An imbalanced current distribution is often observed in cable-in-conduit (CIC) superconductors which are composed of multi-staged, triplet type sub-cables, and hence deteriorates the performance of the coils. Therefore, since it is very important to obtain a homogeneous current distribution in the superconducting strands, we propose a coaxial multi-layer type CIC conductor. We use a circuit model for all layers in the coaxial multi-layer CIC conductor, and derive a generalized formula governing the current distribution as explicit functions of the superconductor construction parameters, such as twist pitch, twist direction, radius of each layer, and number of superconducting (SC) strands and copper (Cu) strands. We apply the formula to design the coaxial multi-layer CIC which has the same number of SC strands and Cu strands of the CIC for Central Solenoid of ITER. We can design three kinds of the coaxial multi-layer CIC depending on distribution of SC and Cu strands on all layers. It is shown that the SC strand volume should be optimized as a function of SC and Cu strand distribution on the layers.

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A multilayer coaxial superconductor has been investigated theoretically and experimentally for realizing homogeneous current distribution. The theory, based on the magnetic conservation law defining net magnetic flux enclosed by the electric center lines of the two filaments in the two strands corresponding to adjacent layers to be zero, gives a generalized formula governing the current distribution. It was shown from the formulation that the current distribution can be homogenized by adjusting the twist pitches of a multilayer superconductor with particular layer radii and twist direction. To verify the theory, we have fabricated a three-layer coaxial superconductor 1m long and thereby carried out its test. The conductor was composed of Ag-sheathed Bi2223 tapes wound spirally onto three tubular FRP (Fiber Reinforced Plastic) formers with pitches of 333.33mm, 166.67mm, and 111.11mm corresponding to the first (innermost), second and third layers, respectively. To minimize contact resistance between the HTS tapes and terminals, a ladder-shaped pretreated Cu block was employed for the end connection. The test was performed at 77K by imposing AC transport current in the range of 0-100 ampere maximum value with a frequency of 100Hz. The measured currents of the first, second, and third layers were 30.25%, 36.82% and 32.935% of total transport current, respectively. This result evidently is in good agreement with that of the theory. This paper describes the design, construction and experimental results of a test superconducting multilayer cable based on the homogeneous current theory.

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High-temperature superconducting (HTS) cables have been studied because of their low loss and compact properties as compared to conventional copper cables. Three-phase cables are usually composed of three single-phase coaxial cables. Recently, a tri-axial cable composed of three concentric phases has been intensively developed, because it has advantages such as reduced HTS tape, small leakage field and small leakage heat loss as compared to three single-phase cables. However, there is an inherent imbalance in the three-phase currents in tri-axial cables due to the differences in the radii of the three-phase current layers. The imbalance of the currents causes additional loss and a large leakage field in the cable, and deteriorates the electric power quality. Therefore, a model composed with two longitudinal sections is proposed. This model allows us to determine cable construction parameters such as winding pitches and the radii of the three concentric phase layers, thereby satisfying both conditions: three balanced concentric phase currents and a homogeneous current distribution in each phase of the tri-axial cable. We formulate and derive general equations for the three-phase current distribution in the tri-axial cable as functions of the winding pitches of the three concentric phase layers. The equations are applied to tri-axial cables composed of one or two layers per phase.

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High Temperature Superconducting (HTS) cables have been studied because of low loss and compactness, compared with conventional copper cables. Three-phase cables are usually composed of three single-phase coaxial cables. Recently, a tri-axial cable, composed of three concentric phases, has been intensively developed, because it has advantages such as reduced amount of HTS tapes, small leakage fields and small heat loss in leak, compared with the three single-phase cables. However, there is an inherent imbalance in the three-phase currents in the tri-axial cable due to the differences in radii of the three-phase current layers. The imbalance of the currents causes additional loss and large leakage field in the cable, and deteriorates the electric power quality. Therefore, we propose a new model which is a tri-axial cable composed of two longitudinal sections with different twist pitches to obtain the solutions of the balanced three-phase currents and homogeneous current distribution in each phase of the tri-axial cable. We derive general equation satisfying both the balanced three-phase currents and homogeneous current distribution, as functions of winding pitches, and finally apply it to the simplest cable. We fabricated and tested a 1m long HTS cable in order to verify the proposed theory that satisfies the balanced distribution. The results demonstrate the theory is right. We also investigate the current distributions along the long tri-axial cable considering the capacitances between the layers in the tri-axial cable.

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