SRF2011 - List of Authors (Tian, H.)

Paper

Title

Page

Sulfur Residues in Niobium Electropolishing

421

L. Zhao
The College of William and Mary, Williamsburg, USA
M.J. Kelley, C.E. Reece, H. Tian
JLAB, Newport News, Virginia, USA

Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
Electropolishing (EP) in sulfuric/hydrofluoric acid mixtures affords significantly greater surface smoothness than the incumbent buffered chemical polishing (BCP), making it attractive as the future baseline technology for SRF cavity manufacture. However, reported observations of particulate sulfur residues raise concern. One hypothesis is sulfate reduction to elemental sulfur at the cathode, where the measured potential drop is thermodynamically sufficient. Alternatively, the low effectiveness of the cathode’s aluminum oxide surface as a hydrogen recombination catalyst could lead to accumulation of atomic hydrogen, a powerful reductant. We explored these possibilities under standard EP conditions in a small three-electrode laboratory cell. We varied aluminum cathode area to obtain different current densities (and thus overpotentials) at constant cell current. We substituted platinum, an excellent hydrogen recombination catalyst, for aluminum in some experiments. Surface of cathodes were examined with Scanning Electron Microscope (SEM). Surface composition was analyzed by Energy Dispersive X-Ray Spectroscopy (EDS) and X-Ray Photoelectron Spectroscopy (XPS).

WEIOA01

Quantitative EP Studies and Results for SRF Nb Cavity Production

565

H. Tian, C.E. Reece
JLAB, Newport News, Virginia, USA

To achieve high performance and reliability, which is essential for Nb SRF cavities production, it is important for us to understand Nb EP in detail so that we can tailor it to the best effect. The analytical tools of electrochemistry and surface topography are the means of developing such understanding. In this talk, the recent incorporation of analytic electrochemical techniques into the development of well controlled protocols for Nb EP will be reported, such as using three electrode method for polarization curve measurements, EIS to understand the mechanism of EP, and RDE to study the diffusion coefficient of active species of F ions, and the related diffusion layers etc. In parallel, investigations for monitoring scale-dependent Nb surface morphology evolution under different Nb EP parameters, which is expected to lead to matrix a best EP parameter and have a predictive power to describe the Nb surface evolution during EP will be demonstrated. Early lessons learned that are relevant to Nb cavities will be introduced, and directions for the future are aimed at well control, high reproducibility, efficient, and geometry independent EP process for coming SRF-based projects.

Slides WEIOA01 [2.129 MB]

THPO038

Detailed Nb Surface Morphology Evolution During Electropolishing for SRF Cavity Production

802

H. Tian, C.E. Reece
JLAB, Newport News, Virginia, USA
S.R. Brankovic, N. Dole
University of Houston, Houston TX, USA

Electropolishing is currently an important part of attaining the best performance of SRF cavities. We endeavor to develop sufficient understanding of the process dynamics to gain predictive power over specific topographies subjected to controlled electropolishing conditions. This work examines the evolution of highly reproducible Nb surface morphology produced by centrifugal barrel polishing of fine-grain and single-crystal material as this material is electropolished under different well-controlled conditions. The morphology evolution of Nb surface has been described using a combined approach of scaling analysis and predictions of the electropolishing theory. Our results shows that electropolishing at low temperature helps to smooth out the surface feature scales within the diffusion layer and to reduce the kinetics-controlled surface etching. A preliminary computational model has been developed that simulates the evolution of specific topography of a niobium surface under parametrized conditions. This work is expected to lead to the direct linking of starting surface morphology specification, specific processing protocol, and consistently attained finished surface condition.

Poster THPO038 [0.895 MB]

THPO046

Characterization of Scale-Dependent Roughness of Niobium Surfaces as a Function of Surface Treatment Processes

832

C. Xu, M.J. Kelley
The College of William and Mary, Williamsburg, USA
C.E. Reece, H. Tian
JLAB, Newport News, Virginia, USA

Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
Micro-roughness is attributed to be a critical issue for realizing optimum performance of Superconducting Radio Frequency (SRF) cavities. Several surface processing methods such as chemical, mechanical and plasma, are used to obtain relatively smooth surfaces. Among those process methods, Buffered Chemical Polish (BCP) and Electro-Polishing (EP) are most commonly used in current niobium cavity production. The Power Spectral Density (PSD) of surface height data provides a more thorough description to the topography than a simple Rq (RMS) measurement and reveals useful information including fractal and superstructure contributions. Polishing duration and temperature can have predictable effects on the evolution of such features at different scale regions in PSD spectrum. 1 dimensional average PSD functions derived from morphologies of niobium surfaces treated by BCP and EP with different controlled starting conditions and durations have been fitted with a combination of fractal, K-correlation and shifted Gaussian models, to extract characteristic parameters at different spatial harmonic scales.

THPO071

Detailed Surface Analysis of Incremental Centrifugal Barrel Polishing (CBP) of Single-Crystal Niobium Samples

908

A.D. Palczewski, C.E. Reece, H. Tian, O. Trofimova
JLAB, Newport News, Virginia, USA

Funding: This work is authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
We performed Centrifugal Barrel Polishing (CBP) on single crystal niobium samples housed in a stainless steel sample holder following a polishing recipe recently developed at FNAL [*]. We were able to obtain a mirror-like finish after the final stage of tumbling, although some defects and imbedded particles remain. Our presentation will discuss the initial results from the coupon study, including qualitative and quantitative analysis of the surface characteristics from each step in the CBP process, followed by HPR and well controlled incremental EP. These will include surface roughness, size and character of contaminants, surface crystal structure, and overall finish. We will discuss how the surface characteristics should guide the SRF community in exploiting or adapting the Fermi recipe; including why minimal subsequent EP is needed, and possible places for modification of the recipe to reduce polishing time.
* CA Cooper, LD Cooley , “Mirror Smooth Superconducting RF Cavities by MechanicalPolishing with Minimal Acid Use,” http://lss.fnal.gov/archive/2011/pub/fermilab-pub-11-032-td.pdf, (May 31, 2011)

Paper	Title	Page
TUPO024	Sulfur Residues in Niobium Electropolishing	421
	L. Zhao The College of William and Mary, Williamsburg, USA M.J. Kelley, C.E. Reece, H. Tian JLAB, Newport News, Virginia, USA
	Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. Electropolishing (EP) in sulfuric/hydrofluoric acid mixtures affords significantly greater surface smoothness than the incumbent buffered chemical polishing (BCP), making it attractive as the future baseline technology for SRF cavity manufacture. However, reported observations of particulate sulfur residues raise concern. One hypothesis is sulfate reduction to elemental sulfur at the cathode, where the measured potential drop is thermodynamically sufficient. Alternatively, the low effectiveness of the cathode’s aluminum oxide surface as a hydrogen recombination catalyst could lead to accumulation of atomic hydrogen, a powerful reductant. We explored these possibilities under standard EP conditions in a small three-electrode laboratory cell. We varied aluminum cathode area to obtain different current densities (and thus overpotentials) at constant cell current. We substituted platinum, an excellent hydrogen recombination catalyst, for aluminum in some experiments. Surface of cathodes were examined with Scanning Electron Microscope (SEM). Surface composition was analyzed by Energy Dispersive X-Ray Spectroscopy (EDS) and X-Ray Photoelectron Spectroscopy (XPS).

WEIOA01	Quantitative EP Studies and Results for SRF Nb Cavity Production	565
	H. Tian, C.E. Reece JLAB, Newport News, Virginia, USA
	To achieve high performance and reliability, which is essential for Nb SRF cavities production, it is important for us to understand Nb EP in detail so that we can tailor it to the best effect. The analytical tools of electrochemistry and surface topography are the means of developing such understanding. In this talk, the recent incorporation of analytic electrochemical techniques into the development of well controlled protocols for Nb EP will be reported, such as using three electrode method for polarization curve measurements, EIS to understand the mechanism of EP, and RDE to study the diffusion coefficient of active species of F ions, and the related diffusion layers etc. In parallel, investigations for monitoring scale-dependent Nb surface morphology evolution under different Nb EP parameters, which is expected to lead to matrix a best EP parameter and have a predictive power to describe the Nb surface evolution during EP will be demonstrated. Early lessons learned that are relevant to Nb cavities will be introduced, and directions for the future are aimed at well control, high reproducibility, efficient, and geometry independent EP process for coming SRF-based projects.
	Slides WEIOA01 [2.129 MB]

THPO038	Detailed Nb Surface Morphology Evolution During Electropolishing for SRF Cavity Production	802
	H. Tian, C.E. Reece JLAB, Newport News, Virginia, USA S.R. Brankovic, N. Dole University of Houston, Houston TX, USA
	Electropolishing is currently an important part of attaining the best performance of SRF cavities. We endeavor to develop sufficient understanding of the process dynamics to gain predictive power over specific topographies subjected to controlled electropolishing conditions. This work examines the evolution of highly reproducible Nb surface morphology produced by centrifugal barrel polishing of fine-grain and single-crystal material as this material is electropolished under different well-controlled conditions. The morphology evolution of Nb surface has been described using a combined approach of scaling analysis and predictions of the electropolishing theory. Our results shows that electropolishing at low temperature helps to smooth out the surface feature scales within the diffusion layer and to reduce the kinetics-controlled surface etching. A preliminary computational model has been developed that simulates the evolution of specific topography of a niobium surface under parametrized conditions. This work is expected to lead to the direct linking of starting surface morphology specification, specific processing protocol, and consistently attained finished surface condition.
	Poster THPO038 [0.895 MB]

THPO046	Characterization of Scale-Dependent Roughness of Niobium Surfaces as a Function of Surface Treatment Processes	832
	C. Xu, M.J. Kelley The College of William and Mary, Williamsburg, USA C.E. Reece, H. Tian JLAB, Newport News, Virginia, USA
	Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. Micro-roughness is attributed to be a critical issue for realizing optimum performance of Superconducting Radio Frequency (SRF) cavities. Several surface processing methods such as chemical, mechanical and plasma, are used to obtain relatively smooth surfaces. Among those process methods, Buffered Chemical Polish (BCP) and Electro-Polishing (EP) are most commonly used in current niobium cavity production. The Power Spectral Density (PSD) of surface height data provides a more thorough description to the topography than a simple Rq (RMS) measurement and reveals useful information including fractal and superstructure contributions. Polishing duration and temperature can have predictable effects on the evolution of such features at different scale regions in PSD spectrum. 1 dimensional average PSD functions derived from morphologies of niobium surfaces treated by BCP and EP with different controlled starting conditions and durations have been fitted with a combination of fractal, K-correlation and shifted Gaussian models, to extract characteristic parameters at different spatial harmonic scales.

THPO071	Detailed Surface Analysis of Incremental Centrifugal Barrel Polishing (CBP) of Single-Crystal Niobium Samples	908
	A.D. Palczewski, C.E. Reece, H. Tian, O. Trofimova JLAB, Newport News, Virginia, USA
	Funding: This work is authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. We performed Centrifugal Barrel Polishing (CBP) on single crystal niobium samples housed in a stainless steel sample holder following a polishing recipe recently developed at FNAL []. We were able to obtain a mirror-like finish after the final stage of tumbling, although some defects and imbedded particles remain. Our presentation will discuss the initial results from the coupon study, including qualitative and quantitative analysis of the surface characteristics from each step in the CBP process, followed by HPR and well controlled incremental EP. These will include surface roughness, size and character of contaminants, surface crystal structure, and overall finish. We will discuss how the surface characteristics should guide the SRF community in exploiting or adapting the Fermi recipe; including why minimal subsequent EP is needed, and possible places for modification of the recipe to reduce polishing time. CA Cooper, LD Cooley , “Mirror Smooth Superconducting RF Cavities by MechanicalPolishing with Minimal Acid Use,” http://lss.fnal.gov/archive/2011/pub/fermilab-pub-11-032-td.pdf, (May 31, 2011)