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This content will become publicly available on February 19, 2019

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

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:
Report Number(s):
FERMILAB-PUB-18-126-TD
Journal ID: ISSN 0953-2048
Grant/Contract Number:
FG02-07ER41451; AC02-07CH11359
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
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 Orgs:
Fermilab; Thomas Jefferson National Accelerator Laboratory
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
OSTI Identifier:
1422033
Alternate Identifier(s):
OSTI ID: 1436710