The critical current of Nb/sub 3/Sn wires for ITER as a function of the axial tension and compression

Supercond. Sci. Technol.

2002

The engineering critical current density (J E ) and the index of transition, N (where E = αJ N ), of a Nb 3 Al multifilamentary strand, mass-produced as a part of the Fusion programme, have been characterized as a function of field (B), temperature (T ) and strain (ε) in the ranges B 15 T, 4.2 K T 16 K and −1.79% ε +0.67%. Complementary resistivity measurements were taken to determine the upper critical field (B C2 (T , ε)) and the critical temperature (T C (ε)) directly. The upper critical field defined at 5%ρ N , 50%ρ N or 95%ρ N , is described by the empirical relation B ρ N C2 (T , ε) = B ρ N C2 (0, ε) 1 − T T ρ N C (ε) ν . The upper critical field at zero 0953-2048/02/070991+20$30.00 © 2002 IOP Publishing Ltd Printed in the UK b 1 2where the Ginzburg-Landau (GL) parameter κ is given by the relation, γ is the Sommerfeld constant and t = T/T C (ε). At an applied field equal to the upper critical field found from fitting the Kramer dependence (i.e. at B C2 (T , ε)), the critical current is non-zero and we suggest that the current flow is percolative. The functional form of F P implies that in high fields the grain boundary pinning does not limit J E , this is consistent with J E -microstructure correlations in other superconducting materials.

Section: Discussionmentioning

confidence: 99%

“…(a) Normalized critical current as a function of intrinsic strain at 13 T and 4.2 K for measurements performed in Durham and FzK[10]. (b) Critical current as a function of field at various temperatures and zero strain between Durham and JAERI[59]. The solid lines shown are the technological fit to the data.…”

mentioning

confidence: 99%

The strain and temperature scaling law for the critical current density of a jelly-roll Nb3Al strand in high magnetic fields

Keys

Koizumi

Supercond. Sci. Technol.

2002

“…Detailed tests are therefore required of the effects of spring material and geometry, as well as comparisons between the different measurement techniques. Some such results have been presented for other types of bending spring [12,34,43], but results for helical springs are very limited.…”

Section: Introductionmentioning

confidence: 99%

“…The influence of thermal prestrain, elasticity of the spring and the transverse and longitudinal strain uniformity will be investigated in detail. We will evaluate the extent to which the intrinsic properties of conductors can be accurately measured (and hence to what degree the different measurement techniques are, in principle, equivalent [12,43]). Based on our experimental and FEA results, we will also make a number of recommendations about the design of helical springs, supplementing previous work by Walters et al [6].…”

Section: Introductionmentioning

confidence: 99%

Properties of helical springs used to measure the axial strain dependence of the critical current density in superconducting wires

Taylor

Supercond. Sci. Technol.

2005

The use of helical (Walters) springs is an effective technique for measuring the critical current density (J C ) of superconducting wires in high fields as a function of both compressive and tensile axial strains. We report J C versus strain measurements for Nb 3 Sn wires on a number of helical springs of different materials and geometries, together with results from finite element analysis (FEA) of these systems. The critical current density, n-value and effective upper critical field data are universal functions of intrinsic strain (to within ±5%) for measurements on four different spring materials. The strains on the wire due to the differential thermal contraction of the spring are equivalent to the applied mechanical strains and hence only produce a change in the parameter ε M (the applied strain at the peak in J C ). Variable-strain J C data for springs having turns with rectangular and tee-shaped cross-sections (and hence different transverse strain gradients across the wire) show good agreement when the strain-gauge calibration data are corrected to give the strain at the midpoint of the wire. The correction factors can be obtained from FEA or analytical calculations. Experimental and FEA results show that the applied strain varies periodically along the wire with an amplitude that depends on the spring material and geometry. We suggest that Ti-6Al-4V springs with an integer number of turns and optimized tee-shaped cross-sections enable highly accurate measurements of the intrinsic properties of superconducting wires.

“…32,33 An excellent bending beam apparatus was also developed at the University of Twente for short samples of LTS wires. [34][35][36] Its design gives access to variable temperature, variable strain J c measurements in the very highest dc magnetic fields (in vertical magnets) available, but has the drawback of relatively high E-field baselines (short samples with finite current transfer lengths 37,38 ). Our research aims broadly include contributing to finding HTS scaling laws analogous to the ones available for LTS materials and to optimizing HTS for high-field applications such as MRI 39 and fusion tokamaks.…”

Section: Introductionmentioning

confidence: 99%

Probes for investigating the effect of magnetic field, field orientation, temperature and strain on the critical current density of anisotropic high-temperature superconducting tapes in a split-pair 15 T horizontal magnet

Sunwong

Higgins

Review of Scientific Instruments

2014

(2014) 'Probes for investigating the eect of magnetic eld, eld orientation, temperature and strain on the critical current density of anisotropic high-temperature superconducting tapes in a split-pair 15 T horizontal magnet.', Review of scientic instruments., 85 (6). 065111.Further information on publisher's website: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. We present the designs of probes for making critical current density (J c ) measurements on anisotropic high-temperature superconducting tapes as a function of field, field orientation, temperature and strain in our 40 mm bore, split-pair 15 T horizontal magnet. Emphasis is placed on the design of three components: the vapour-cooled current leads, the variable temperature enclosure, and the springboardshaped bending beam sample holder. The vapour-cooled brass critical-current leads used superconducting tapes and in operation ran hot with a duty cycle (D) of ∼0.2. This work provides formulae for optimising cryogenic consumption and calculating cryogenic boil-off, associated with current leads used to make J c measurements, made by uniformly ramping the current up to a maximum current (I max ) and then reducing the current very quickly to zero. They include consideration of the effects of duty cycle, static helium boil-off from the magnet and Dewar (b ), and the maximum safe temperature for the critical-current leads (T max ). Our optimized critical-current leads have a boil-off that is about 30% less than leads optimized for magnet operation at the same maximum current. Numerical calculations show that the optimum cross-sectional area (A) for each current lead can be parameterized by . A split-current-lead design is employed to minimize the rotation of the probes during the high current measurements in our high-field horizontal magnet. The variable-temperature system is based on the use of an inverted insulating cup that operates above 4.2 K in liquid helium and above 77.4 K in liquid nitrogen, with a stability of ±80 mK to ±150 mK. Uniaxial strains of −1.4% to 1.0% can be applied to the sample, with a total uncertainty of better than ±0.02%, using a modified bending beam apparatus which includes a copper beryllium springboard-shaped sample holder. © 2014 AIP Publishing LLC. [http://dx