Critical current degradation in rebco and BI-2223 superconductors under torsional and tensile strain

Kosse, Jaap (2013) Critical current degradation in rebco and BI-2223 superconductors under torsional and tensile strain.

Abstract:Measurements were done on superconducting Bi-2223 and ReBCO tapes, investigating the behaviour of the critical current, , under torsional and tensile strains. For this purpose a new insert was made that would allow these strains to be applied while cooling the samples to using liquid nitrogen. The new probe will be used for testing different ReBCO tapes from various manufacturers aiming for the selection of the most suitable tape for magnet coils in DEMO, the next nuclear fusion reactor after ITER. The samples were soldered to copper contacts on the sample holder in order to secure the sample and to provide the current. The sample holder is designed to allow stress-free contraction while cooling down to measurement temperatures. is determined from measured curves using the standard four probe method. After every measurement the applied strain is removed and a measurement is performed to determine the reversibility of the degradation. The n-index gives an indication of how the electric field depends on the current in the transition region; it is obtained by fitting measured data to the equation: ( ) where is the electric field criterion. Simulations in COMSOL using the finite element method (FEM) help to provide an understanding of stress profiles in the ReBCO tape resulting from torsional strain, valid in the elastic stress-strain region. From comparing the stress-to-yield stress ratios across the different materials in the tape, it can be concluded that the copper sheets are the first to show plastic deformation, after which a redistribution of stress will occur. A Bi-2223 tape was measured first to examine if the insert works properly. The torsion test done on this sample showed that this old Bi-2223 tape permanently damages on applying torsional strain, with almost no reversibility observed. The ReBCO tape used is the SCS4050 tape, manufactured by SuperPower. For the ReBCO tape under strain, the homogeneity of the across taps is good up to the critical strain, , after which divergent behaviour can be observed. Reversibility is very high until around , after which it drops sharply. The -index shows a slow decrease at higher strain which is still present after the strain has been removed. It is very likely that damage resulting from tensile strain will occur because of the formation of cracks at localized sites, while damage occurring from torsional strain occurs more gradually because cracks can propagate more uniformly along the tape. This explains the more sudden -degradation under tensile strain compared to torsional strain. For the Nb3Sn wire the plan was to measure the tensile strain at which the reaches it’s maximum with a parallel magnetic field of different magnitudes; this strain is taken as . After this, a torsion measurement would be done, at and the found in the preceding measurement. For this purpose the insert was changed for wire-testing; when this modified insert was tested, it was found that the wire fixture was misaligned resulting in a large Lorentz force on the sample. Also, the copper tubes providing the current caused too high helium evaporation. Modifications to this setup are being performed for further testing at high magnetic field.
Item Type:Essay (Bachelor)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Applied Physics BSc (56962)
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