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

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Nov 15, 2013 (3 years and 8 months ago)

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The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


1
st

SSTIN

Suppressed superconductivity on the surface

of 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


The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


1
st

SSTIN

Outline

Issues


GBs are defect responsible for
Q
0

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
(
θ
)

Local magnetic characteristics at the GBs

Summary

Current search
-

local magnetic characteristics on the PITs


The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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GBs are defect responsible for
Q
0

drop at high field regime?

G.
Ciovati

et al. Proc. of the 2006 LINAC, paper TUP033

No major difference of
Q
0

between grain sizes

G.
Eremeev

et al. Proc. of EPAC’06 , Edinburgh,
MOPCHI176



40% of all hot
-
spots associated with GB

G.
Ciovati

et al. Phys. Rev. ST
Accel

Beam 13, 2010

So far… Not clear about the GBs effects on SRF Nb cavity

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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Quench locations: Z111 TESLA 6 cell cavity

G.
Ciovati

et al. SRF Material Workshop at MSU, 2009

Courtesy of W. Singer. ILC Cavity Group 9th Meeting. January 27
th
. 2009

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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Very weak dissipation at
H

<
H
c1

(
Q
0

= 10
10
-
10
11
)

Q

drop due to vortex dissipation at
H

>
H
c1


Nb has the highest lower critical field
H
c1











5
.
0
ln
4
2
0
1




c
H


2
2
0

c
H
Gurevich et al. PRL 88, 097001 (2002)

GBs can locally reduce superconducting gap (
Δ
)
and the
depairing

current density,
J
b.gb

→ suppress
the onset of vortex penetration → increase R
s


increase power dissipation

GB is a hot spot site

Thermodynamic critical field
H
c


(surface barrier for vortices disappears)

Gurevich et al. Physica C, 441 (2006)

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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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

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


1
st

SSTIN

Premature flux penetration at the grain boundary

when the GB plane // H
ext
(by MO imaging)

P. Lee, et al.,
Physica

C, 441 (1), (2006)

H = 58
mT

FC T = 6 K

H = 0
mT

ZFC T = 6 K

H = 72
mT

H = 0
mT

BCP’ed

EP’ed

ZFC T = 6.5 K

FC T = 6.5 K

1 mm

H

Both: GB//
H
ext

Top & Bottom

100min BCP

H = 80
mT

ZFC

Lack of flux penetration at the tilted GB

3D image: 23.6
°

tilted GB

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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SSTIN

~20
-
30
mT

200mT

As
-
received

Mechanically ground

Surface image of I
-
shape sample after BCP treatment

1 mm

100
μ
m

Expand the gap between H
c1

(170mT) and H
c2

(200mT)
at 4.2 K → Make vortex penetration at lower
H
ext

1.
Cut samples into I
-
shape with wire
-
EDM

2.
Mechanically grind down the bottom of the
sample surface to ~150
-
250
μ
m, so the top
surface remain as
-
received condition

3.
Ultra fine polish with vibratory polisher
(Vibromet

® Buehler)

4.
Finalize all surfaces with either BCP or EP

-

Make surfaces representative of real cavity surface

5.
Further reduce the bridges of some I
-
shape
single
-

& bi
-

crystals with extra BCP

6.
Artificially groove with FIB and mechanically
smear away the grooved produced by the
chemical treatments

The procedures

DC transport
V
-
I

characterization with 1T Electromagnet

Higher
-
H
c

SC

Nb

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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Preferential flux flow on the deep GB groove

A deep (3
-
5
μ
m) and highly inclined groove

No groove (~0.5

2.0
μ
m roughness)



The
V
-
J

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



Flux flow evidence from H = 0.08 T to 0.28 T



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

0.05T

0.08T

0.10T

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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Field dependent GB resistivity : R
f
(H)



A
H
H
J
H
W
H
R
c
n
f




2
,
)
(

0



B
W
a
W
dV
row
The width of the FF channel (
W
) & The number of vortices flowing on the channel (
dV
row
) 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

Single vortex row

Collective
depinning

of
multiple vortex rows
along GB

Multi vortex rows

Gurevich’s model

7
°

[001] tilted YBa
2
Cu
3
O

7
-
δ
GB

1
st

linear fitting

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


1
st

SSTIN

High GB flux flow tendency when H
ext

lies on the GB plane

A deep (3
-
5
μ
m) and highly inclined groove

Preferential flux flow H
ext

= 0.08 T to 0.28 T

At the GB plane // H
ext

The # is the angle between the GB plane and
H
ext

0.08T

0.05T

0.10T

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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SSTIN

Mechanisms of suppression of flux flow on the GB

a




A
-
J core size:


The A
-
J cores overlap if


㸠愬a潲




Viscous flux motion


R(B) is independent of B, if a single vortex


chain moves along GB, while


㸠a

Gurevich et al. PRL 88, 097001 (2002)

V = (I
-

I
b
)R

H > (
J
b
/
J
d
)
2

H
c2



=

J
2
/




J
d
/J
b

1. Split A vortex

2. Increase of GB area

J

θ

GB

α
A

>
α
B


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

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


1
st

SSTIN

Do BCP and EP have different effects on flux penetration?

Linear coordinates

Flux flow?



No distinct flux flow evidence at the electropolished GB, similar to BCP’ed Single crystal



However, traces of flux flow along the electropolished GB are visible

Linear coordinates

H = 58
mT

ZFC T = 6 K

BCP’ed

EP’ed

H = 72
mT

ZFC T = 6.5 K

GB flux flow

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


1
st

SSTIN

GB flux flow as f(
θ
) :
crystallographic misorientation angle

More misoriented GB retards the progress of preferential flux flow

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

36
°

bi
-
crystal (the aspect ratio ~ 24.3)

22
°

bi
-
crystal (the aspect ratio ~ 12.5)

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


1
st

SSTIN

Field enhancement by severe surface topology @GB

H
ext
//surface

As
-
received

As
-
received

10
-
20
μ
m @GB

Magnetic field enhancement model at the slope of GB

2
)
(
2
1
'
H
w
R
Q
m
nc
nc
diss


crit
H
H

m

if

E
-
beam WELDED AREA

Machine marks, large grains

and height steps at GBs

J. Knobloch, 8
th

SRF workshop

Β
m


height, angle, aspect ration and radius of
curvature at the corner of the grain boundary

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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Local field enhancement characterization using micro Hall sensor

15

Micro Hall probe

Micro Hall probe

Hall probe sensor


Fabricated by Dr. Milan Polak (2009)

Al
2
O
3


BCP’ed Nb bi
-
crystal

GB

H


Glass (t~126
μ
m)

Spec: ~ 2mm by 2mm



Total thickness: ~0.4mm



0.4mm thick GaAs substrate



5
μ
m thin InSb layer



Activation area: 50
μ
m by 50
μ
m



Minimum distance between the sample surface and the Hall
probe is ~0.2mm

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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st

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Field vs response voltage on Hall probe

Field enhancements at highly inclined GB

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
H
ext

≈ 50mT, the induced

H
gb
, ≈ 274.6 mT, and at
H
ext

≈ 100 mT,
H
gb

≈ 635.2 mT, accounting
that local areas of the grain boundary region is normal state.

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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st

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GB // H
RF

GBs

Cavity wall

μ

structure and chemical properties ?

GB

Oxide

Au
-
Pd

Preferential GB flux penetration

Preferential GB flux flows

GB Field enhancement

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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st

SSTIN

Summary


GBs can preferentially admit magnetic flux before it is admitted to grains
when the GB plane //
H
ext


Topological features introduced by BCP or EP were not the cause of the
preferential nucleation of magnetic flux when
H
ext

// the GB plane


Transport measurements on BCP’ed 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
H
ext
vector.


The highly inclined slope of grain boundary produced by BCP causes
localized field enhancement when
H
ext

// the sample surface.


High resolution analytical microscopy is highly recommended to further
understanding of intrinsic grain boundary properties

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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st

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Investigation of SC breakdown on the PITs

Cold spot

Courtesy of Dr. Romanenko

from BCP’ed cavity

#3


SE2 Image

PIT
-
BSD image

After Diamond cutting

EBSD scanned area

PIT on this triple point did not
cause excessive heating

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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st

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PIT
-
BSD image

#3
-
PIT : 3D topology

20.8
μ
m

Surface topology

Surface topological features of the PIT

Surface profiles

By scanning laser confocal microscope

( ~ few tens of nm z
-
direction resolution)

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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st

SSTIN

Local magnetic characterizations using micro Hall Array Sensor

1. Micro
-
Hall Sensor

Using 7
-
8 Channels

~ 420
m
m

2. Micro
-
Hall Array Sensor

Enough resolution to detect a single vortex quantum (
Φ

≈ 2.0672
×

10
-
15

T/m
2
)

7
-
8 channels of 21 channels the entire length of the array (~420
μ
m)

Provided by Dr. Milan Polak (2009)

Provided by Dr. Eli Zeldov (2009)

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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Local vortex avalanches in superconducting Nb

E.
Altshuler
, Phys Rev B, 70 R, & Physica C, 408
-
410, (2004)

MO imaging on Nb thin film

HAS

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

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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

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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st

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Voltage responses of PIT on the single Hall sensor

Kink by field enhancements

Kink by field enhancements:

due to the topological effect
of a pit

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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st

SSTIN

Voltage responses of the PIT on the HAS

Kink by field enhancements

Kink by field enhancements

due to the topological effect of a pit

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

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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st

SSTIN

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


The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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st

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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)

GB

Au
-
Pd

Oxide

GB

Au
-
Pd

Oxide

Λ

~ 40nm

GB

GB

Oxide

Oxide

Au
-
Pd

Au
-
Pd

Inclusions

Native oxide : Nb
2
O
5


5
-
10 nm

Interface : sub oxides
+ interstitial oxygen :
some monolayers.

interstitials : what
concentration, what
depth profile ?

Grain boundaries

Chemical
residue


~ 40nm



No oxide indentation at the GB



Thickness of Nb oxide; ~ 5
-
7nm

Λ

~ 40nm

A

B

Halbritter’s widely accepted model

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The National High Magnetic Field Laboratory

Florida State University

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J
c

enhancements on meandered GBs of YBCO

Planar GB (PLD)

Feldmann et al., JAP 102 (2007): J.Am.Ceram.Soc (2008)

Meandered GB (PLD & MOD)

Planar GB

B.C. Cai et al., Phil. Mag. B. (1987): A. Dasgupta et al, Phil. Mag. B. (1978)

I
c

enhancement when H
ext

// the GB plane of Nb

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

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Further efforts for the GB depairing critical current

BCP’ed Nb Bi
-
crystal (26
°

misoriented GB)

B
perp

= H
-

η·
M

η

= demagnetization factor

M = magnetization

Multi vortices rows

Single vortex row

7
°

[001] tilted YBa
2
Cu
3
O

7
-
δ
GB

Micro
-
Hall Sensor Arrays

GaAs/AlGaAs heterostructure

Courtesy of Dr. Eli Zeldov

~ 420
m
m

The Applied Superconductivity Center

The National High Magnetic Field Laboratory

Florida State University

Z.H. Sung


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Field dependent GB resistivity : R
f
(H)

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

Local magnetization: B
prep

on the BCP’ed bi
-
crystal

Multi vortices rows

Single vortex row