Suppressed superconductivity on the surface of SRF quality niobium for particle accelerating cavities

1
Suppressed superconductivity on the surfaceof
SRF quality niobium for particle accelerating
cavities

• Z.H. Sung, A.A. Polyanskii, P.J. Lee, A.
  Gurevich,
• and D.C. Larbalestier
• Applied Superconductivity Center
• National High Magnetic Field Laboratory
• Florida State University

2
Outline

• Issues GBs are defect responsible for Q0 drop
  at high field regime?
• Previous work - Premature flux penetration by MO
  imaging technique
• DC transport measurements - Preferential flux
  flow at the GB
• Field dependent flux flow resistivity of the GB
  and its angular dependence, and GB flux flow as
  f(?)
• Local magnetic characteristics at the GBs
• Summary
• Current search - local magnetic characteristics
  on the PITs

3
GBs are defect responsible for Q0 drop at high
field regime?
So far Not clear about the GBs effects on SRF Nb
cavity
No major difference of Q0 between grain sizes
? 40 of all hot-spots associated with GB
G. Eremeev et al. Proc. of EPAC06 , Edinburgh,
MOPCHI176
G. Ciovati et al. Proc. of the 2006 LINAC, paper
TUP033
G. Ciovati et al. Phys. Rev. ST Accel Beam 13,
2010
4
G. Ciovati et al. SRF Material Workshop at MSU,
2009
5
GB is a hot spot site
GBs can locally reduce superconducting gap (?)
and the depairing current density, Jb.gb ?
suppress the onset of vortex penetration ?
increase Rs ? increase power dissipation
Gurevich et al. Physica C, 441 (2006)
Thermodynamic critical field Hc (surface barrier
for vortices disappears)
Very weak dissipation at H lt Hc1 (Q0
1010-1011) Q drop due to vortex dissipation at H
gt Hc1 Nb has the highest lower critical field Hc1
Gurevich et al. PRL 88, 097001 (2002)
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Description of specific grains and grain
boundaries
Large grain Nb sheet from JLAB (P. Kneisel and
Co-workers)

• Thinner GBs have planes which are closer to the
  surface perpendicular. Thicker GBs are inclined
  20-30 from the perpendicular.
• This as-received slice (RRR 280, 3.1mm thick)
  has a very large grain size (50-100mm), which
  allowed us to isolate multiple bi-crystals.

OIM GBs are 25-36 misoriented
Overview of the as-received niobium slice
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Premature flux penetration at the grain
boundarywhen the GB plane // Hext (by MO
imaging)
Lack of flux penetration at the tilted GB
Top Bottom
P. Lee, et al., Physica C, 441 (1), (2006)
100min BCP
3D image 23.6 tilted GB
EPed
BCPed
ZFC T 6.5 K
ZFC T 6 K
H
H 58 mT
H 72 mT
Both GB//Hext
FC T 6.5 K
FC T 6 K
H 0 mT
H 0 mT
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DC transport V-I characterization with 1T
Electromagnet
Expand the gap between Hc1 (170mT) and Hc2
(200mT) at 4.2 K ? Make vortex penetration at
lower Hext
Higher-Hc SC
Nb
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Preferential flux flow on the deep GB groove
A deep (3-5µm) and highly inclined groove
No groove (0.52.0µm roughness)

• Flux flow evidence from H 0.08 T to 0.28 T

• The V-J characteristics show that the grain
  boundary is a channel of preferential flux flow
  (FF) by weakly pinned vortices.

• However, the slightly non-ohmic V-I response
  suggests that flux flow is not just confined to a
  single vortex row flowing along the grain boundary

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Field dependent GB resistivity Rf(H)
Multi vortex rows
7 001 tilted YBa2Cu3O 7-dGB
Gurevichs model
Single vortex row
Collective depinning of multiple vortex rows
along GB
The width of the FF channel (W) The number of
vortices flowing on the channel (dVrow) at H
0.08T are 0.185 µm and 0.489 µm, representing
1.15 and 3.04 rows, respectively from the low and
high V portions of the V-I curve
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High GB flux flow tendency when Hext lies on the
GB plane
The is the angle between the GB plane and Hext
A deep (3-5µm) and highly inclined groove
Preferential flux flow Hext 0.08 T to 0.28 T At
the GB plane // Hext
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Mechanisms of suppression of flux flow on the GB
Transformation of Abrikosov vortex to mixed
Abrikosov Josepshon vortex at the grain boundary
of SRF-quality niobium is still unclear but GB
weakness is shown at V-I responses
1. Split A vortex
2. Increase of GB area
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Do BCP and EP have different effects on flux
penetration?
EPed
BCPed
ZFC T 6.5 K
ZFC T 6 K
H 72 mT
H 58 mT
Linear coordinates
Linear coordinates
GB flux flow

• No distinct flux flow evidence at the
  electropolished GB, similar to BCPed Single
  crystal
• However, traces of flux flow along the
  electropolished GB are visible

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GB flux flow as f(?) crystallographic
misorientation angle
More misoriented GB retards the progress of
preferential flux flow
36 bi-crystal (the aspect ratio 24.3)
22 bi-crystal (the aspect ratio 12.5)
The aspect ratio is defined as the ratio of the
width to length of the bridge of I-shape , so it
means the higher the ratio is, the more
demagnetization is
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Field enhancement by severe surface topology _at_GB
E-beam WELDED AREA
Magnetic field enhancement model at the slope of
GB
Machine marks, large grains and height steps at
GBs
Hext//surface
As-received
As-received
10-20µm _at_GB
?m ? height, angle, aspect ration and radius of
curvature at the corner of the grain boundary
J. Knobloch, 8th SRF workshop
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Local field enhancement characterization using
micro Hall sensor
H
Micro Hall probe
BCPed Nb bi-crystal
Al2O3
Glass (t126µm)
GB
Hall probe sensor
Micro Hall probe

• Spec 2mm by 2mm
• Total thickness 0.4mm
• 0.4mm thick GaAs substrate
• 5µm thin InSb layer
• Activation area 50µm by 50 µm
• Minimum distance between the sample surface and
  the Hall probe is 0.2mm

Fabricated by Dr. Milan Polak (2009)
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Field enhancements at highly inclined GB
Field vs response voltage on Hall probe
Roughness profile at the GB
Slope change from 10.5 ? 25.7

• The field enhancement at the grain boundary
  region is almost 5 times higher than in-grain
• For example, at Hext 50mT, the induced Hgb,
  274.6 mT, and at Hext 100 mT, Hgb 635.2 mT,
  accounting that local areas of the grain boundary
  region is normal state.

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µ structure and chemical properties ?
GB // HRF
Preferential GB flux flows
GB Field enhancement
Preferential GB flux penetration
Cavity wall
GBs
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Summary

• GBs can preferentially admit magnetic flux before
  it is admitted to grains when the GB plane //
  Hext
• Topological features introduced by BCP or EP were
  not the cause of the preferential nucleation of
  magnetic flux when Hext // the GB plane
• Transport measurements on BCPed samples showed
  clear evidences of preferential flux flow on the
  GB.
• Consistent with the results of MO imaging, the
  V-I characterizations showed that the grain
  boundary weakness is greatly enhanced when the
  plane of GB is parallel to the Hext vector.
• The highly inclined slope of grain boundary
  produced by BCP causes localized field
  enhancement when Hext // the sample surface.
• High resolution analytical microscopy is highly
  recommended to further understanding of intrinsic
  grain boundary properties

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Investigation of SC breakdown on the PITs
Cold spot
from BCPed cavity
After Diamond cutting
PIT on this triple point did not cause excessive
heating
PIT-BSD image
EBSD scanned area
Courtesy of Dr. Romanenko
3 SE2 Image
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Surface topological features of the PIT
Surface topology
PIT-BSD image
3-PIT 3D topology
By scanning laser confocal microscope ( few
tens of nm z-direction resolution)
Surface profiles
20.8 µm
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Local magnetic characterizations using micro Hall
Array Sensor
2. Micro-Hall Array Sensor
1. Micro-Hall Sensor
Provided by Dr. Milan Polak (2009)

• Enough resolution to detect a single vortex
  quantum (F 2.0672 10-15 T/m2)
• 7-8 channels of 21 channels the entire length of
  the array (420 µm)

Provided by Dr. Eli Zeldov (2009)
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Local vortex avalanches in superconducting Nb
MO imaging on Nb thin film
HAS
E. Altshuler, Phys Rev B, 70 R, Physica C,
408-410, (2004)
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PIT from cold spot sample on HAS
PIT on the HAS
PITS
Hall array sensors
Decreased the dimension of 3 part (see slide 4)
with a precision diamond, then carefully
mechanically reduced its thickness up to half of
the original
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Voltage responses of PIT on the single Hall sensor
Kink by field enhancements due to the
topological effect of a pit
Kink by field enhancements
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Voltage responses of the PIT on the HAS
Kink by field enhancements due to the topological
effect of a pit
Kink by field enhancements
The tendency of voltage responses are very
similar to the plots by the single Hall sensor,
but the onsets of Kinks by field enhancements are
delayed up to 20 50 mT
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Acknowledgement

• We would like to thank Peter Kneisel and Ganapati
  Rao Myneni and their colleagues at TJNL for
  providing the Nb slice.
• Lance Cooley, SRF Materials Group Leader, at
  FNAL.
• Special thanks to Ian Winger (Physic department,
  FSU) for wire-EDM
• Dr. Milan Polak and Dr. Eli Zeldov for supplying
  a micro-Hall sensors
• This work was supported by the US-DOE under
  grants DE-FG02-05ER41392 and DE-FG02-07ER41451
  and by the State of Florida support for the
  National High Magnetic Field Laboratory

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What is the real microstructure at the GB after
BCP?
HRTEM used to observe the vicinity of the GB
(with FIB technique)
A
GB
? 40nm
GB

• No oxide indentation at the GB
• Thickness of Nb oxide 5-7nm

Au-Pd
Au-Pd
Oxide
Oxide
40nm
B
GB
GB
? 40nm
Au-Pd
Halbritters widely accepted model
Au-Pd
Oxide
Oxide
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Jc enhancements on meandered GBs of YBCO
Meandered GB (PLD MOD)
Planar GB (PLD)
Planar GB
Feldmann et al., JAP 102 (2007) J.Am.Ceram.Soc
(2008)
Ic enhancement when Hext // the GB plane of Nb
B.C. Cai et al., Phil. Mag. B. (1987) A.
Dasgupta et al, Phil. Mag. B. (1978)
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Further efforts for the GB depairing critical
current
Micro-Hall Sensor Arrays
BCPed Nb Bi-crystal (26 misoriented GB)
Courtesy of Dr. Eli Zeldov
Multi vortices rows
7 001 tilted YBa2Cu3O 7-dGB
GaAs/AlGaAs heterostructure
Single vortex row
Bperp H - ?M
? demagnetization factor M magnetization
420 mm
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Local magnetization Bprep on the BCPed
bi-crystal
Field dependent GB resistivity Rf(H)
Multi vortices rows
Single vortex row
Fields Ch1 Ch2 Ch3 Ch4 Ch5 Ch6 Ch7 Ch8
10mT 8.34 9.09 7.99 6.39 3.26 2.03 4.97 20.88
20mT 18.48 19.87 18.45 16.08 11.70 11.94 22.59 35.56
30mT 29.68 31.55 29.45 27.22 23.49 25.54 35.35 47.10
40mT 41.65 43.45 40.95 38.96 36.03 38.71 47.78 58.53
50mT 53.53 55.56 52.79 50.91 48.58 51.53 60.06 69.87
60mT 65.61 67.69 64.60 63.03 61.07 64.32 72.24 81.12
70mT 77.85 80.11 76.57 75.08 73.56 76.89 84.27 92.30
80mT 90.17 92.61 88.79 87.25 85.99 89.37 96.13 103.49
90mT 102.86 105.72 101.51 99.56 98.44 101.60 107.95 114.70