B. ten Haken

The strain dependence of the critical properties of Nb3Sn conductors

The critical current (Ic) of six different Nb3Sn multifilamentary wires is investigated as a function of temperature, magnetic field, and strain. A relation for a critical temperature (Tc) that depends on the deviatoric strain is proposed and applied to interpret the results. First, a short review is given on the flux-pinning relations that are used to introduce a strain dependent Tc in a relation for the Ic as a function of field and temperature. The conductor samples are investigated in two different deformation states, namely, in a spiraled shape on a Ti sample holder and a straight section soldered onto a brass substrate. The brass substrate is used to apply a compressive or tensile axial strain to the conductor. The Ic in the different samples prepared from a single conductor type can be described very well with a single set of critical properties and strain parameters. In particular, in the strain regime where the matrix deformation is limited and the superconductor is axially compressed, the propos...

Strain effects in Nb3Al multifilamentary conductors prepared by phase transformation from bcc supersaturated-solid solution

Strain effects on critical current densities have been examined for conductors containing nearly stoichiometric Nb3Al filaments with fine grains. The Nb3Al phase in these multifilamentary conductors are prepared by phase transformation from supersaturated Nb(Al) bcc solid solution and show high-field critical current densities much larger than those for conventionally prepared Nb3Al conductors, where the Nb3Al phase is known to be off-stoichiometric. The degradation of critical current densities with −0.7% intrinsic strain is ca. 20% at 12 T, comparable with those for conventional Nb3Al conductors of high strain tolerance.

A device to investigate the axial strain dependence of the critical current density in superconductors

We have developed an instrument to study the behavior of the critical current density (Jc) in superconducting wires and tapes as a function of field (μ0H), temperature (T), and axial applied strain (ea). The apparatus is an improvement of similar devices that have been successfully used in our institute for over a decade. It encompasses specific advantages such as a simple sample layout, a well defined and homogeneous strain application, the possibility of investigating large compressive strains and the option of simple temperature variation, while improving the main drawback in our previous systems by increasing the investigated sample length by approximately a factor of 10. The increase in length is achieved via a design change from a straight beam section to an initially curved beam, placed perpendicular to the applied field axis in the limited diameter of a high field magnet bore. This article describes in detail the mechanical design of the device and its calibrations. Additionally initial Jc(ea) dat...

Magnetisation and transport current loss of a BSCCO/Ag tape in an external AC magnetic field carrying an AC transport current

In practical applications, BSCCO/Ag tapes are exposed to external AC magnetic field and fed with an AC transport current. The total AC loss can be separated in two contributions: first, the transport current loss influenced by an external AC magnetic field, and second, the magnetisation loss that depends on the transport current running through the conductor. In this paper the total AC loss is considered and the role of the electric and magnetic components is compared. This comparison is made with an available analytical model for the AC loss in an infinite slab and verified experimentally for a BSCCO/Ag tape conductor. For small transport currents the magnetisation loss dominates the total loss. When the current increases, a field dependent crossover occurs, after which the transport current loss also plays a role. Qualitatively the measurements can be described well in terms of the critical state model. For magnetic field parallel to the wide side of the conductor the CSM for an infinite slab describes the measurements also quantitatively.

Descriptive model for the critical current as a function of axial strain in Bi-2212/Ag wires

A descriptive model is presented for the critical current of a Bi-2212 conductor that is axially deformed. This model is based on previously published critical current measurements on axially compressed and elongated Bi-2212 wires. The validity of the model is investigated by applying an additional pre-strain at room temperature. The experimental results on the pre-strained wires are in reasonable agreement with the prediction. The description for the critical-current reduction in Bi-2212 is applied in a model for a bend conductor. This model predicts the critical current as a function of the bending radius in a wire and a tape conductor.

Numerical calculation of current density distributions in high temperature superconducting tapes with finite thickness in self field and external field

The current density distribution of high temperature superconducting (HTS) tapes is modeled for the combined case of an alternating self and applied magnetic field. This numerical analysis is based on the two-dimensional Poisson equation for the vector potential. A one-dimensional current (z-direction) and a one-dimensional applied field (y-direction) are assumed. The vector potential is rewritten into an equation of motion for the current density J(x,y,t). The model covers the finite thickness of the conductor and an n-power E–J relation. The magnetic field dependence of Jc is also included in this E–J relation. A time-dependent two-dimensional current distribution that is influenced by the aspect ratio of the conductor and the material properties in E=f(J,B) is calculated numerically. The numerical results are compared with the experimental results for the AC loss of a tape driven by a transport current. Finally, a total AC loss factor is given for two cases in magnetic field direction, perpendicular and parallel to the conductor broad side.

Performance of an ITER CS1 model coil conductor under transverse cyclic loading up to 40,000 cycles

The large currents in the cable-in-conduit conductors (CICC) destined for the high field magnets in the International Thermonuclear Experimental Reactor (ITER), cause huge transverse forces on the strands compressing the cable against one side of the conduit. This load causes transverse compressive strain in the strands at the crossovers contacts. Moreover, the strands are also subjected to bending and contact surfaces micro-sliding, which results into friction and anomalous contact resistance versus force behavior. Three Nb/sub 3/Sn central solenoid model coil (CSMC) conductors were tested previously in the Twente Cryogenic Cable Press up to 40 cycles with a transverse peak load of 650 kN/m. This press can transmit a variable (cyclic) transverse force directly to a cable section of 400 mm length at a temperature of 4.2 K (or higher). To explore life-time cycling, we tested a CSMC Nb/sub 3/Sn conductor up to 40,000 cycles. The coupling loss and the associated interstrand resistance between various strands and strand bundles are measured at various loads. The force on the cable and the displacement are monitored in order to determine the effective cable Youngs modulus and the mechanical heat generation. Some aspects of strand deformation in CICCs are discussed. The test results are discussed in view of previous press results and data extracted from the ITER model coil tests.

Dynamic resistance in a slab-like superconductor with Jc(B) dependence

The dynamic resistance in a slab-like superconductor is calculated, taking into account a field-dependent critical current density. In superconductors carrying DC transport current in an AC external magnetic field, the dynamic resistance causes a transport loss which depends on the amplitude and frequency of this field as well as on the transport current. This resistance is calculated analytically in a critical-state model applied to a superconducting slab in a parallel field. The field has a general periodic time dependence and for the superconductor a relation between critical current and momentary magnetic field as in the Kim model is assumed. The dynamic resistance appears only at field amplitudes larger than the so-called threshold field, which depends on the transport current. The model predictions are compared with experimental results obtained with a Bi-2223/Ag tape at liquid-nitrogen temperature. At small field amplitude and at low transport current, the derived model predicts the observed dependence of dynamic resistance on field amplitude, field frequency and transport current. For a larger field amplitude and simultaneous high transport current, the resistance is found to be larger than the model predicts. This is probably due to the not completely slab-like geometry and/or to a different field dependence of the critical current density in a high AC field.

Critical current versus strain research at the University of Twente

At the University of Twente a U-shaped spring has been used to investigate the mechanical properties of a large variety of superconducting tapes and wires. Several mechanisms are responsible for the degradation of critical current as a function of applied strain. A change in its intrinsic parameters causes a reversible critical current dependence in Nb3Sn. The critical current reaches a maximum at a wire-dependent tensile strain level, and decreases when this tensile strain is either released or further increased. In Bi-based tapes the critical current is virtually insensitive to tensile strain up to a sample-dependent irreversible strain limit. When this limit is exceeded, the critical current decreases steeply and irreversibly. This behaviour is attributed to microstructural damage to the filaments. This cracking of the filaments is verified by a magneto-optical strain experiment. Recent experiments suggest that in MgB2 the degradation of critical current is caused by a change in intrinsic properties and damage to the microstructure. Magneto-optical imaging can be used to investigate the influence of applied strain on the microstructure of MgB2, as is done successfully with Bi-based tapes. In all these conductors the thermal precompression of the filaments plays an important role. In Nb3Sn it determines the position of the maximum and in Bi-based and MgB2 conductors it is closely related to the irreversible strain limit.

A new experimental method to determine the local critical current density in high-temperature superconducting tapes

The lateral critical current density (Jc) distribution in high-temperature superconducting tapes is determined with a new experimental method. With this method it is possible to determine a Jc-profile in a non-destructive manner. The experiment utilizes the strong suppression that occurs in the critical current (Ic) when a perpendicular magnetic field is applied to a Bi-2223 tape. A perpendicular magnetic field with a strong gradient that passes through zero is applied to a tape, to select a zone where the perpendicular field is small. This magnetically selected zone then determines the Ic of the entire tape. By moving this field gradient along the tape it is possible to observe the spatial variations in Jc. For a quantitative analysis of the critical current density a deconvolution of the measured Ic-profile with the system response is required. The local superconducting area is obtained from an optical micrograph of the tape cross-section. Finally a position dependent Jc is determined for two different types of superconducting tape.