Compressive and tensile axial strain reduced critical currents in Bi-2212 conductors

Mono and multifilamentary wires of BSCCO-2212 in Ag matrix are investigated in an axial strain experiment. The superconducting samples are soldered to a substrate that is bend in order to achieve a compressive or tensile axial strain. The I/sub c/-strain dependence is measured in magnetic fields up to 16 T at 4.2 K and the strain is varied from -2% to +1.2%. In these Bi-2122 samples any strain-induced I/sub c/ reduction is irreversible. Moreover a significant rise in I/sub c/ was never observed after changing the strain. Special attention is paid to the tensile axial strain regime (0 to 0.4%). A small but significant reduction in I/sub c/ is found in this case. The strain behaviour of these wires indicates that the I/sub c/ reduction is due to fractures in the superconducting filament

Toward an accurate scaling relation for the critical current in niobium-tin conductors

Until a few years ago, a set of equations commonly referred to as the Summers relations gave the most accurate description of the critical current in Nb/sub 3/Sn conductors as a function of applied field, temperature and axial strain. Although highly empirical, they describe reasonably well the critical current data of past Nb/sub 3/Sn conductors. New data from various types of Nb/sub 3/Sn conductors, as well as recent analysis of the ITER CS model coil results reveal however, that this description lacks the precision, required to correlate the conductor data to the model coil results. This discrepancy, attributed to the highly empirical background for the relations, manifests itself mainly in the strain- and temperature dependence. The development of an alternative, more accurate description of the behavior of the critical current, starting from a more fundamental description of the strain dependence, has been initiated. At the moment, the development concentrates around the improvement of the temperature dependency relations to achieve a better accuracy of the overall descriptions, especially in the high temperature region

Scaling of the critical current in ITER type niobium-tin superconductors in relation to the applied field, temperature and uni-axial applied strain

The three dimensional surface of the critical current density versus field and temperature Jc(B,T) of niobium-tin is a function of the strain state of the superconductor. A brief review of literature on this subject is presented. The Jc(B) function is described by the relations for flux pinning. The temperature and strain dependencies are added to this relation, This results in a unifying scaling law for A15 materials, which is verified for different niobium-tin conductors with respect to all the relevant variables, i.e. field, temperature and uni-axial strain. Nb3Sn conductors from 9 manufacturers are measured in the frame work of the third ITER benchmark tests on critical current. The investigated ranges are: applied field from 7 to 13 T, temperature from 4.2 to 8 K and applied strain from -0.4 to +0.8%. Special attention is paid to the region of compressive axial strain, which is the most relevant state of strain for superconductors under thermal compression in practical application

Calculation of the Critical Current Reduction in a Brittle Round Multifilamentary Wire due to External Forces

A simple model is presented that can describe the electro-mechanical state of a multifilamentary wire. An elastic cylinder model is used to derive the strain state analytically. Axial and transverse forces came a position dependent critical current density in the wire. The integral critical current of such a wire is calculated. The effect of two different parameters, the hydrostatic and the deviatoric strain, on the critical current is compared. The critical current reduction of a model wire due to various external loads in a Nb 3Sn wire is analysed. Finally the position of the superconducting filaments is considere

Small and repetitive axial strain reducing the critical current in BSCCO/Ag superconductors

The critical current in two types of axially deformed BSCCO/Ag tape conductors is investigated. An Ic reduction is observed for small axial strains (ranging from 0 to 0.3%) with a characteristic slope dic/d&epsiv;=-5±1 (relative Ic, change per relative change in length). In the case of an axial compression there is a more pronounced Ic reduction. For small axial strains (<0.3%) a certain reversible change in Ic is observed. This reversible behaviour occurs in combination with an irreversible reduction that increases when the number of strain cycles is increased. The reversible part of the Ic change remains for a large number of strain cycles (>10000) and has a similar negative slope for both compressive and tensile strains. It is proposed that the reversible Ic change is correlated to a non-hydrostatic lattice deformation. The Ic versus strain behaviour is in good agreement with an earlier proposed mode

Modeling of strain in multifilamentary wires deformed by thermal contraction and transverse forces

A previously published analytical model that describes a simplified wire geometry with three stacked cylinders is compared with finite element model calculations. The thermal strain from the matrix on the superconducting filaments is considered first. It appears that the analytical model is able to describe the strain that occurs in the filaments relatively accurate. Especially the radial dependence of the strain if a central core of normal material is present, is described quit well by the analytical model. The strain inside a wire surrounded by epoxy and subjected to a transverse load is almost uniform and can be approximated with an analytical model too. When yielding is involved to simulate a more localised transverse load inside a multifilamentary wire it is necessary to consider a numerical model

Strain and grain connectivity in Bi2223/Ag superconducting tapes

The critical current reduction in silver-sheathed (Bi,Pb)2 Sr2Ca2Cu3O10 superconducting tapes (Bi2223/Ag) is investigated when loaded with uni-axial strains in combination with a magnetic field perpendicular to the tape surface. The number and quality of the grain-to-grain connections and the alignment of the superconducting cores mainly determine the critical current in Bi2223/Ag tapes. It is assumed that the transport current flows simultaneously through two current carrying paths in the tape: one through the network of Josephson junctions and the other is through the well-connected grains. The model describes well the magnetic field dependence of the critical current at various strains. A detailed analysis has shown that strain deteriorates grain connectivity, induces cracking and hence changes the current carrying path. Furthermore, strain may introduce new defects inside the grains along the strong-link current path and increase intra-granular pinning strengt

Coupling currents in Rutherford cables under time varying conditions

A network model is presented to simulate fully transposed Rutherford cables under time varying conditions. The intrinsic properties of the cable and the external applied conditions can be changed spatially. Several statistical distributions of the contact resistances are built in to investigate local differences in the coupling loss and in the eddy currents. The average loss is quite independent of the resistance distribution but locally both the loss and the eddy currents can increase significantly. The self field distribution of the cable is included, resulting in a saturation of the strands which depends on the relative direction between the magnetic field, the field sweep rate, and the transport current. Mutual inductances between strands are introduced, allowing the use of the model for nonstationary problems. Time constants can be calculated for both the coupling currents in the strands and for the local and global dissipatio

The influence of Lorentz force on the ac loss in sub-size cable-in-conduit conductors for ITER

The cable-in-conduit superconductors for the ITER coils have operating current in excess of 40 kA and function under last ramp conditions and fields up to 13 T. The transverse Lorentz force acting on strands may reduce the effective contact resistance between strands in the cable and as a consequence, the coupling loss will increase. This influence is investigated with a sub-size jacketed cable having 81 Cr-coated Nb3Sn strands. The AC loss is measured with a sinusoidal and trapezoidal magnetic field superimposed to a stationary background field of 1 or 2 T while the cable carries a constant transport current up to about 30 kA. The AC loss is determined by a pick-up coil system and partly with a calorimeter for calibration purposes. The nτ at 0 current declines after cyclic loading, from 9 ms in the virgin state to 2 ms after several loads. The increase of the interstrand coupling loss due to Lorentz effects, accompanied by resistance-hysteresis and relaxation effects as observed in the loss are discussed. The total loss increases considerably due to interference of transport current and induced coupling currents with rising transport current and DC field

Super Coupling Currents in Rutherford Type of Cables due to Longitudinal Non-homogeneities of dB/dt

In this paper it is shown that nonhomogeneities in the field sweep rate dB/dt along the length of a Rutherford cable provoke a nonhomogeneous current distribution during a field sweep. This process can be described by means of super coupling currents (SCCs) flowing through the strands over lengths far larger than the cable pitch. These SCCs can be characterised by a characteristic length, a characteristic time, and a propagation velocity. The dependence of these three parameters on the strand resistance and the contact resistance between strands is illustrated. Two longitudinal nonhomogeneities in dB/dt are considered which are present in accelerator magnets. Firstly, an increase in dB/dt from 0 to a certain value simulating that part of the cable where the cable enters the magnet field. Secondly, a longitudinal decrease in dB/dt which occurs mainly in the heads of the magnet. It is shown that in accelerator magnets a nonhomogeneous current distribution induced by the field sweep can not be avoided. However, it seems to be very difficult to estimate the amplitude of the effec