# Presentation: 465820

## Flux Penetration in SRF Cavity Quality Nb – especially at Grain Boundaries 

**Anatoly Polyanskii, Peter Lee, Alex Gurevich,  Alex Squitieri, David Larbalestier**
** ****Applied Superconductivity Center at the University of Wisconsin-Madison**
** **
**Pierre Bauer, Leo Bellantoni, Christian Boffo and Helen Edwards **
** ****Fermilab, Batavia, IL**

-   International Linear Collider Meeting, Fermilab October 5-7, 2005 

- Special thanks to Peter Kneisel and Ganapati Myneni for supply of a very large grain Nb ingot slice
- Experimental work has been supported by Fermilab
- Modeling study of Gurevich was supported by DOE-HEP

## Motivation, methods and approach

- In 2004 we found by magneto-optical (MO) imaging that some Nb grain boundaries (GBs) showed preferential flux penetration below that seen in the grains
- Was this evidence for depressed superconductivity at some Nb GBs and if so what causes it?
- Nb has a long coherence length (~40nm), as compared to all high field superconductors, yet is seemed to be showing some characteristics of a high-Tc superconductor with low carrier density and much shorter  (1-3nm)
- Plan:  Observe the MO flux penetration, the magnetization and the surface topology of a through-process sample set made at Fermilab
- In early 2005 ingot slices from the JLab single crystal cavities became available
- we cut slices from large grain material so as to observe single GB properties in transport too
- Model the RF performance of Nb (Alex Gurevich)

## MO method

- Working range is down to 6K and up to about 130 mT – to above Hc2

## Experiment Sequence

- Experiment I uses nominally (but not) identical samples
- Regular fine grain Nb Cavity sheet
- Simulated welds in the same sheet
- Experiment II uses the same sample throughout
- Samples cut from large-grain JLab ingot
- Same sample is being taken through the whole cavity optimization treatment sequence

## MO images and magnetization on same samples

**Fine grain**

**Weld**

## 20 mT

- Hp=32 mT

- 40 mT

- 60 mT

- 80 mT

- 1

- Weld 1--- cold work

- 100 mT

- 0 mT

**Weld sample 1**
**As cut from plate**

## Hp=26 mT

- 2

- 40 mT

- 60 mT

- 48 mT

- Weld 2--- 100 min etch

- 0 mT

- 20 mT

**Weld sample 2**
**100 minute etch**

- Fairly uniform flux entry beyond Hp

## 20 mT

- 3

- Weld 3--- +7500C

- 32 mT

- jumps

- 32.4 mT

- 36 mT

- 40 mT

- 38 mT

- 0 mT

**Weld sample 3**
**After 750 C HT**

- Less uniform flux entry beyond Hp

## 20 mT

- 3

- Weld 3--- +7500C

- 32 mT
- Flux did not trapped at H=0 and Hp-?

- jumps

- 32.4 mT

- 36 mT

- 40 mT

- 38 mT

- 0 mT

**Weld sample 3**
**Virgin magnetization loop after 750C HT**

- Clearly non-uniform flux entry

## 16 mT

- 4

- Weld 4--- 20 min etch

- jumps

- 20 mT

- 30.8 mT

- 34 mT

- 40 mT

- 0 mT

**Weld sample 4**
**20 min BCP etch**

- Flux enters to the center very early – through “leaky” GB (we believe)

## 24 mT

- 5

- Weld 5--- +1200C bake

- 32 mT

- 33.6 mT

- 34 mT

- 40 mT

- jumps

**Weld sample 5**
**After 120 C bake**

- Hysteresis is further reduced but central flux appearance again occurs

## Regular samples

- Optical images of surface machine marks

- ZFC images show increasingly irregular flux penetration

- FC images after cooling to 7 K in H  Hc2 = 110 mT show more uniform flux distribution

**Regular samples**

## Weld samples

- GBs are visible through the surface machining marks

- ZFC images show increasingly irregular flux penetration

- H=0 FC images after cooling to 7 K in H  Hc2 = 110 mT show initially uniform flux distribution, which is progressively more perturbed in later process steps

**Weld samples**

## Large Grain Ingot Slice Experiments

- Ingot slice courtesy of Peter Kneisel and Ganapati Myneni (Reference Metals Nb)

## Some GBs admit flux, others do not.............

- Uniform roof top pattern for sample #4 with inclined GB

- Non-uniform roof top pattern for sample #5 with almost straight GB

- 7.5 K , 12.4 mT

*J*c=2.7x103A/cm2
*J*GB=8.8x102A/cm2

## Field dependence of flux penetration

- Sample #5 ZFC T=7.5K

- Grain boundary clearly distorts the field penetration

## RRR across GB vs. the grain

- Intragrain
- ~3 mm

- Intergrain
- ~5 mm

- B

**RRR across grain boundary –   187**
**Within grain - 211**

***Measured at Room (291K) Temp and  4.19K using the zero field resistance  back extrapolated from in-field measurements between 2-5T***

## Critical current in grain and across GB

- Superconducting signal at 500 mT shows effect of cold work during cutting

- ~ohmic flux flow across GB occurs before grain and triggers grain – suggestive of AJ vortex flow at GB

## Summary

- There is abundant evidence that *some* Nb grain boundaries show early flux penetration
- Sensitivity is greatest close to Hp ~ Hc1
- Penetration has some dependence on the treatment given the Nb
- Optimization treatments seem to be enhancing the variability of properties
- Crystallography of the GB may be important
- Topology on the micron scale does not seem to be driving the penetration
- It is very striking that we can reproduce some aspects of High-Tc performance in Nb!
- A near-term goal is to apply our well developed understanding of HTS GBs to Nb
- AJ vortices, atomic scale segregation (Song et al. Nature Materials 4, 470-475  2005)
- Plans - MO to sort good and bad GBs, topography using SEM or confocal, crystallography by EBSD, transport across single GBs, TEM of the good and bad GBs

## Large peaks Ca segregation in tensile regions, troughs in compressive regions

- X. Song, G. Daniels, D. M. Feldmann, A. Gurevich, and D. C. Larbalestier, "Electromagnetic, Atomic-Structure and Chemistry Changes Induced by Ca-doping of Low Angle YBCO Grain Boundaries," Nature Materials, 4, 470-475, 2005.

- The HTS analog - Ca segregation at a YBCO GB

## Josephson core size: 
The Josephson cores overlap if   > a 
 	 (Gurevich, PRB48, 12857 (1993); PRB46, R3187 (1992)): 

Viscous flux motion 
R(B) is independent of B, if a single vortex
	chain moves along GB, while  > a

**vortex rows along GB:**
**R(B) = w(B)********n****B/B****c2**

**Josephson core size: **
**The Josephson cores overlap if  ******** ****> a**** **
** ****	**** ****(Gurevich, PRB48, 12857 (1993); PRB46, R3187 (1992))****: **
**Viscous flux motion **
**R(B) is ****independent of B****, if a single vortex**
**	****chain moves along GB, while ******** ****> a**

**Single vortex-chain motion along pure YBCO GB**

**V = (I - I****b****)R**

**H > (J****b****/J****d****)****2**** H****c2**

****** ****= ********J****2****/******** = ********J****d****/J****b**

**Gurevich et al. PRL 88, 097001 (2002)**