Magneto-Optical Study of High Purity Nb for SRF Application

A.A. Polyanskii, Z.H. Sung, P.J. Lee, A. Gurevich, D.C. Larbalestier

ASC, NHMFL, FSU,Tallahassee, USA

SSTIN-2010, Jefferson Lab, NewPort News, Virginia, September 22-24, 2010

This work was supported by the US DOE under grants DE-FG02-07ER41451, FNAL PO 570362, and the Florida State

3D Parametric Surface

1

2

3D Parametric Surface

1

2

MOTIVATION• The recent studies showed that superconducting cavity

performance is very sensitive to quality of Nb surface.

• The penetration of small numbers of flux lines into the cavity surface during RF operation is leading to breakdown of the superconducting state and causing Q-drop or quench.

• Q-drop can be triggered by early flux penetration in hotsports leading to thermal breakdown on the cavity surface (1 and 2). (Model by A. Gurevich).

1

4

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10

13

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25

28

31

34

S1 S

3 S5 S

7 S9

S1

1

S1

3

S1

5

0

0 .1

0 .2

0 .3

0 .4

0 .5

0 .6

0 .7

T ( K )

A z im u t hB o t t o m I r is

T o p Ir i s

E q u a t o r

Q 0 = 2 .8 1 0 9

B p = 1 1 0 m T

1

2

1

4

7

10

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16

19

22

25

28

31

34

S1 S

3 S5 S

7 S9

S1

1

S1

3

S1

5

0

0 .1

0 .2

0 .3

0 .4

0 .5

0 .6

0 .7

T ( K )

A z im u t hB o t t o m I r is

T o p Ir i s

E q u a t o r

Q 0 = 2 .8 1 0 9

B p = 1 1 0 m T

1

2

• Local study of flux penetration is important

MOTIVATION

Magnetization and Magneto-Optical Imaging (MOI) techniques can detect local magnetic flux penetrate in Nb

EXPERIMENTALTECHNIQUE :

1. MAGNETO-OPTICAL2. VSM3. SEM4. ZYGO LIGHT MICROSCOPY by Z. H. Sung(See:www.zygo.com/?/products/nv6000)

Protective layer

F=V Bz 2d

dBa,z GGG

YIG: Bi

Polarized light

M

Ba,x

PRINCIPILE OF MAGNETO-OPTICAL TECHNIQUE MAGNETO-OPTICAL INDICATOR ON BASE OF GARNET

FILM DOPED BY Bi DETECTS MAGNETIC FIELD

Reflective layerNb

M

AP

H

H

H=0

THE FARADAY ROTATION IN A FERRIMAGNETIC Bi-DOPED IRON GARNET INDICATOR FILMS WITH IN-PLANE

MAGNETIZATION

LIGHT POLARIZATION

MAGNETO-OPTICAL CONTRAST PICKS UP ON PLASTIC REFRIGIRATE CARD:

DISTRIBUTION OF STRAY FIELD AROUND MAGNETIC STRIPS

5.25 inch disc 3.5 inch disc Audio tape

MO IMAGES OF MAGNETIC FIELD IN AUDIO TAPE AND COMPUTER DISCETS

500m 100m

(AAA)

American Automobile

Association Card

Library Vendor

Card

Visa Credit

Card

500m

MO IMAGES OF MAGNETIC CODES IN DIFFERENT PLASTIC CARDS

MAGNETO-OPTICAL IMAGE OF MAGNETIC SECURITY CODE IN US $100

He

CRYOSTAT

TV

cryostat

MAGNETO-OPTICAL IMAGING SETUP FOR SUPERCONDUCTING RESEARCH: electromagnetic

system creates magnetic field in two direction: X and Z

Polarized Optical Microscope

Hext

ZHext

X

LHe

MAGNETO-OPTICAL SETUP ON THE BASE OF A POLARIZING MICROSCOPE IN REFLECTIVE MODE

Hz=(1/1-Nz) x Hext

Hext

Z

X

GEOMETRY OF MAGNETO-OPTICAL EXPERIMENT ON Nb SAMPLES FOR CAVITY APPLICATION IN

PERPENDICULAR-Z AND PARALLEL-X (IN-PLANE) FIELDS

Demagnetization factor Nz

NbNbNb

MO indicator

MO indicator with in-plane magnetization detects the normal component of magnetic flux

MAGNETIC FLUX DISTRIBUTION IN RECTANGULAR SAMPLE: EFFECT OF SAMPLE SHAPE WITH DEMAGNETIZATION

FACTOR Nz>0.

MOI of magnetic flux distribution in uniform square

Nb sample

1 mm

Maximum field enhancement at the center of each side.

(“roof pattern” or “pillow”)

MSC

ZERO FIELD COOLED (ZFC) IN PERPENDICULAR FIELD: MO images in uniform rectangular and circle Nb samples

1 mm

Calculated current streamlines

MO images

(“roof pattern” or “pillow”)

SC

FIELD COOLED (FC) IN PERPENDICULAR FIELD: MO images trapped magnetic flux in uniform rectangular and circle Nb

samples

Calculated current streamlines

MO images(“roof pattern”

SAMPLES

Thomas Jefferson Lab National Accelerator Facility (JLAB):• Nb samples were cut from 1.8 mm thick large slice from the extremely large grain ingot fabricated by CBMM - Brazil for Jlab: GBs were randomly oriented • In case of cavities some planes of GBs could be parallel to direction of RF magnetic field

Fermi National Accelerator Laboratory (FNAL): • Nb pure polycrystalline samples taken through a typical cavity optimization process and cut from regular and weld regions• Samples with artificial grooves and deformed samples• Samples with PIT in weld seam

JLAB samples with big grains were cut from extremely large-

grain ingotSamples rectangular and circle shapes were cut from large

grain 1.8 mm thick material fabricated by CBMM-Brazil

Disc shape sample- to avoid flux penetration due to sample shape

Bi-crystal and Tri-crystal samples

Plane of GBs were perpendicular and random oriented to face of sample: some plane of GBs in cavity wall parallel to RF field

Tri-crystalTri-crystal

Tri-crystal

Tri-crystal

Bi-crystalGB (#1)

Normal to Surface

Bi-crystalGB (#1)

Normal to SurfaceBi-crystalGB (#2)

Angle to Surface

Bi-crystalGB (#2)

Angle to Surface

Nb slice was cut from ingot with big grains. Sample shapes: rectangular and circle (disc) with big variety of GBs

Tri-crystalTri-crystal

Thickness of sheet is 1.88 mm

GB #2GB #1

RF field in-plane

Overlap top and bottom surfaces of Nb sheet with traces of GBs, Sample #9

Bottom face

Top face

GB #1 and GB #2 form triple-point. Plane of of GB #1 twisted in the vicinity of the triple point: Orientation of RF field in cavity may be parallel the plane of any GBs. Samples were taken from different part of GB#1 and GB#2 and in triple-point

#9

Perpendicular and random angle orientation GBs in bi- and tri-crystals samples rectangular and round shapes

GB #2 has angle about 30-35 degree to wide face of sample

bi-crystal

GB#1 perpendicular to wide face of sample

bi-crystal

MO imaging sample on different surface: when GB - 330 angle and after 900 rotation - GB perpendicular to surface

GB trace on top face of sample

No GB trace on side face

GB trace on bottom face of sample

This face has been imaged by MO, when sample was turned by 900

H

MO indicator

2.78mm

2.17

mm1.89mm

H=80 mT T=5.4K

H=100 mT

Bi-crystal #6 with the 330 angle GB #2 to surface, no remarkable flux penetration along GB

1 mm

Optical, GB#2, angle to surface 330

MO Image and current distribution in sample with perpendicular plane , ZFC, T=5.5K. 170GB #1 admits magnetic flux and shows

obstacle to current flow: distortion of current stream lines

1 mm

GB #1

T=5.5K H=80 mT

17.80 GB17.80 GB 17.80 GB

Current streamline : GB is obstacle to

current flow Jb=0.5Jc

Misorientation angle between two grains ≈17.8o Orientation Imaging Microscopy (OIM): from D. Abraimov

17.80 GBMisorientation profile

Nb JLAN #3A bicrystal, Jc in grain and across GB

T, K

5 6 7 8 9 10

Jc,

kA/c

m2

0

10

20

30

40

50

60

Jc, Grain Jc, GB

Critical current vs T : bulk Jc and Jb across 17.80 GB (from MO measurement)

Ratio Jgb/Jb

T, K

5 6 7 8 9 10

R=

Jgb

/Jb

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

Nb 17.80 GB T=5.5 K Jb ~ 0.5Jc

YBCO 50 GB T=7 K Jb ~ 0.5JcA.A. Polyanskii, at el, Phys. Rev. B, 53, 8687, (1996).

MAGNETIC FLUX BEHAVIOR AROUND 3o, 5o, and 10o GRAIN BOUNDARIES in YBCO bi-crystals T = 7 K, H ~ 40 mT, H||c

10o GB

Jb/Jc =0.95

A.A. Polyanskii, at el, Phys. Rev. B, 53, 8687, (1996).

5o GB3o GB

Jb=0.95Jc Jb=0.5Jc Jb=0.065Jc

#9 H=28 mT

#11 H=32 mT #13 H=40 mT

After 900 rotation GB is perpendicular to surface. GB is a weak link in ZFC and FC. T=6K

#23 H=0 FC T=6K

#7 H=24 mT #8 H=26 mT

1 mm

GB#2

GB#2

GB#2

GB perpendicular to surface: Top and bottom surfaces of Nb sample after electro-polishing (EP) has different roughness

Top surface. After Mechanical polishing + Electropolishing

Bottom surface. After diamond saw + Electropolishing

GB is not visible GB hardly

visible

TOP BOTTOM

MO image of top and bottom surfaces of Nb sample after (EP): good flux penetration on both surfaces

ZFC H=60 mT T=6.5KCurrent streamline on top side: GB is obstacle to current flow

TOP BOTTOM

Nb sample with GB perpendicular to surface.

ROUND JLAB SAMPLE, with random oriented GBs in triple-point

Bottom face

Top face

#81 H=80 mT, ZFC T=5.5K

Tri-crystal #9 fully processed: 5 steps, top face, no preferential flux penetration along GBs, but flux penetrates faster in some additional local places ??

GB #2

GB #1

GB #2

GB #1

GB #2

Optical, top face. Shape is not perfectly round.

1 mm

JLAB #11 with GB #2 with angle 30-350 , ZFC T=5.6K. No flux penetration along GB #2. Flux penetrates in some additional places much faster. What is a reason??

#33 H=86 mTSurface, bottom face

GB #2

1 mm

Tri-crystal #12, fully processed: 5 steps, top face, no preferential flux penetration due to GBs. Asymmetric flux penetration

GBs

Optical, top surface #15 ZFC H=72 mT

Optical, Surface of tri-crystal 3D MODEL of GBs on the base GB traces on top and bottom. Random orientation (Peter Lee)

1 mm

Nb tri-crystal JLAB #4: GBs with random orientation, thickness is 1.88 mm

1 mm

T=5.5 K H=120 mT Remn T=7 K H=0 after H=24 mT

Visible traces of GB

Nb tri-crystal JLAB #4: no penetration at to GB with random orientation, no evidence of weak link, but traces of GBs are visible

1. GBs can accept magnetic flux in case when plane of GB is parallel to external magnetic field2. Some additional flux penetration has been found on surface of many fully processed samples.

MO conclusion on Jlab samples with big grains

SAMPLES FROM FNAL FNAL samples-square shape 3.75 x 3.75 x 1.5 mm were cut from

the same 2.8 mm thick sheet (RRR~ 450) but in two different places:

• Samples from fine-grained Nb sheet (regular), where grain sizes were small ~ 50 m,

• Samples from weld region, where grain sizes were big > 1 mm

• Both types of samples taken through a typical cavity optimization process:

• The processing sequence includes 5 steps:

• (1) cold work produced by the sample machining process, followed by degreasing,

• (2) ~100 m BCP etch,

• (3) HT 5 hours at 750°C in a vacuum < 10-6 Torr,

• (4) ~20 m BCP etch

• (5) bake 50 hours at 120°C in a vacuum < 10-6 Torr

SOME EXAMPLE: OPTICAL IMAGES OF Nb samples cut feom REGULAR AND WELD regions

1 mm

Machine marks (like grooves), large grains and steps at GBs are well visible

REGULAR AREA: small grain size

WELDED AREA: large grain size

VSM Magnetization of Regular and Weld samples taken through a typical cavity optimization process: 5 steps. Magnetization hysteresis is significantly reduced after step 4, the large reduction Hc2 and the small increase in the first field of flux penetration

Regular: small grains Weld: big grains

MOI of Regular samples with small grains taken through a typical cavity optimization process: 5 steps

1 mm

80 мТ 80 мТ 34 мТ 48 мТ40 мТ

60 мТ 60 мТ 30 мТ 40 мТ34 мТ

Optical

ZFC Then H applied

ZFC Then H applied

FC in 110 mT, then H=0 applied

2. 100 µm etch1. CW, degrease 3. HT 5 hr/7500C 5. Bake 50 hr/1200C4. 20 µm etch

MOI of Weld samples with big grains taken through a typical cavity optimization process: 5 steps

1 mm

Optical

ZFC Then H applied

ZFC Then H applied

FC in 110 mT then H=0 H applied

80 мТ 80 мТ 40 мТ 48 мТ40 мТ

60 мТ 48 мТ 36 мТ 31 мТ31 мТ

2. 100 µm etch1. CW, degrease 3. HT 5 hr/7500C 5. Bake 50 hr/1200C4. 20 µm etch

We have observed unusual central flux penetration in weld sample after HT and 20

min BCP (second etching), step 3, 4

Details of flux penetration

32 mT T=5.6KHx

Hz

Details of a flux nucleation on surface weld sample in perpendicular field: demagnetization factor of sample Nz 0

Optical, machinery marks (grooves) and large grains are well visible

Enhancement of magnetic flux at surface defect

1 mm

#28 H=54 mT

ZFC T=5.6 K

Details of a flux nucleation on surface W2-5 in perpendicular field: Expansion of central flux penetration vs H

1 mm

#27 H=52 mT

#30 ZFC H=56 mT

Details of a flux nucleation on surface W2-5 in perpendicular field: Farther expansion of central flux penetration in weld sample ZFC T=5.6 K

X, m

0 1000 2000 3000 4000

Inte

nci

ty (

a.u

.)

500

1000

1500

2000

2500

3000

3500

4000

1 mm

Flux profile taken across sample

T=5.6K 48 mT

Surface profile taken along marked line (ZYGO microscope). Max step height is about 10 m

Form of penetrated flux correlates with surface shape

Surface weld sample and profile: “dome” shape. Good correlation with MO central flux penetration.

1 mm

• MO STUDY IN IN-PLANE FIELD.

• OPTICAL STUDY THE SURFACE OF WELD SAMPLE BY USING ZYGO LIGHT MICROSCOPE (See:www.zygo.com/?/products/nv6000) (by SUNG).

Topological defects on surface weld sample

H in-plain =60 mT

H

H

MO contrast in in-plane field on topological defects, when field changes direction. MO contrasts are different on different defects: double and monochrome.

Machinery marks have double MO contrast (black and white)

Steps at GBs have only monochrome contrast

1 mm

H

1

32

4

5

In-plane field 60 mT, T=5.7KX, m

0 1000 2000 3000 4000

Inte

nsity

(a.

u.)

40

60

80

100

120

140

160

180

200

220

1

32

4

52 3

Enhancement of normal component Hz on steps at GBs, well visible on profile by Zygo microscope

1 mm

H in H in

MECHANICAL POLISHING AND BCP ON WELD SAMPLE

Mechanical polishing with 0.05 m Alumina Suspension

BCP condition

-HF(49%):HNO3(69.5%):H3PO4(85%) = 1:1:2

-Temp; ~13˚C, Time; ~7 min

Before mechanical polishing

After mechanical polishing: traces of GBs are still visible, but machinery marks (grooves) disappeared

Weld sample after mechanical polishing with 0.05 m Alumina Suspension and BCP

After BCP: grains and GBs are well visible, but machinery marks (grooves) disappeared

Zygo light microscope images and surface profiles of weld sample before and after mechanical polishing with 0.05 m Alumina Suspension and BCP

Before mechanical polishing

After mechanical polishing

After BCP

Mechanical polishing BCP

Weld sample after mechanical polishing and BCP

H=60 mT H=60 mTH=60 mT

TOP FACE

Nb O Al Si 41.29 % : 56.73% : 1.62% : 0.36%

SEM on BCP weld sample : BCP produces steps at GBs . As longer BCP as bigger steps

Our study found influence of topological features which are on the surface of fully processed Nb weld sample:

• The second chemical etch (BCP) creates steps at GBs (10-15 m ) and cause the surface to “dome” shape.

• Concentration of magnetic flux much stronger at surface steps than at grooves due to different sizes. The surface steps may be are one of the reasons the hotsport on the cavity surface, where nonuniform thermal breakdown actually happened.

• Sample with demagnetization factor Nz>0 in perpendicular field deforms external magnetic field and creates in-plane components. In-plane components concentrate magnetic flux at GB steps and ignites unusual flux penetration into the center of weld Nb. The “dome” surface contributes the propagation flux toward the edge.

• Mechanical polishing and BCP removed steps and “dome” surface and restored classical flux behavior.

JLAB single crystal with artificial defect (groove) on the surface

Surface, artificial notch

0.5 mm

Depth of notch is not the same on both edges

0.5 mm

Remn H=80 mT T=7KZFC H=40 mT T=7K

Artificial defect (groove) has small impact on flux distribution

no deformation

1 mm35% deformation 46% deformation

MO images of trapped flux in Nb discs deformed by compression at T=6K (samples deformed at Fermi Lab and received from A. Romanenko)

Sample for MO

1 mm

Weld Pit 1

Slice cut from welding area with PIT for MO

Pit in weld seam (Fermi from L. Cooly)

Pit 1 contour map: “moat” around the peaks

1 mm

#20 T=7K ZFC H=40 mT

#27 T=7K ZFC H=100 mT

#28 T=7K H=0 Remn after H=100 mT

MO images in PIT area: obviously more stronger resistivity to flux penetration

Pit 1 in weld seam

Optical, cross-section area with PIT

SUMMARY• MOI is a sensitive tool to check superconducting property of Nb for cavity application

• GBs can admit magnetic flux and depress superconductivity, if magnetic field parallel to plane of GBs

• GBs do not admit magnetic flux in case of random orientation of GB.

• Some additional flux penetration has been found on surface of many fully processed samples. We do not know the reason. It will takes some more study.

• In-plane orientation of magnetic field: detect enhancement of magnetic field on topological defects

• Topological defects stimulate the nucleation of magnetic flux in Nb and may depress superconductivity and Q-drop can be triggered by early flux penetration