Investigations of Instability in High Jc Nb3Sn Strands and Cable

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Investigations of Instability in High Jc Nb3Sn
Strands and Cable

• Arup Ghosh, Lance Cooley and Arnold Moodenbaugh

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Acknowledgements

• BNL SSTF Personnel
• J. DAmbra
• P. Philipsberg
• A. Werner
• E. Sperry
• Jeff Parrell (OST) for strand samples.

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Cable testing

• Testing Nb3Sn Cable sample is far more
  challenging than NbTi cable.
• Samples are assembled similar to coil
  fabrication.
• Epoxy impregnation
• careful sizing of the sample composite
• uniform application of transverse pressure

Cable test station 75mm bore dipole magnet with
field to 7.5T(4.2K) , test currents to 25kA
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Status of cable testingDec03

• 12.3mm wide cables using ITER strand (Jc 450-750
  A/mm2 _at_12T) have been successfully tested in the
  5-7T applied field range.
• Similar cable using MJR strand (Jc 2000 A/mm2
  _at_12T) have reached 85 of the expected Ic after
  considerable training, (background field of 7T).
  Further cable tests are being made to understand
  these results.
• Conductor instability (magnetic ?) was examined
  for a 28-strand FNAL cable made using 1.0mm
  strand. Quench currents were observed to be
  independent of applied field with quenching
  occurring in the very low field region, (more on
  this by G. Ambrosio this afternoon)

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Sample test configuration
BA
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Longitudinal Cable Configuration
Field Profile in 7T Applied Field
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ITER Cable in // Configuration
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MJR-30 Strand Cable, Cu/non-Cu 1.5Strands not
Sintered
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Quench History and RR-Effect
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Limitation of cable performance

• Sample training due to mechanical motion of
  strands within the cable
• Ramp rate dependence suggests that
  current-sharing could be a problem in un-sintered
  cable high RC and RA
• Test cable with sintered strands ( mimic
  wind-and-react) to offset this problem.
• Another possibility is some form of instability
  in the cable
• FNAL magnet experience and test of MJR cable at
  BNL suggests instability of the cable at low
  fields.
• Calculations by Vadim Kashikhin indicate that
  flux-jump instability can limit cable performance
  in magnets ?(cable tests can suffer from similar
  effect)

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Flux-Jump Instability
The individual sub-elements of high Jc
internal-Sn Nb3Sn multi-filamentary strands
behave as a solid tube of superconductor of large
diameter 60-100 mm . This leads to magnetic
instability at low fields as seen in
magnetization measurements.
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Flux-Jump Instability

• The critical state of the superconducting
  filaments may become unstable because of two
  inherent properties of high-field high current
  superconductors
• Jc decreases with increasing current (-dJc/dT)
• Flux-motion within the superconductor generates
  heat.
• The Stability parameter
• has to be less than 3 to ensure conductor
  stability from flux-jumps.
• Jc is the current density of the filaments
• d is the filament diameter
• C is the heat-capacity (increases with increasing
  temperature and
• Tc is the critical temperature (field-dependent)

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BNLs Strand Test Barrel
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Traditional V-I curveOST RRP 7054, 0.72 mm
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Non-Traditional V-H plot
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Transient Quench Data
QV 2 m/s
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OST 7054 0.72 mm Dsub-element 70 mm
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OST 7054 ISET at Quench-Threshold
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Voltage Recovery
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OST 6555 0.82 mm Dsub-elm 80 mm
Courtesy Jeff Parrell OST
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Ramp Rate Effect on Flux-Jump
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Quench at dB/dt 10 mT/s
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Sintered MJR 12 mm Wide Cable
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Sintered MJR 12 mm Wide Cable
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Example of a V-H plot for Cable
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Quench Threshold
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Sample Configuration
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Summary

• Flux-jumps can induce quenching of strand and
  cable at low fields.
• Quench threshold for the current OST RRP strand
  is 1800-2000 A/mm2
• Plan to measure other high Jc strands
• Question of Dynamic Stability
• Effect of Cu RRR and cooling conditions
• Question How does one stabilize the cable at low
  fields in the presence of this magnetic
  instability
• Need to develop high-Jc strands that have higher
  low-field-quench-Jc thresholds ( i.e.. lower Deff
  )