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Title: Evidence for preferential flux flow at the grain boundaries of superconducting RF-quality niobium

Abstract

Here, the question of whether grain boundaries (GBs) in niobium can be responsible for lowered operating field (B RF) or quality factor (Q 0) in superconducting radio-frequency (SRF) cavities is still controversial. Here, we show by direct DC transport across planar grain boundaries isolated from a slice of very large-grain SRF-quality Nb that vortices can preferentially flow along the grain boundary when the external magnetic field lies in the GB plane. However, increasing the misalignment between the GB plane and the external magnetic field vector markedly reduces preferential flux flow along GB. Importantly, we find that preferential GB flux flow is more prominent for a buffered chemical polished than for an electropolished bi-crystal. The voltage-current characteristics of GBs are similar to those seen in low angle grain boundaries of high temperature superconductors where there is clear evidence of suppression of the superconducting order parameter at the GB. While local weakening of superconductivity at GBs in cuprates and pnictides is intrinsic, deterioration of current transparency of GBs in Nb appears to be extrinsic, since the polishing method clearly affect the local GB degradation. The dependence of preferential GB flux flow on important cavity preparation and experimental variables, particularly, the final chemicalmore » treatment and the angle between the magnetic field and the GB plane, suggests two more reasons why real cavity performance can be so variable.« less

Authors:
ORCiD logo [1]; ORCiD logo [2]; ORCiD logo [3]; ORCiD logo [2]
  1. National High Magnetic Field Lab., Tallahassee, FL (United States); Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
  2. National High Magnetic Field Lab., Tallahassee, FL (United States)
  3. Old Dominion Univ., Norfolk, VA (United States)
Publication Date:
Research Org.:
Florida State Univ., Tallahassee, FL (United States); Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
Contributing Org.:
Fermilab; Thomas Jefferson National Accelerator Laboratory
OSTI Identifier:
1422033
Alternate Identifier(s):
OSTI ID: 1436710
Report Number(s):
FERMILAB-PUB-18-126-TD
Journal ID: ISSN 0953-2048
Grant/Contract Number:  
FG02-07ER41451; AC02-07CH11359
Resource Type:
Accepted Manuscript
Journal Name:
Superconductor Science and Technology
Additional Journal Information:
Journal Volume: 31; Journal Issue: 4; Journal ID: ISSN 0953-2048
Publisher:
IOP Publishing
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; 43 PARTICLE ACCELERATORS; 36 MATERIALS SCIENCE; SRF; Niobium; Grain Boundaries

Citation Formats

Sung, Z. -H., Lee, P. J., Gurevich, A., and Larbalestier, D. C. Evidence for preferential flux flow at the grain boundaries of superconducting RF-quality niobium. United States: N. p., 2018. Web. doi:10.1088/1361-6668/aaa65d.
Sung, Z. -H., Lee, P. J., Gurevich, A., & Larbalestier, D. C. Evidence for preferential flux flow at the grain boundaries of superconducting RF-quality niobium. United States. doi:10.1088/1361-6668/aaa65d.
Sung, Z. -H., Lee, P. J., Gurevich, A., and Larbalestier, D. C. Mon . "Evidence for preferential flux flow at the grain boundaries of superconducting RF-quality niobium". United States. doi:10.1088/1361-6668/aaa65d. https://www.osti.gov/servlets/purl/1422033.
@article{osti_1422033,
title = {Evidence for preferential flux flow at the grain boundaries of superconducting RF-quality niobium},
author = {Sung, Z. -H. and Lee, P. J. and Gurevich, A. and Larbalestier, D. C.},
abstractNote = {Here, the question of whether grain boundaries (GBs) in niobium can be responsible for lowered operating field (BRF) or quality factor (Q0) in superconducting radio-frequency (SRF) cavities is still controversial. Here, we show by direct DC transport across planar grain boundaries isolated from a slice of very large-grain SRF-quality Nb that vortices can preferentially flow along the grain boundary when the external magnetic field lies in the GB plane. However, increasing the misalignment between the GB plane and the external magnetic field vector markedly reduces preferential flux flow along GB. Importantly, we find that preferential GB flux flow is more prominent for a buffered chemical polished than for an electropolished bi-crystal. The voltage-current characteristics of GBs are similar to those seen in low angle grain boundaries of high temperature superconductors where there is clear evidence of suppression of the superconducting order parameter at the GB. While local weakening of superconductivity at GBs in cuprates and pnictides is intrinsic, deterioration of current transparency of GBs in Nb appears to be extrinsic, since the polishing method clearly affect the local GB degradation. The dependence of preferential GB flux flow on important cavity preparation and experimental variables, particularly, the final chemical treatment and the angle between the magnetic field and the GB plane, suggests two more reasons why real cavity performance can be so variable.},
doi = {10.1088/1361-6668/aaa65d},
journal = {Superconductor Science and Technology},
number = 4,
volume = 31,
place = {United States},
year = {2018},
month = {2}
}

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Figures / Tables:

Figure. 1 Figure. 1: (a) Overview of the large-grain (grain size > ~5-10 cm), high-purity (RRR ≥ 250) niobium ingot slice (2.5mm thickness), provided by TJNAF (Thomas Jefferson National Accelerator Facility) for this study. (b) The top and (c) bottom surface view of an I-shape (dog-bone) bi-crystal sample used for DC transportmore » measurement after ~50 μm layer removal with BCP. The GB groove indicated by blue dotted arrows in (b) & (c) appeared after BCP. (d) Schematic diagram of the angular direction ($θ$) between applied magnetic field (H) and the plane of GB and the direction of the transport current (I) on the I-shape sample. This bi-crystal sample was extracted from the regions indicated by the dotted blue rectangles in (a). The disorientation angle of the GB plane to the surface is ~ 5° and the crystallographic misorientation across the GB is ~ 26°.« less

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