1. The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University 7 th SRF MW Investigation: Variation of Surface Superconductivity with cold-work deformations of Superconducting RF quality Niobium ZH. Sung 1, P.J. Lee 1, and D.C. Larbalestier 1 A. Dzyuba 2 and L. Cooley 2 1. NHMFL-ASC, FSU 2. FNAL Acknowledgements: The work is based upon work supported by FNAL Award Number 570362 and the State of Florida. B. Starch for wire fabrication

2. Applied Superconductivity Center National High Magnetic Field Laboratory Florida State Universit y Can we quantify the deterioration of surface RF superconducting properties by different level of defects? What are the surface defects? Grain boundaries, Oxides, Hydrides, Dislocations, Vacancies, or severe topology?... etc Here, we compare surface SC properties (λ≈40nm) with bulk properties on the same measurement. We mechanically deformed a high purity Nb rod as a wire shape in order to increase density of undefined-surface defects. We explored variation of surface SC properties by healing these surface damages.

3. Applied Superconductivity Center National High Magnetic Field Laboratory Florida State Universit y Nb-wire was fabricated by a wire-drawing! Nb Cu clad As-drawn Roughness ~25-35μm Local strain map of cross section of an as- drawn wire by EBSD 2° threshold 3° threshold Annealed Nb rod starting Ø~0.428”(10.9 mm) & 6 ”in length (152.4 mm): starting with ɛ 0 = 0 Final Nb size.044 (1.1 mm) ~ 90% reduction / ɛ 0 = 4.17 Higher possibility of lower misorientation strained structures at GBs by SLCM Cu removed by HNO 3

4. As- drawn. (No EP) 50m EP 2h EP3h EP 50m EP+ 120°C/ 48 hrs 2hrs EP +120°C/ 48 hrs 3hrs EP +120°C/ 48 hrs 50m EP +800°C/ 2 hrs 2hrs EP +800°C/ 2 hrs 3hrs EP +800°C/ 2 hrs Variation of Surface roughness, micro-structure, and micro-chemistry 50min EP’ed 2hrs EP’ed 3hrs EP’ed 50m EP’ed: 18.07 ± 0.19 μm Roughness by SLCM: Valley to peak 2hrs EP’ed: 2.21 ± 0.14 μm 3hrs EP’ed: 1.67 ± 0.08 μm Local strain map of cross section of an as-drawn+EP+120°C HT wire by EBSD High density of strained structures on the top layer 2° threshold 50min EP’ed+120°C 2hrs EP’ed +120°C HT 3hrs EP’ed+ 120°C HT 2° threshold Removed?Why? More strained structure? Localized?

5. Applied Superconductivity Center National High Magnetic Field Laboratory Florida State Universit y S. Casalbuoni showed H c3 ( 1.695 and B 3 variation: here we investigate this ratio (possible oxide or hydride source, or preferential diffusion channel). 2.5mm by 2.8mm disk-shape by MPMS2 10 Hz, 10m Oe, 5K by S. Casalbuoni (2005) Ø~0.8mm Ɩ~8mm wire-shape by PPMS 100 Hz, 0.1 Oe, 5K – Imaginary part H c2 H c3 Increase of pinning in surface sheath?

6. Applied Superconductivity Center National High Magnetic Field Laboratory Florida State Universit y @ 5KH c2 [Oe]H c3 [Oe]r 32 50 min EP359869901.943 2 hrs EP382573591.924 3 hrs EP398667191.686 50 min EP+120°C HT431476311.769 2 hrs EP+120° C HT417575761.814 3 hrs EP+120°C HT355372812.049 50 min EP+800°C HT393176311.941 2 hrs EP+800C° HT409269311.694 3 hrs EP+800C° HT447069031.544 Imaginary part @ 0.1 Oe & 100 Hz Onset of H c3 : deviation from the linear fitting of normal state (@10K) r 32 increases by longer EP on deformed Nb, perhaps because of impurities in surface sheath. Mild heat (120 0 C) treatment may enhance migration of O or H toward surface along dislocations and GBs. @5K @10K H c3 ( ≤ λ = 40nm) and H c2 (bulk) H c3

7. What is the main medium for immigration of impurities? GBs? Or Dislocations at GBs? Or Oxide layers along GBs? GB Dislocations HR TEM image of As-drawn-50m EP Nb wire Dislocations Dislocations Nb oxide Nb sub oxide layer Atomic structure by Cs- corrected STEM RRR 316 Nb film by CED Nb Oxide layer is always unordered structure Oxide Nb (001) Oxide Nb Nb Au-Pd Sub-Oxide

8. Applied Superconductivity Center National High Magnetic Field Laboratory Florida State Universit y Discussion! This technique looks very promising! But.....??? How long should we heat-treat or EP cavities? Would be better study on single crystals. How representative can we make TEM samples? …… etc.

9. Applied Superconductivity Center National High Magnetic Field Laboratory Florida State Universit y Thanks!

10. Applied Superconductivity Center National High Magnetic Field Laboratory Florida State Universit y Below is the back-up slide!

11. Applied Superconductivity Center National High Magnetic Field Laboratory Florida State Universit y AC susceptibility (within external DC field) is a direct way to trace H c3, but significantly relies on AC amplitude and frequency L. V. Sawilski PhD thesis, Hamburg, 2002 R.W. Rollins and J. Silcox. Phy. Rev. 1967 Frequency dependence f(AC ampl.) f(Hz)

12. Applied Superconductivity Center National High Magnetic Field Laboratory Florida State Universit y Wire Drawing Schedule & strain ratio Die sizeɛ0ɛ0 0.409642 0.3647960.232 0.3248610.464 0.2892970.696 0.2576270.928 0.2294231.16 0.20431.39 0.18191.62 0.1621.86 0.14432.09 0.12852.32 0.11442.55 0.10192.78 0.09073.02 0.08083.25 0.0723.48 0.06413.71 0.05713.94 0.05084.17 Nb rod starting dia..428”(10.87mm)and 6”in length (152.39mm) Copper tube.500”dia.(12.7mm) 6”(152.39mm)length.032”(.812mm) wall Final Nb size.044 (1.11mm) ~ 90% reduction/ ɛ 0 =4.17 Nb wires were produced by a deep drawing!: severe work hardening Nb Cu clad

13. Applied Superconductivity Center National High Magnetic Field Laboratory Florida State Universit y 2° threshold IFP IFP IFP Cross sectional EBSD 50min EP’ed +800°C HT/2hrs 2hrs EP’ed +800°C HT/2hrs 3hrs EP’ed +800°C HT/2hrs

14. Applied Superconductivity Center National High Magnetic Field Laboratory Florida State Universit y EP’ed samples @ 0.1 Oe & 100 Hz EP’ed +120°C HT samples @ 0.1 Oe & 100 Hz AC susceptibility characterization shows response of surface and bulk SC properties within external DC field. H c2 H c3 H c2 H c3