust tube on the top plate. The displacement of the cavity is calculated using the motor rotation and drive screw pitch. The observed forces and displacements will be used as inputs for an FEA structural model of single-cell cavities to estimate local strains on the cavity's surface.

The design of a cavity typically depends on an FEA simulation for mechanical stability. The FEA model can be used to predict the distribution of strain within a cavity for the given amount of applied stress. The routinely used tool will be used to devise a model to predict strain conditions in different areas of the cavity under the given loading conditions. The model will be calibrated with experimental data from the mock-tuning experiments described above. The model will be further refined with an RF simulation interpreting the degradation of RF performance and frequency shifts.

The progression timeline interdependencies of the main tasks for the proposed research are summarized in Table 1.

Table 1: Progression timeline with main activities

Goals for FY2021

· Quarter 1

· Material procurement

· Cavity Fabrication

· Quarter 2

· Qualification of cavities for Nb3Sn coating

· Nb3Sn coating of cavities

· Quarter 3

· Qualification of Nb3Sn-cavities

· Mechanical system for mock-tuning

· Qualify coated cavities for tuning tests

· First cycle tuning tests

· FEA modeling framework

· Quarter 4

· Renormalize and recoat cavities

· Second cycle tuning test

· FEA modeling and calibration

· Report

Required Resources

All the experimental and computational work will be performed at Jefferson Lab using the following resources:

· Cavity fabrication facility: machine shop, chemistry, e-beam welding, heat treatment furnace, optical inspection system, material characterization facilities, etc.

· Cavity Testing Facility: Chemistry, HPR, clean room, VTA, etc.

· Nb3Sn coating deposition facility.

.

Anticipated Outcomes/Results

The project will quantify the tuning limitations of Nb3Sn-coated cavities at 4.2 K. The frequency shift at which the material fails, leading to a major RF performance degradation will be determined. The FEA model interpreting the experimental data would be another vital outcome that will allow characterizing the strain conditions on Nb3Sn-coated cavities. The future design of a cavity may use this FEA analysis for mechanical stability for a robust technology.

It should be noted that Jefferson Lab is in the process of using Nb3Sn-coated CEBAF five-cell cavities to build a quarter cryomodule, and test it with the beam in UITF, the successful result from the proposed study would be beneficial for similar opportunities in the future. The experimental setup and the model may be upgraded to test CEBAF shape five-cell cavities and/or Nb3Sn coatings produced with alternate coating protocols or techniques.

Budget Explanation

i. The following procurements are expected for the project:

· Two single-cell cavities

· Machine shop fabrication of fixtures and tooling

· Hardware needed for RF testing of each cavity

· Hardware required to build a mechanical system for load suspension and variation

· Cryogenic strain gauges

ii. The proposed budget includes the following labor allocations:

· U. Pudasaini will be involved with fabrication, Nb3Sn coating, RF, and tuning testing of each cavity.

· C. Reece will be advising the project.

· A mechanical engineer will be involved in the design and building a mechanical system to suspend and vary axial stress loadings.

· G. Cheng will be involved in FEA modeling.

· F. Marhauser will be involved in RF simulation.

· Technician support for cavity processing and RF testing, and mechanical system and strain gauge installation.

This proposal seeks a total budget of $156.7k for the one-year research program.

References

[1] H. Padamsee, J. Knobloch, and T. Hays, "RF Superconductivity for Accelerators" John Wiley & Sons," Inc., New York, pp. 199, 1998.

[2] G.V. Eremeev, M.J. Kelley, U. Pudasaini, C.E. Reece, and J. Tuggle, "Progress With Multi-Cell Nb3Sn Cavity Development Linked With Sample Materials Characterization", in Proc. 17th International Conference on RF Superconductivity, Whistler, BC, Canada, 13-18, 2015, paper TUBA05..

[3] U. Pudasaini, G.V. Eremeev, M.J. Kelley, C.E. Reece, and J. Tuggle, G. Ciovati, I. Parajuli, and Md. N. Sayeed "Nb3Sn Multi-cell cavity coating at Jefferson Lab", in Proc. 9th Int. Particle Accelerator Conf. (IPAC'18), Vancouver, Canada (May 2018), paper WEYGBF3.

[4] U. Pudasaini, G.V. Eremeev, C.E. Reece, J. Tuggle, and M.J. Kelley, M.J., 2020. "Analysis of RF losses and material characterization of samples removed from a Nb3Sn-coated superconducting RF cavity". Superconductor Science and Technology, 33(4), p.045012 (2020).

[5] U. Pudasaini, G.V. Eremeev, M.J. Kelley, C.E. Reece, J. Angle, and J. Tuggle, "Growth of Nb3Sn coating in tin vapor-diffusion process" Journal of Vacuum Science & Technology A 37, 051509 (2019).

[6] U. Pudasaini, G.V. Eremeev, C.E. Reece, J. Tuggle, M.J. Kelley "Initial Growth of Tin on Niobium for Vapor Diffusion Coating of Nb3Sn." Superconductor Science and Technology, Volume 32, Number 4 (2019)

[7] U. Pudasaini, G.V. Eremeev, C.E. Reece, and M.J. Kelley "Recent Developments of Nb3Sn at Jefferson Lab for SRF Accelerator Application," in Proc. North American Particle Accelerator conference, Lansing, MI, Sep 2019, paper WEPLM52.

[8] U. Pudasaini, G.V. Eremeev, C.E. Reece, and M.J. Kelley "Insights into Nb3Sn Coating of CEBAF Cavities from Witness Sample Analysis," in Proc. 19th International Conference on RF Superconductivity, Dresden, Germany, July 2019, paper MOP016.

[9] U. Pudasaini, G.V. Eremeev, M.J. Kelley, C.E. Reece, J. Angle, and J. Tuggle, "Nb3Sn Films for SRF Cavities: Genesis and RF Properties," in Proc. 19th International Conference on RF Superconductivity, Dresden, Germany, July 2019, paper THFUA6

[10]U. Pudasaini, G. Ciovati, G.V. Eremeev, C.E. Reeece, M.J. Kelley, I.P. Parajuli, and N. Sayeed "Recent Results From Nb3Sn Single Cell Cavities Coated at Jefferson Lab," in Proc. 19th International Conference on RF Superconductivity, Dresden, Germany, July 2019, paper MOP018.

[11] R. C. Dhuley, S. Posen, M. I. Geelhoed, O. Prokofiev, and J. C. T. Thangaraj. "First demonstration of a cryocooler conduction cooled superconducting radiofrequency cavity operating at practical cw accelerating gradients." Superconductor Science and Technology 33, no. 6 (2020): 06LT01.

[12] G. Ciovati, J. Anderson, B. Coriton, J. Guo, F. Hannon, L. Holland, M. LeSher et al. "Design of a cw, low-energy, high-power superconducting linac for environmental applications." Physical Review Accelerators and Beams 21, no. 9 (2018): 091601.

[13]G.V. Eremeev, K. Macha, U. Pudasaini, C.E. Reece, and A-M. Valente-Feliciano, "Progress Towards Nb3Sn CEBAF Injector Cryomodule", in Proc. 28th Linear Accelerator Conf. (LINAC'16), East-Lansing, MI, USA, Sep. 2016, paper MOPLR024..

[14]M. G. T. Mentink, "An experimental and computational study of strain sensitivity in superconducting Nb3Sn", Faculty of Science and Technology, University of Twente, Netherlands, Ph.D. dissertation, 2014.

[15]A. Godeke. "A review of the properties of Nb3Sn and their variation with A15 composition, morphology and strain state." Superconductor Science and Technology 19.8 (2006): R68.

[16] G.V. Eremeev and U. Pudasaini, "Development of Nb₃Sn Multicell Cavity Coatings," in Proc. IPAC'19, Melbourne, Australia, May 2019, pp. 3070-3073. doi:10.18429/JACoW-IPAC2019-WEPRB111.

[17] G.V. Eremeev, W. Crahen, J. Henry, F. Marhauser, C.E. Reece, and U. Pudasaini, "RF Performance Sensitivity to Tuning of Nb3Sn Coated CEBAF Cavities," in Proc. 19th International Conference on RF Superconductivity, Dresden, Germany, July 2019, paper MOP015.

[18] S. Posen, O. S. Melnychuk, A. S. Romanenko, D. A. Sergatskov, B. Tennis, and J. Lee, "Nb3Sn SRF Cavity Development at Fermilab", presented at the 19th Int. Conf. RF Superconductivity (SRF'19), Dresden, Germany, Jun.-Jul. 2019, paper MOP042.

[19] R. D. Porter et al., "Next Generation Nb3Sn SRF Cavities for Linear Accelerators", in Proc. 29th Linear Accelerator Conf. (LINAC'18), Beijing, China, Sep. 2018, pp. 462-465. doi:10.18429/JACoW-LINAC2018-TUPO055.

3 of 13JLab LDRD Proposal

12 of 13JLab LDRD Proposal

P

ROPOSAL

FOR

FY2021

L

ABORATORY

D

I

RECTED

R

ESEARCH AND

D

EVELOPMENT

F

UNDS

T

ITLE

:

QUANTIFICATION

OF

TUNING

LIMITATIONS

OF

NB

3

SN

COATED

CAVITIES

T

OPIC

:

A

DDRESSING

R&D

ISSUES RELEVANT

FOR NEW RESEARCH

DIRECTIONS USING EXI

STING

JL

AB

FACILITIES

LEAD SCIENTIST

OR

ENGINEER

:

UTTAR PUDASAINI

Phone:

X7445

Email:

uttar@jlab.org

Date:

05/20

/2020

Department/Division

:

SRFR&D/Accelerator

Other Personnel

:

Charles Reece

Gary Chang

Frank

Marhauser

Proposal Term

:

From:

10/2020

Through:

09/2021

If continuation

,

indicate year (2

nd

/3

rd

)

: No

Division

B

udget Analyst

Annie Ungaro

Phone:

atran@jlab.org

Email:

X5752

PROPOSAL FOR FY2021 LABORATORY DIRECTED RESEARCH AND

DEVELOPMENT FUNDS

TITLE: QUANTIFICATION OF TUNING LIMITATIONS OF NB

3

SN

COATED CAVITIES

TOPIC: ADDRESSING R&D ISSUES RELEVANT FOR NEW RESEARCH

DIRECTIONS USING EXISTING JLAB FACILITIES

LEAD SCIENTIST OR

ENGINEER:

UTTAR PUDASAINI

Phone:

X7445

Email:

uttar@jlab.org

Date:

05/20/2020

Department/Division:

SRFR&D/Accelerator

Other Personnel:

Charles Reece

Gary Chang

Frank Marhauser

Proposal Term:

From: 10/2020

Through: 09/2021

If continuation, indicate year (2

nd

/3

rd

): No

Division Budget Analyst

Annie Ungaro

Phone:

atran@jlab.org

Email:

X5752