1. 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

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). Local study of flux penetration is important

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

4. EXPERIMENTALTECHNIQUE : 1. MAGNETO-OPTICAL 2. VSM 3. SEM 4. ZYGO LIGHT MICROSCOPY by Z. H. Sung (See:www.zygo.com/?/products/nv6000)www.zygo.com/?/products/nv6000

5. Protective layer   F =V B z 2d d Ba,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 layer Nb

6. THE FARADAY ROTATION IN A FERRIMAGNETIC Bi-DOPED IRON GARNET INDICATOR FILMS WITH IN-PLANE MAGNETIZATION LIGHT POLARIZATION

7. MAGNETO-OPTICAL CONTRAST PICKS UP ON PLASTIC REFRIGIRATE CARD: DISTRIBUTION OF STRAY FIELD AROUND MAGNETIC STRIPS

8. 5.25 inch disc 3.5 inch disc Audio tape MO IMAGES OF MAGNETIC FIELD IN AUDIO TAPE AND COMPUTER DISCETS 500  m100  m

9. (AAA) American Automobile Association Card Library Vendor Card Visa Credit Card 500  m MO IMAGES OF MAGNETIC CODES IN DIFFERENT PLASTIC CARDS

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

11. cryostat MAGNETO-OPTICAL IMAGING SETUP FOR SUPERCONDUCTING RESEARCH: electromagnetic system creates magnetic field in two direction: X and Z Polarized Optical Microscope H ext Z X LHe

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

13. Hz=(1/1-Nz) x H ext H ext 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  Nb MO indicator MO indicator with in-plane magnetization detects the normal component of magnetic flux

14. 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”)

15. M SC 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”)

16. 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”

17. 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

18. JLAB samples with big grains were cut from extremely large- grain ingot Samples 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

19. Tri-crystal Bi-crystal GB (#1) Normal to Surface Bi-crystal GB (#1) Normal to Surface Bi-crystal GB (#2) Angle to Surface Bi-crystal GB (#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-crystal Thickness of sheet is 1.88 mm GB #2 GB #1 RF field in-plane

20. 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

21. 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

22. MO imaging sample on different surface: when GB - 33 0 angle and after 90 0 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 90 0 H MO indicator 2.78mm 2.17mm 1.89mm

23. H=80 mT T=5.4K H=100 mT Bi-crystal #6 with the 33 0 angle GB #2 to surface, no remarkable flux penetration along GB 1 mm Optical, GB#2, angle to surface 33 0

24. MO Image and current distribution in sample with perpendicular plane, ZFC, T=5.5K. 17 0 GB #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 17.8 0 GB Current streamline : GB is obstacle to current flow J b =0.5J c

25. Misorientation angle between two grains ≈17.8 o Orientation Imaging Microscopy (OIM): from D. Abraimov 17.8 0 GB Misorientation profile

26. Critical current vs T : bulk Jc and Jb across 17.8 0 GB (from MO measurement) Nb 17.8 0 GB T=5.5 K J b ~ 0.5J c YBCO 5 0 GB T=7 K J b ~ 0.5J c A.A. Polyanskii, at el, Phys. Rev. B, 53, 8687, (1996).

27. MAGNETIC FLUX BEHAVIOR AROUND 3 o, 5 o, and 10 o GRAIN BOUNDARIES in YBCO bi-crystals T = 7 K, H ~ 40 mT, H||c 10 o GB J b /J c =0.95 A.A. Polyanskii, at el, Phys. Rev. B, 53, 8687, (1996). 5 o GB 3 o GB J b =0.95J c J b =0.5J c J b =0.065J c

28. #9 H=28 mT #11 H=32 mT#13 H=40 mT After 90 0 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

29. 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 TOPBOTTOM

30. MO image of top and bottom surfaces of Nb sample after (EP): good flux penetration on both surfaces ZFC H=60 mT T=6.5K Current streamline on top side: GB is obstacle to current flow TOPBOTTOM Nb sample with GB perpendicular to surface.

31. ROUND JLAB SAMPLE, with random oriented GBs in triple-point Bottom face Top face

32. #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

33. JLAB #11 with GB #2 with angle 30-35 0, 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 mT Surface, bottom face GB #2 1 mm

34. 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

35. 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

36. 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

37. 1. GBs can accept magnetic flux in case when plane of GB is parallel to external magnetic field 2. Some additional flux penetration has been found on surface of many fully processed samples. MO conclusion on Jlab samples with big grains

38. 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

39. 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

40. 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

41. MOI of Regular samples with small grains taken through a typical cavity optimization process: 5 steps 1 mm 80 мТ 34 мТ48 мТ40 мТ 60 мТ 30 мТ40 мТ34 мТ Optical ZFC Then H applied FC in 110 mT, then H=0 applied 2. 100 µm etch1. CW, degrease3. HT 5 hr/750 0 C5. Bake 50 hr/120 0 C4. 20 µm etch

42. MOI of Weld samples with big grains taken through a typical cavity optimization process: 5 steps 1 mm Optical ZFC Then H applied FC in 110 mT then H=0 H applied 80 мТ 40 мТ48 мТ40 мТ 60 мТ48 мТ36 мТ31 мТ 2. 100 µm etch1. CW, degrease3. HT 5 hr/750 0 C5. Bake 50 hr/120 0 C4. 20 µm etch

43. 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

44. 32 mT T=5.6K Hx 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

45. #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

46. #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 1 mm Flux profile taken across sample

47. 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

48. 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).www.zygo.com/?/products/nv6000 Topological defects on surface weld sample

49. 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

50. 1 32 4 5 In-plane field 60 mT, T=5.7K 1 32 4 523 Enhancement of normal component Hz on steps at GBs, well visible on profile by Zygo microscope 1 mm H in

51. MECHANICAL POLISHING AND BCP ON WELD SAMPLE Mechanical polishing with 0.05  m Alumina Suspension BCP condition -HF(49%):HNO 3 (69.5%):H 3 PO 4 (85%) = 1:1:2 -Temp; ~13˚C, Time; ~7 min

52. 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

53. 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

54. Mechanical polishing BCP Weld sample after mechanical polishing and BCP H=60 mT

55. 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

56. 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.

57. 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

58. Remn H=80 mT T=7KZFC H=40 mT T=7K Artificial defect (groove) has small impact on flux distribution

59. no deformation 1 mm 35% 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)

60. 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

61. 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

62. 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