T. Kawashima

Interstrand coupling effect on losses and current distributions in superconducting cable conductors

Abstract We have successfully quantitatively estimated the interstrand coupling effect on fundamental electromagnetic properties such as losses and current distributions in a superconducting cable. A ·model conductor’, composed of test strands with various surface conditions, is used to obtain the value of the contact resistance between non-insulated strands in the conductor, a parameter which is closely related to the fundamental properties. The contact resistance is educed by comparing the measured transverse field loss in the model conductor with the numerically calculated value. The interstrand coupling effect increases the coupling loss of a cable-in-conduit conductor with a multi-stage twisting structure. This can theoretically be explained by an approximate analysis in which the specific structure is taken into account and the value of the contact resistance, as determined above, is used. Our measurement shows that the current distribution in a twisted conductor for a.c. uses is uniform among the strands. It can also be explained theoretically by considering the characteristic length necessary to make the transport current distribution uniform when the interstrand coupling effect is quantitatively taken into account. Extension of our estimates provides the basis for analysing the current sharing problems involving thermal effects caused when the conductors are quenched.

Numerical calculation method of inter-strand coupling current losses in superconducting conductors

Abstract We provide a new method to calculate numerically inter-strand coupling current losses in superconducting cabled conductors with both complicated structures and non-linear electromagnetic properties. In this calculation, we solve Laplaces equation over the normal metal region in the conductor by means of the finite element method under the analytically obtained boundary condition characterizing such a superconducting cabled conductor. Based on preliminary results of an error of numerical calculation, we apply the present method to loss calculation for some practical cabled conductors. Validity and the usefulness are shown for the present method with good prospect of expansion.

Losses in cable-in-conduit superconductors used for the poloidal coil system of the large helical device

The authors discuss the loss features of the NbTi cable-in-conduit (CIC) superconductor for the Large Helical Device (LHD) poloidal coil system. Eight conductors were prepared. The conductors differ in the matrix or in the surface condition of the strands. The interstrand coupling losses tend to increase during the pulsive operation of the poloidal coil. Short and straight samples of about 500 mm in length were provided for this experiment since bending may change the interstrand contact condition. The authors measured the loss-frequency characteristics of the short sample conductors with various types of strands to clarify their loss properties. The measurement was carried out in the transverse AC ripple field superposed on bias fields, in the frequency range from 0.1 Hz to 200 Hz. The measurements showed that the replacement of the CuNi matrix strands in the conductor by the Cu-matrix strands did not increase the intrinsic coupling loss significantly, but that noninsulation of the strands caused considerably higher interstrand coupling losses.<<ETX>>

Using magnetic field pulses with a slow sweep rate to produce uniform current distribution in multifilamentary superconducting wire

Abstract A detailed discussion is presented on the change in current distribution which takes place in a twisted multifilamentary superconducting wire induced by successive magnetic field pulses with a slow sweep-rate. The number of the field pulses required for a localized current-distribution to become uniform is estimated. Some modification of existing theories is necessary to describe the phenomenon. The results obtained are confirmed by systematic measurements of the net flux penetration into the wire during each cycle of field pulses.

On the distribution of a transport current inside a multifilamentary superconducting wire in a rapidly changing transverse magnetic field

Abstract For the current distribution inside a multifilamentary superconducting wire carring a dc transport current in a rapidly changing transverse magnetic field, inconsistencies with the existing models are shown by the following experimental evidence: when a transverse magnetic field is applied, the distribution of transport current is not unaltered but is forced to concentrate into the inner circular cross section region during a characteristic time constant τ c , called the coupling time constant. Secondly, the characteristic time constant for the transport current distribution inside the inner region to approach a uniform distribution is not τ c but a new time constant τ 1 called the ‘uniforming time constant’, though the variation in the distribution does not occur unless the external magnetic field changes with time. It is shown that the model of the current distribution based on the above experimental evidence exhibits a remarkable difference from the existing models, especially for the wires containing very fine superconducting filaments.

3D FEM analysis of inter-strand coupling losses in Rutherford cables with composite core

Improved Rutherford cables have been proposed for magnets for experiments of high energy physics. They have either a sheet of stainless-steel or a composite metal sheet as a core put in the usual Rutherford cable. The cable with this composite core sheet in it is a candidate for a new cable with both low losses and high stability. In order to estimate the interstrand coupling losses produced in this cable, three dimensional finite element method (3D FEM) analysis is carried out. The results obtained show that the existence of 3D shielding current in it determines the profile of loss properties under changing magnetic fields of face-on orientation. The effect of spatial period of the core structure along the cable axis on the loss is also elucidated. This will be a useful aid for the design of the cable.

Losses in a multifilamentary superconducting wire caused by a simultaneous sweep of current and magnetic field

Abstract A theoretical discussion is presented on the energy loss in a multi-filamentary superconducting wire when an applied transport current and an external transverse magnetic field are varied simultaneously with a repeating pulsive wave form. In the present calculation, the effects of the ‘uniforming time constant’ which has been introduced by the authors as a characteristic time constant for the change in the transport-current distribution inside the wire is taken into account, together with the field dependence of the critical current density of superconducting filaments. Thus the present analytic expression for the energy loss of multi-filamentary wire is available to the whole range of the external magnetic field. It is shown that the contribution of the dynamic resistance loss to the total loss is strongly dependent on the position of the wire inside a coil.

The change of coupling losses in aluminum-stabilized superconductors due to the Hall effect

The change of coupling losses in aluminum-stabilized superconductors due to the Hall effect is investigated experimentally and numerically. The Hall effect is here taken to be an interaction between the inter-strand coupling current and the external DC magnetic field. The loss measurement of the R&D conductor for the helical coil of the Large Helical Device is carried out under the coexistence of the transverse and the longitudinal magnetic fields. The measured coupling loss of the real conductor induced by the changing transverse magnetic field is increased by the existence of the longitudinal DC field. These loss features are compared with those by numerical analysis, based on the two dimensional finite element method, of a real conductor with complex cross-sectional structure.<<ETX>>