ABSTRACT MOI Nb bi-crystals

The MO technique enables both local and global magnetic fluxbehavior to be visualized using the double Faraday effect in a garnetindicator film in the reflective mode with in-plane magnetization,and hence reveals the relationship between microstructural defectsand magnetic flux penetration and trapping below the criticaltemperature (Tc = 9.3 K in Nb).

SUMMARY

MOI is a powerful tool for the visualization of magnetic fluxbehavior and current distribution in Nb for SRF cavity application.

Acknowledgement: Supported by US Dept. of Energy award DE-SC0009960. A portionof this work was performed at NHMFL, which is supported by NSF CooperativeAgreement No. DMR-1644779 and the State of Florida

Magneto-Optical Imaging of SRF NbAnatolii Polyanskii1, Shreyas Balachandran1, Peter J. Lee1, Mingmin Wang2,, Thomas R. Bieler2

1Applied Superconductivity Center, National High Magnetic Field Laboratory, Tallahassee, FL 32310, USA 2 Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824, USA

Magneto-optical imaging (MOI) is an important tool for themeasurement of the spatial distribution of magnetic andsuperconducting properties in the presence of microstructuralchanges and defects. This technique is based on ferrimagneticgarnet indicator films, which have a high angle Faraday rotation ofvisible light in a polarized microscope. We have been able to applythe MOI techniques to evaluate high purity SRF Nb with varyinglevels of microstructure and surface changes. The MOI techniquehas been able to capture changes resulting from variations inprocessing recipes for SRF Nb for high-efficiency RF applications. Inthis presentation, we review MOI as a diagnostic tool for high purityNb for SRF accelerator applications and show examples of MOI onSRF Nb on coupon samples processed with the latest recipes forhigh gradient and high-quality factor applications.

Hext

Z

NbNbNb

MO indicator

MSC

Calculated current streamlines

MO images

MAGNETO-OPTICAL IMAGING (MOI)

1 mm

1 mm

GB #1

T=5.5 K H=80 mT

17.8° GB17.8° GB 17.8° GB

Current streamline : GB is obstacle to current flow:

Jb = 0.5JcNb JLAN #3A bicrystal, Jc in grain and across GB

T, K

5 6 7 8 9 10

Jc,

kA

/cm

2

0

10

20

30

40

50

60

Jc, Grain

Jc, GB

Ratio Jgb/Jb

T, K

5 6 7 8 9 10

R=

Jgb/J

b

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

#9 H=28 mT

#11 H=32 mT #13 H=40 mT #23 H=0 FC T=6 K

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

1 mm

GB#2

GB#2

GB#2

Nb bi-crystal thin film Jlab#1, t=1.2 μm

H=0 (FC in H=120 mT)H=8 mT H=20 mTT=6 K H=5 mT

T=6 K H=8 mT H=16 mT H=24 mT

Nb bi-crystal thin film Jlab#2, t=~1.0 μm

H=0 (FC in H=24 mT)

Two bi-crystals grown on STO substrate at TJNAF (by A-M. Valente @ TJNAF)

GB

GB

MOI Nb bi-crystal thin film

Top surface. After Mechanical polishing + Electropolishing

At T below 7 K flux triggers to dendritic instability

MOPFDV005

See poster THPCAV015 for more details