SRF2011 - List of Authors (Ciovati, G.)

Paper

Title

Page

Summary of the Symposium on Ingot Nb and New Results on Fundamental Studies of Large Grain Nb

319

G. Ciovati, P. Dhakal, R. Myneni
JLAB, Newport News, Virginia, USA

Funding: This manuscript has been authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177
The First International Symposium on the Superconducting Science and Technology of Ingot Niobium was held at Jefferson Lab in September 2010. Significant activities are taking place at laboratories and universities throughout the world to address several aspects related to the science and technology of Ingot Nb: from ingot production to mechanical, thermal and superconducting properties. A summary of the results presented at the Symposium is given in this contribution. New results on the superconducting properties and interstitial impurities content measured in large-grain Nb samples and cavities are briefly highlighted.

TUPO051

High-Temperature Heat Treatment Study on a Large-Grain Nb Cavity

508

G. Ciovati, P. Dhakal, R. Myneni
JLAB, Newport News, Virginia, USA
P. Maheshwari, F.A. Stevie
NCSU AIF, Raleigh, North Carolina, USA

Funding: This manuscript has been authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177
Improvement of the cavity performance by a high-temperature heat-treatment without subsequent chemical etching have been reported for large-grain Nb cavities treated by buffered chemical polishing, as well as for a fine-grain cavity treated by vertical electropolishing [1]. Changes in the quality factor, Q0, and maximum peak surface magnetic field achieved in a large-grain Nb single-cell cavity have been determined as a function of the heat treatment temperature, between 600 °C and 1200 °C. The highest Q0 improvement of about 30% was obtained after heat-treatment at 800 °C-1000 °C. Measurements by secondary ion mass spectrometry on large-grain samples heat-treated with the cavity showed large reduction of hydrogen concentration after heat treatment.
[1] G. Ciovati, G. Myneni, F. Stevie, P. Maheshwari, and D. Griffis, Phys. Rev. ST Accel. Beams 13, 022002 (2010)

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.

THPO016

Preliminary Results on the Laser Heating Investigation of Hotspots in a Large-Grain Nb Cavity

745

G. Ciovati, C. Baldwin, G. Cheng, R. J. Flood, K. Jordan, P. Kneisel, M.L. Morrone, L. Turlington, K.M. Wilson, S. Zhang
JLAB, Newport News, Virginia, USA
S. M. Anlage
UMD, College Park, Maryland, USA
A.V. Gurevich
Old Dominion University, Norfolk, Virginia, USA
G. Nemes
Astigmat, Santa Clara, USA

Magnetic vortices pinned near the inner surface of SRF Nb cavities are a possible source of RF hotspots, frequently observed by temperature mapping of the cavities outer surface at RF surface magnetic fields of about 100 mT. Theoretically, we expect that the thermal gradient provided by a 10 W green laser shining on the inner cavity surface at the RF hotspot locations can move pinned vortices to different pinning locations. The experimental apparatus to send the beam onto the inner surface of a photoinjector-type large-grain Nb cavity is described. Preliminary results on the changes in thermal maps observed after applying the laser heating are also reported.

Poster THPO016 [0.983 MB]

THPO028

SIMS and TEM Analysis of Niobium Bicrystals

776

P. Maheshwari, A.D. Batchelor, D.P. Griffis, F.A. Stevie, C. Zhou
NCSU AIF, Raleigh, North Carolina, USA
G. Ciovati, R. Myneni, J.K. Spradlin
JLAB, Newport News, Virginia, USA
M. Rigsbee
Materials Science and Engineering, Raleigh, USA

The behaviour of interstitial impurities(C,O,N,H) on the Nb surface with respect to grain boundaries may affect cavity performance. Large grain Nb makes possible the selection of bicrystal samples with a well defined grain boundary. In this work, Dynamic SIMS was used to analyze two Nb bicrystal samples, one of them heat treated and the other non heat treated (control). H levels were found to be higher for the non heat treated sample and a difference in the H intensity and sputtering rate was also observed across the grain boundary for both the samples. TEM results showed that the bicrystal interface showed no discontinuity and the oxide layer was uniform across the grain boundary for both the samples. TOF-SIMS imaging was also performed to analyze the distribution of the impurities across the grain boundary in both the samples. C was observed to be segregated along the grain boundary for the control sample, while H and O showed a difference in signal intensity across the grain boundary. Crystal orientation appears to have an important role in the observed sputtering rate and impurity ion signal differences both across the grain boundary and between samples

THPO032

TOF-SIMS Analysis of Hydrogen in Niobium, From 160°K to 475°K

788

P. Maheshwari, A.D. Batchelor, D.P. Griffis, F.A. Stevie, C. Zhou
NCSU AIF, Raleigh, North Carolina, USA
G. Ciovati, R. Myneni
JLAB, Newport News, Virginia, USA
M. Rigsbee
Materials Science and Engineering, Raleigh, USA

Niobium (Nb) is the material of choice for superconducting radio frequency (SRF) cavities due to its high critical temperature and critical magnetic field. Interstitial impurity elements such as H directly influence the efficiency of these cavities. Quantification of H in Nb is difficult since H is extremely mobile in Nb with a very high diffusion coefficient even at room temperature. In the presented work, Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) was used to characterize H in Nb over a wide temperature range (160°K to 475°K) in situ to check for changes in mobility. Multiple experiments showed that as the specimen temperature is decreased below 300 °K, the H/Nb intensity changes by first increasing and then decreasing drastically at temperatures below 200°K. As specimen temperature is increased from 300°K to 450°K, the H/Nb intensity decreases. Remarkably, the H intensity with respect to Nb increases with time at 475°K (approximately 200oC). Correlation between this data and the H-Nb phase diagram appears to account for the H behaviour.

Poster THPO032 [0.125 MB]

THPO057

Superconducting DC and RF Properties of Ingot Niobium

856

P. Dhakal, G. Ciovati, P. Kneisel, R. Myneni
JLAB, Newport News, Virginia, USA

Recently [1, 2], the DC and low frequency magnetic and thermal properties of large-grain niobium samples subjected to different chemical and heat treatment were measured. Here, we extend the similar study to the cylindrical hollow rods of larger diameter, fabricated from new niobium ingots, manufactured by CBMM. The results confirm the influence of chemical and heat-treatment processes on the superconducting properties, with no significant dependence on the impurity concentrations in the original ingots. Furthermore, RF properties such as the surface resistance and quench field of the niobium rods were measured using a TE011 cavity. The hollow niobium rod is the center conductor of this cavity, converting it to a coaxial cavity. The quench field is limited by the critical heat flux through the rods’ cooling channel.
[1] Mondal et al., SRF 2009, Berlin, 2009.
[2] Dhavale et al.,Proc. of the First Int. Symp. on the Superconducting Sci. and Tech. of Ingot Niobium, AIP Conference Proceedings 1352, p. 119 (2011).

Poster THPO057 [1.238 MB]

THPO067

Characterization of Large Grain Nb Ingot Microstructure Using OIM and Laue Methods

890

D. Kang, D.C. Baars, T.R. Bieler
Michigan State University, East Lansing, USA
G. Ciovati
JLAB, Newport News, Virginia, USA
C. Compton
FRIB, East Lansing, Michigan, USA
T.L. Grimm, A.A. Kolka
Niowave, Inc., Lansing, Michigan, USA

Funding: This work was supported by the U.S. Department of Energy, Office of High Energy Physics, through Grant No. DE-S0004222.
Large grain niobium is being examined for fabricating superconducting radiofrequency cavities as an alternative to using rolled sheet with fine grains. It is desirable to know the grain orientations of a niobium ingot slice before fabrication, as this allows heterogeneous strain and surface roughness effects arising from etching to be anticipated. Characterization of grain orientations has been done using orientation imaging microscopy (OIM), which requires destructive extraction of pieces from an ingot slice. Use of a Laue camera allows nondestructive characterization of grain orientations, a process useful for evaluating slices and deformation during the manufacturing process. Five ingot slices from CBMM, Ningxia, and Heraeus are compared. One set of slices was deformed into a half cell and the deformation processes that cause crystal rotations have been investigated and compared with analytical predictions. The five ingot slices are compared in terms of their grain orientations and grain boundary misorientations, indicating no obvious commonalities, which suggests that grain orientations develop randomly during solidification.

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
TUIOB02	Summary of the Symposium on Ingot Nb and New Results on Fundamental Studies of Large Grain Nb	319
	G. Ciovati, P. Dhakal, R. Myneni JLAB, Newport News, Virginia, USA
	Funding: This manuscript has been authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177 The First International Symposium on the Superconducting Science and Technology of Ingot Niobium was held at Jefferson Lab in September 2010. Significant activities are taking place at laboratories and universities throughout the world to address several aspects related to the science and technology of Ingot Nb: from ingot production to mechanical, thermal and superconducting properties. A summary of the results presented at the Symposium is given in this contribution. New results on the superconducting properties and interstitial impurities content measured in large-grain Nb samples and cavities are briefly highlighted.

TUPO051	High-Temperature Heat Treatment Study on a Large-Grain Nb Cavity	508
	G. Ciovati, P. Dhakal, R. Myneni JLAB, Newport News, Virginia, USA P. Maheshwari, F.A. Stevie NCSU AIF, Raleigh, North Carolina, USA
	Funding: This manuscript has been authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177 Improvement of the cavity performance by a high-temperature heat-treatment without subsequent chemical etching have been reported for large-grain Nb cavities treated by buffered chemical polishing, as well as for a fine-grain cavity treated by vertical electropolishing [1]. Changes in the quality factor, Q0, and maximum peak surface magnetic field achieved in a large-grain Nb single-cell cavity have been determined as a function of the heat treatment temperature, between 600 °C and 1200 °C. The highest Q0 improvement of about 30% was obtained after heat-treatment at 800 °C-1000 °C. Measurements by secondary ion mass spectrometry on large-grain samples heat-treated with the cavity showed large reduction of hydrogen concentration after heat treatment. [1] G. Ciovati, G. Myneni, F. Stevie, P. Maheshwari, and D. Griffis, Phys. Rev. ST Accel. Beams 13, 022002 (2010)

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.

THPO016	Preliminary Results on the Laser Heating Investigation of Hotspots in a Large-Grain Nb Cavity	745
	G. Ciovati, C. Baldwin, G. Cheng, R. J. Flood, K. Jordan, P. Kneisel, M.L. Morrone, L. Turlington, K.M. Wilson, S. Zhang JLAB, Newport News, Virginia, USA S. M. Anlage UMD, College Park, Maryland, USA A.V. Gurevich Old Dominion University, Norfolk, Virginia, USA G. Nemes Astigmat, Santa Clara, USA
	Magnetic vortices pinned near the inner surface of SRF Nb cavities are a possible source of RF hotspots, frequently observed by temperature mapping of the cavities outer surface at RF surface magnetic fields of about 100 mT. Theoretically, we expect that the thermal gradient provided by a 10 W green laser shining on the inner cavity surface at the RF hotspot locations can move pinned vortices to different pinning locations. The experimental apparatus to send the beam onto the inner surface of a photoinjector-type large-grain Nb cavity is described. Preliminary results on the changes in thermal maps observed after applying the laser heating are also reported.
	Poster THPO016 [0.983 MB]

THPO028	SIMS and TEM Analysis of Niobium Bicrystals	776
	P. Maheshwari, A.D. Batchelor, D.P. Griffis, F.A. Stevie, C. Zhou NCSU AIF, Raleigh, North Carolina, USA G. Ciovati, R. Myneni, J.K. Spradlin JLAB, Newport News, Virginia, USA M. Rigsbee Materials Science and Engineering, Raleigh, USA
	The behaviour of interstitial impurities(C,O,N,H) on the Nb surface with respect to grain boundaries may affect cavity performance. Large grain Nb makes possible the selection of bicrystal samples with a well defined grain boundary. In this work, Dynamic SIMS was used to analyze two Nb bicrystal samples, one of them heat treated and the other non heat treated (control). H levels were found to be higher for the non heat treated sample and a difference in the H intensity and sputtering rate was also observed across the grain boundary for both the samples. TEM results showed that the bicrystal interface showed no discontinuity and the oxide layer was uniform across the grain boundary for both the samples. TOF-SIMS imaging was also performed to analyze the distribution of the impurities across the grain boundary in both the samples. C was observed to be segregated along the grain boundary for the control sample, while H and O showed a difference in signal intensity across the grain boundary. Crystal orientation appears to have an important role in the observed sputtering rate and impurity ion signal differences both across the grain boundary and between samples

THPO032	TOF-SIMS Analysis of Hydrogen in Niobium, From 160°K to 475°K	788
	P. Maheshwari, A.D. Batchelor, D.P. Griffis, F.A. Stevie, C. Zhou NCSU AIF, Raleigh, North Carolina, USA G. Ciovati, R. Myneni JLAB, Newport News, Virginia, USA M. Rigsbee Materials Science and Engineering, Raleigh, USA
	Niobium (Nb) is the material of choice for superconducting radio frequency (SRF) cavities due to its high critical temperature and critical magnetic field. Interstitial impurity elements such as H directly influence the efficiency of these cavities. Quantification of H in Nb is difficult since H is extremely mobile in Nb with a very high diffusion coefficient even at room temperature. In the presented work, Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) was used to characterize H in Nb over a wide temperature range (160°K to 475°K) in situ to check for changes in mobility. Multiple experiments showed that as the specimen temperature is decreased below 300 °K, the H/Nb intensity changes by first increasing and then decreasing drastically at temperatures below 200°K. As specimen temperature is increased from 300°K to 450°K, the H/Nb intensity decreases. Remarkably, the H intensity with respect to Nb increases with time at 475°K (approximately 200oC). Correlation between this data and the H-Nb phase diagram appears to account for the H behaviour.
	Poster THPO032 [0.125 MB]

THPO057	Superconducting DC and RF Properties of Ingot Niobium	856
	P. Dhakal, G. Ciovati, P. Kneisel, R. Myneni JLAB, Newport News, Virginia, USA
	Recently [1, 2], the DC and low frequency magnetic and thermal properties of large-grain niobium samples subjected to different chemical and heat treatment were measured. Here, we extend the similar study to the cylindrical hollow rods of larger diameter, fabricated from new niobium ingots, manufactured by CBMM. The results confirm the influence of chemical and heat-treatment processes on the superconducting properties, with no significant dependence on the impurity concentrations in the original ingots. Furthermore, RF properties such as the surface resistance and quench field of the niobium rods were measured using a TE011 cavity. The hollow niobium rod is the center conductor of this cavity, converting it to a coaxial cavity. The quench field is limited by the critical heat flux through the rods’ cooling channel. [1] Mondal et al., SRF 2009, Berlin, 2009. [2] Dhavale et al.,Proc. of the First Int. Symp. on the Superconducting Sci. and Tech. of Ingot Niobium, AIP Conference Proceedings 1352, p. 119 (2011).
	Poster THPO057 [1.238 MB]

THPO067	Characterization of Large Grain Nb Ingot Microstructure Using OIM and Laue Methods	890
	D. Kang, D.C. Baars, T.R. Bieler Michigan State University, East Lansing, USA G. Ciovati JLAB, Newport News, Virginia, USA C. Compton FRIB, East Lansing, Michigan, USA T.L. Grimm, A.A. Kolka Niowave, Inc., Lansing, Michigan, USA
	Funding: This work was supported by the U.S. Department of Energy, Office of High Energy Physics, through Grant No. DE-S0004222. Large grain niobium is being examined for fabricating superconducting radiofrequency cavities as an alternative to using rolled sheet with fine grains. It is desirable to know the grain orientations of a niobium ingot slice before fabrication, as this allows heterogeneous strain and surface roughness effects arising from etching to be anticipated. Characterization of grain orientations has been done using orientation imaging microscopy (OIM), which requires destructive extraction of pieces from an ingot slice. Use of a Laue camera allows nondestructive characterization of grain orientations, a process useful for evaluating slices and deformation during the manufacturing process. Five ingot slices from CBMM, Ningxia, and Heraeus are compared. One set of slices was deformed into a half cell and the deformation processes that cause crystal rotations have been investigated and compared with analytical predictions. The five ingot slices are compared in terms of their grain orientations and grain boundary misorientations, indicating no obvious commonalities, which suggests that grain orientations develop randomly during solidification.

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