SRF2011 - List of Authors (Cooley, L.D.)

Paper

Title

Page

Development and Scale-Up of an HF Free Electropolishing Process in Single-Cell Niobium SRF Cavities

397

M.E. Inman, H.M. Garich, S.T. Snyder, E.J. Taylor
Faraday Technology, Inc., Clayton, USA
L.D. Cooley, C.A. Cooper, A.M. Rowe
Fermilab, Batavia, USA

The performance of niobium SRF cavities is strongly dependent on a microscopically smooth and clean surface, achieved using buffered chemical polishing or electropolishing, which require a viscous electrolyte containing hydrofluoric acid to achieve niobium oxide breakdown and current distribution control. An ideal polishing process would include: electrolyte free of hydrofluoric acid; control of surface roughness to less than 0.1 micron; surface free from contamination; current distribution control enabling uniform polishing; removal of at least 100 microns. Faraday is working with Fermilab to develop and scale-up the FARADAYIC Electropolishing process to achieve these conditions. FARADAYIC Electropolishing combines pulse reverse electric fields and low viscosity aqueous electrolytes to control current distribution and oxide formation during metal removal. Recent results on coupon polishing will be presented including polishing rates up to 1 micron/min, control of electrolyte temperature to below 20 C, and surface finishes less than 0.2 microns over 4 mm length scales. Construction of a single-cell cavity electropolishing apparatus at Faraday are discussed.

Poster TUPO017 [1.931 MB]

TUPO025

Integrated Cavity Processing Apparatus at Fermilab: SRF Cavity Processing R&D

424

C.A. Cooper, M.S. Champion, L.D. Cooley, V. Poloubotko, O. Pronitchev, A.M. Rowe, M. Wong
Fermilab, Batavia, USA

A center for cavity processing R&D at Fermilab, called the Integrated Cavity Processing Apparatus, is currently in the final stages of installation and commissioning. This facility contains centrifugal barrel polishing, a horizontal electropolishing tool, a 1000°C vacuum furnace, a high pressure rinse tool utilizing ultrapure water, ISO class 4, 5 and 6 clean rooms for cavity assembly work and various other associated pieces of support equipment. All the operations are designed for single cell and nine cell 1.3 GHz Tesla type cavities except for the electropolishing tool which will initially be only for single cell use. Upgrades are currently being examined for single and five cell 650 MHz cavities. The current status of the facility and plans for future work are discussed.

THPO008

Post-Baking Losses in Niobium Cavities Studied by Dissection

710

A. Romanenko, L.D. Cooley, G. Wu
Fermilab, Batavia, USA
G. Ciovati
JLAB, Newport News, Virginia, USA

Thermometry investigations on electropolished cavities, which underwent mild baking, and are limited by a localized quench at 150-200 mT, show that in the absence of the high field Q-slope there are still a few localized sources of dissipation. Identification of these areas along with the high field quench location followed by dissection and surface analysis of the resulting coupons allowed to gain insight into possible mechanisms of these effects, and will be reported in this contribution.

THPO015

Repair SRFCavity by Re-Melting Surface Defects via High Power Laser Technique

740

G.M. Ge
CLASSE, Ithaca, New York, USA
E. Borissov, L.D. Cooley, D.T. Hicks, T.H. Nicol, J.P. Ozelis, J. Ruan, D.A. Sergatskov
Fermilab, Batavia, USA
G. Wu
ANL, Argonne, USA

As the field emission is gradually under control in recent SRF activities, cavity performance is limited by hard quench in the most case. Surface defect has been identified as one of main reasons caused cavity quench scattering cavity accelerating gradient from 12 MV/m to 40 MV/m. Laser processing is able to re-shape the steep flaws to be flat and smooth surface. In Fermilab, a sophisticated laser repair system has been built for 1.3GHz low performance SRF cavity which is limited by surface defect. The pit in a 1.3GHz single-cell cavity was re-melted by high power laser pulse, cavity took 30 μm light Electropolishing after that. The gradient achieved 39MV/m in initial run; after another 30 μm Electropolishing, it achieved 40 MV/m. The improved laser repair system is able to re-melt the surface defect in one meter long 9-cell SRF cavity. It successfully re-melted a pit in 9-cell SRF cavity TB9ACC017.

THPO051

Laser Re-Melting Influence on Nb Properties: Geometrical and Chemical Aspects

846

A.V. Dzyuba, L.D. Cooley, E. Toropov
Fermilab, Batavia, USA
A.V. Dzyuba
NSU, Novosibirsk, Russia
G.M. Ge
CLASSE, Ithaca, New York, USA
G. Wu
ANL, Argonne, USA

We present recent results on Laser re-melting system used to smoothen niobium surfaces of superconducting RF cavities in order to overcome quench. In the work we studied both chemical and geometrical aspects of the melting by means of electron backscattered diffraction microscopy and laser confocal microscopy. BCP, EP and HF impacts have been investigated on both single and large grain niobium samples. Appropriate post processing has been suggested.

THPO060

First Principles Investigation of Hydrogen in Niobium

868

D.C. Ford, L.D. Cooley
Fermilab, Batavia, USA
D.C. Ford
Northwestern University, Evanston, USA
D.N. Seidman
NU, Evanston, Illinois, USA

Niobium hydride is a contributor to degraded niobium SRF cavity performance by Q-slope and Q-disease. Hydrogen is easily absorbed into niobium when the protective oxide layer is disturbed, such as during electropolishing and chemical treatments, and the structure and distribution of hydrogen in niobium is altered during other processing steps such as baking. To optimize cavity performance and production efficiency, it is important to understand the structures of hydrogen in niobium, including the interactions of hydrogen with structural defects and other impurities such as oxygen. In this study density functional theory was used to evaluate these interactions. Hydrogen was examined as a dissolved interstitial impurity and in ordered niobium-hydride phases; and the interactions between hydrogen, niobium, vacancies on niobium sites, and oxygen dissolved in niobium were evaluated. The results yield information about the thermodynamic, electronic, magnetic, and geometric properties of these systems, which lead to important implications concerning the mobilities of impurities and vacancies in niobium and the precipitation of phases that are detrimental to cavity performance.

Poster THPO060 [1.167 MB]

THPO072

Raman Spectroscopy as a Probe of Surface Oxides and Hydrides on Niobium

912

J. Zasadzinski
IIT, Chicago, Illinois, USA
B. Albee, S. Bishnoi, C. Cao
Illinois Institute of Technology, Chicago, IL, USA
G. Ciovati
JLAB, Newport News, Virginia, USA
L.D. Cooley, D.C. Ford
Fermilab, Batavia, USA
Th. Proslier
ANL, Argonne, USA

Funding: ANL, FNAL
Raman microscopy/spectroscopy has been used in conjunction with AFM, tunneling and magnetic susceptibility to identify surface oxides and hydrides on annealed, recrystallized foils of high purity Nb and on single crystals of cavity grade Nb. Cold worked regions of the Nb foil as well as rough regions near grain boundaries showed clear evidence of ordered hydride phases which were identified by VASP phonon calculations. Cold worked regions also displayed enhanced surface paramagnetism. Surface enhanced Raman spectra have also been obtained using 1.0 nm Au depositon. The SERS spectra reveal hydride molecular species which are not observable by conventional Raman. These results indicate that Raman is a useful probe of Nb surfaces relevant for cavity performance

Paper	Title	Page
TUPO017	Development and Scale-Up of an HF Free Electropolishing Process in Single-Cell Niobium SRF Cavities	397
	M.E. Inman, H.M. Garich, S.T. Snyder, E.J. Taylor Faraday Technology, Inc., Clayton, USA L.D. Cooley, C.A. Cooper, A.M. Rowe Fermilab, Batavia, USA
	The performance of niobium SRF cavities is strongly dependent on a microscopically smooth and clean surface, achieved using buffered chemical polishing or electropolishing, which require a viscous electrolyte containing hydrofluoric acid to achieve niobium oxide breakdown and current distribution control. An ideal polishing process would include: electrolyte free of hydrofluoric acid; control of surface roughness to less than 0.1 micron; surface free from contamination; current distribution control enabling uniform polishing; removal of at least 100 microns. Faraday is working with Fermilab to develop and scale-up the FARADAYIC Electropolishing process to achieve these conditions. FARADAYIC Electropolishing combines pulse reverse electric fields and low viscosity aqueous electrolytes to control current distribution and oxide formation during metal removal. Recent results on coupon polishing will be presented including polishing rates up to 1 micron/min, control of electrolyte temperature to below 20 C, and surface finishes less than 0.2 microns over 4 mm length scales. Construction of a single-cell cavity electropolishing apparatus at Faraday are discussed.
	Poster TUPO017 [1.931 MB]

TUPO025	Integrated Cavity Processing Apparatus at Fermilab: SRF Cavity Processing R&D	424
	C.A. Cooper, M.S. Champion, L.D. Cooley, V. Poloubotko, O. Pronitchev, A.M. Rowe, M. Wong Fermilab, Batavia, USA
	A center for cavity processing R&D at Fermilab, called the Integrated Cavity Processing Apparatus, is currently in the final stages of installation and commissioning. This facility contains centrifugal barrel polishing, a horizontal electropolishing tool, a 1000°C vacuum furnace, a high pressure rinse tool utilizing ultrapure water, ISO class 4, 5 and 6 clean rooms for cavity assembly work and various other associated pieces of support equipment. All the operations are designed for single cell and nine cell 1.3 GHz Tesla type cavities except for the electropolishing tool which will initially be only for single cell use. Upgrades are currently being examined for single and five cell 650 MHz cavities. The current status of the facility and plans for future work are discussed.

THPO008	Post-Baking Losses in Niobium Cavities Studied by Dissection	710
	A. Romanenko, L.D. Cooley, G. Wu Fermilab, Batavia, USA G. Ciovati JLAB, Newport News, Virginia, USA
	Thermometry investigations on electropolished cavities, which underwent mild baking, and are limited by a localized quench at 150-200 mT, show that in the absence of the high field Q-slope there are still a few localized sources of dissipation. Identification of these areas along with the high field quench location followed by dissection and surface analysis of the resulting coupons allowed to gain insight into possible mechanisms of these effects, and will be reported in this contribution.

THPO015	Repair SRFCavity by Re-Melting Surface Defects via High Power Laser Technique	740
	G.M. Ge CLASSE, Ithaca, New York, USA E. Borissov, L.D. Cooley, D.T. Hicks, T.H. Nicol, J.P. Ozelis, J. Ruan, D.A. Sergatskov Fermilab, Batavia, USA G. Wu ANL, Argonne, USA
	As the field emission is gradually under control in recent SRF activities, cavity performance is limited by hard quench in the most case. Surface defect has been identified as one of main reasons caused cavity quench scattering cavity accelerating gradient from 12 MV/m to 40 MV/m. Laser processing is able to re-shape the steep flaws to be flat and smooth surface. In Fermilab, a sophisticated laser repair system has been built for 1.3GHz low performance SRF cavity which is limited by surface defect. The pit in a 1.3GHz single-cell cavity was re-melted by high power laser pulse, cavity took 30 μm light Electropolishing after that. The gradient achieved 39MV/m in initial run; after another 30 μm Electropolishing, it achieved 40 MV/m. The improved laser repair system is able to re-melt the surface defect in one meter long 9-cell SRF cavity. It successfully re-melted a pit in 9-cell SRF cavity TB9ACC017.

THPO051	Laser Re-Melting Influence on Nb Properties: Geometrical and Chemical Aspects	846
	A.V. Dzyuba, L.D. Cooley, E. Toropov Fermilab, Batavia, USA A.V. Dzyuba NSU, Novosibirsk, Russia G.M. Ge CLASSE, Ithaca, New York, USA G. Wu ANL, Argonne, USA
	We present recent results on Laser re-melting system used to smoothen niobium surfaces of superconducting RF cavities in order to overcome quench. In the work we studied both chemical and geometrical aspects of the melting by means of electron backscattered diffraction microscopy and laser confocal microscopy. BCP, EP and HF impacts have been investigated on both single and large grain niobium samples. Appropriate post processing has been suggested.

THPO060	First Principles Investigation of Hydrogen in Niobium	868
	D.C. Ford, L.D. Cooley Fermilab, Batavia, USA D.C. Ford Northwestern University, Evanston, USA D.N. Seidman NU, Evanston, Illinois, USA
	Niobium hydride is a contributor to degraded niobium SRF cavity performance by Q-slope and Q-disease. Hydrogen is easily absorbed into niobium when the protective oxide layer is disturbed, such as during electropolishing and chemical treatments, and the structure and distribution of hydrogen in niobium is altered during other processing steps such as baking. To optimize cavity performance and production efficiency, it is important to understand the structures of hydrogen in niobium, including the interactions of hydrogen with structural defects and other impurities such as oxygen. In this study density functional theory was used to evaluate these interactions. Hydrogen was examined as a dissolved interstitial impurity and in ordered niobium-hydride phases; and the interactions between hydrogen, niobium, vacancies on niobium sites, and oxygen dissolved in niobium were evaluated. The results yield information about the thermodynamic, electronic, magnetic, and geometric properties of these systems, which lead to important implications concerning the mobilities of impurities and vacancies in niobium and the precipitation of phases that are detrimental to cavity performance.
	Poster THPO060 [1.167 MB]

THPO072	Raman Spectroscopy as a Probe of Surface Oxides and Hydrides on Niobium	912
	J. Zasadzinski IIT, Chicago, Illinois, USA B. Albee, S. Bishnoi, C. Cao Illinois Institute of Technology, Chicago, IL, USA G. Ciovati JLAB, Newport News, Virginia, USA L.D. Cooley, D.C. Ford Fermilab, Batavia, USA Th. Proslier ANL, Argonne, USA
	Funding: ANL, FNAL Raman microscopy/spectroscopy has been used in conjunction with AFM, tunneling and magnetic susceptibility to identify surface oxides and hydrides on annealed, recrystallized foils of high purity Nb and on single crystals of cavity grade Nb. Cold worked regions of the Nb foil as well as rough regions near grain boundaries showed clear evidence of ordered hydride phases which were identified by VASP phonon calculations. Cold worked regions also displayed enhanced surface paramagnetism. Surface enhanced Raman spectra have also been obtained using 1.0 nm Au depositon. The SERS spectra reveal hydride molecular species which are not observable by conventional Raman. These results indicate that Raman is a useful probe of Nb surfaces relevant for cavity performance