SRF2011 - List of Authors (Kelley, M.J.)

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).

THPO073

Laser Melt Smoothing of Niobium Superconducting Radio-Frequency Cavity Surfaces

917

S. Singaravelu, M.J. Kelley, J.M. Klopf, G.A. Krafft
JLAB, Newport News, Virginia, USA
C. Xu
The College of William and Mary, Williamsburg, USA

Superconducting Radio Frequency (SRF) niobium cavities are at the heart of an increasing number of particle accelerators. Their performance is dominated by a several nm thick layer at the interior surface. Maximizing the smoothness of this surface is critical and aggressive chemical treatments are now employed to this end. We describe laser-induced surface melting as an alternative “greener” approach. Modeling predicts the surface temperature as a function of per-pulse energy density. Guided selection of laser parameters achieves melting that reduces the surface roughness and may also mitigate surface damage from the fabrication process. The resulting topography was examined by SEM, and AFM. PSD spectra computed from AFM data were used for studying the topography of the treated niobium.

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.

THPO048

RF Surface Impedance of MgB2 Thin Films at 7.5 GHz

838

B. Xiao, M.J. Kelley
The College of William and Mary, Williamsburg, USA
H.L. Phillips, C.E. Reece
JLAB, Newport News, Virginia, USA
T. Tajima
LANL, Los Alamos, New Mexico, USA

Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177
The Surface Impedance Characterization (SIC) system in Jefferson Lab can presently make direct calorimetric RF surface impedance measurements on the central 0.8 cm² area of 5 cm diameter disk samples from 2 to 20 K exposed to RF magnetic fields up to 14 mT at 7.5 GHz. MgB2 thin films from STI/LANL were deposited on 5 cm diameter Nb disks using reactive evaporation technique. We will report the results of measurements on these samples using the SIC system. The data will be interpreted based on BCS theory as the temperature-dependent properties suggest evaluation of the Tc, energy gap, penetration depth, mean free path and coherence length.

Poster THPO048 [0.352 MB]

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).

THPO073	Laser Melt Smoothing of Niobium Superconducting Radio-Frequency Cavity Surfaces	917
	S. Singaravelu, M.J. Kelley, J.M. Klopf, G.A. Krafft JLAB, Newport News, Virginia, USA C. Xu The College of William and Mary, Williamsburg, USA
	Superconducting Radio Frequency (SRF) niobium cavities are at the heart of an increasing number of particle accelerators. Their performance is dominated by a several nm thick layer at the interior surface. Maximizing the smoothness of this surface is critical and aggressive chemical treatments are now employed to this end. We describe laser-induced surface melting as an alternative “greener” approach. Modeling predicts the surface temperature as a function of per-pulse energy density. Guided selection of laser parameters achieves melting that reduces the surface roughness and may also mitigate surface damage from the fabrication process. The resulting topography was examined by SEM, and AFM. PSD spectra computed from AFM data were used for studying the topography of the treated niobium.

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.

THPO048	RF Surface Impedance of MgB2 Thin Films at 7.5 GHz	838
	B. Xiao, M.J. Kelley The College of William and Mary, Williamsburg, USA H.L. Phillips, C.E. Reece JLAB, Newport News, Virginia, USA T. Tajima LANL, Los Alamos, New Mexico, USA
	Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177 The Surface Impedance Characterization (SIC) system in Jefferson Lab can presently make direct calorimetric RF surface impedance measurements on the central 0.8 cm² area of 5 cm diameter disk samples from 2 to 20 K exposed to RF magnetic fields up to 14 mT at 7.5 GHz. MgB2 thin films from STI/LANL were deposited on 5 cm diameter Nb disks using reactive evaporation technique. We will report the results of measurements on these samples using the SIC system. The data will be interpreted based on BCS theory as the temperature-dependent properties suggest evaluation of the Tc, energy gap, penetration depth, mean free path and coherence length.
	Poster THPO048 [0.352 MB]