Technological aspects and recent advances in the current leads incorporating high temperature superconductor (HTS) are described. HTS of sintered Bi-2223, melt cast Bi-2212, unidirectional Y-123 and Ag- or Ag alloy-sheethed Bi-2223 are being investigated and tested in models of current lead systems. Reduction of refrigerator power by a factor of 3 to 5 is demonstrated by model tests. Easy operation and a cryogen-free superconducting magnet has become available using HTS current leads. Researches concerning safety and reliability are being made, ac operation models are tested, and design studies for specific application in large systems are underway. We can expect that all of the Cu current leads will be replaced by HTS current leads.

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General design equations are derived from one-dimensional energy balance equations for gas-cooled current leads of high temperature superconducting (HTSC) material. The equations are solved to evaluate general characteristics for cooling with a laminar flow gas. General characteristics arranged in the design chart are applicable to the design of current leads with a wide variety of geometrical dimensions. Thermal runaway in the case of accidental stoppage of the cooling gas and the transition to the normal state of part of the current leads is analyzed. The current leads optimized under the condition of self-sustained cooling are usable in a refrigerator cooling system, almost minimizing the total refrigerator load.

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We have designed and tested a conduction-cooled high-T_c current lead for an AC 1kA or 1kArms (root mean square) fault current limiter, using tubular (Bi, Pb)₂Sr₂Ca₂Cu₃O_10+x superconductors. A tubular lead has the advantages not only of a high critical current density but also of low AC loss. Preliminary, self-field AC loss was measured at 77K. It was 0.22W for a transporting current of 1kArms for the lead with the dimensions of 34mm in outer diameter, 30mm in inner diameter and 190mm in length. The loss was in good agreement with the numerical calculation by the critical state model, and was much smaller than that calculated for a cylindrical superconductor. In the actual lead system where the lead had a temperature distribution from 77K to 4.2K, the AC loss was less than 0.1W per pair of leads. The lead successfully transported 1kArms for more than 2 hours without any thermal runaway or instability. The total 4.2K heat load of the pair of leads, 1.8W, also satisfied the refrigeration restriction for the fault current limiter.

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We have developed a Bi₂Sr₂CaCu₂O_8+x(Bi-2212) current lead for a 10T NbTi/Nb₃Sn magnet cooled by a cryocooler. The Bi-2212 bulk used in the lead was fabricated by a floating zone melting method with a CO₂ laser. The grain-aligned microstructure along the longitudinal direction of the bulk, exhibited better magnetic field dependence of the critical current density J_c below 50K. This implies that the Bi-2212 lead is useful even in a high leakage field, when the high temperature end is connected to the 50K stage of a cryocooler. Moreover, an inner flexible copper portion was designed to reduce the thermal contraction difference. In the 10T magnet, a pair of these leads successfully transported 130 Amp. at a leakage field of 0.35T, which corresponded to a 10T field. The heat load to the 4K stage was 45mW, which was 1/10 of conventional Cu leads and was enough small for a cryocooler.

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Melt-textured YBa₂Cu₃O_x superconductors made by the QMG process were applied as current lead. The current lead consisted of the QMG-YBa₂Cu₃O_x, pure silver flexible current lead sheets, a protective stainless pipe and copper electrodes. The YBa₂Cu₃O_x was cooled by thermal conduction through the silver sheets. These current leads were used for a critical current measuring holder to demonstrate the performance of these materials in a magnetic field. The consumption rate of liquid helium was extremely reduced to about 25% in comparison with that of a conventional holder using seven φ2mm Cu current leads, which is available for measuring six samples for a transport current up to 200A. These current leads were operated in a stray field of 0.4T at 50K when the hybrid magnet generated 23T. It was found that QMG bulk superconductor is surely as effective as current leads used in high magnetic fields at high temperature. The advantage of QMG current leads lies in high performance in the magnetic field. In addition, compact cryogenic equipment can be realized by adopting YBa₂Cu₃O_x superconductors without a weak-link.

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The Bi (2223) bulk current lead formed of a thin-walled sintered cylindrical tube has been developed and applied in a cryocooler cooled type superconducting magnet without using liquid helium. The critical current density of the bulk is over 1, 000A/cm² under self magnetic field at 77K. The heat leakage from 60K to 10K of the bulk is estimated at 79.1mW and the bending strength of the Bi (2223) bulk current leads are 14-23MPa. The strength of the bulk are double or treble according to the Stycast coating on the bulk surface. Due to these superior properties, the Bi (2223) bulk current lead has been successfully used in a cryocooler magnet. The magnet system is running for 26, 000 hours without any degradation of the current lead.

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We have designed and tested superconducting current leads using Bi(2223) oxide superconducting bulk material. Bi(2223) leads were supported by a glass fiber-reinforced plastic (GFRP) which was chosen in consideration of thermal conductivity and thermal expansion. The thermal conductivity of GFRP was one order of magnitude less than that of Bi(2223) bulk material. The difference of the thermal expansion between GFRP and Bi(2223) bulk material was less than 0.1% in the temperature range from 293K to 20K. The critical current of the Bi(2223) leads did not change by the heat cycle from room temperature to 77K. The current lead was cooled by a two-stage Gifford-McMahon cycle refrigerator and operated in a conduction-cooled superconducting magnet successfully.

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(Y, Ho)-Ba-Cu-O bulk samples have been prepared by the unidirectional solidification method. Since these samples exhibit low thermal conductivity and high critical current density, it is desirable to apply Y-based bulk sample to current lead, especially for a large-scale superconducting apparatus. A high-T_c superconducting current lead model using (Y, Ho)-Ba-Cu-O bulk has successfully transported a high current of 7, 550A. Warm ends of bulk samples were 50.3K and 45.8K, respectively, at 7, 550A. The liquid helium boil-off rate was 4.39l/h at 5, 560A, which corresponds to 3.1W heat leakage. The cryogenic heat load was reduced to about half the value of a conventional copper current lead.

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We are developing high-temperature superconducting (HTS) current leads for a 1MW/1kWh module type SMES system. The module type SMES can be easily adapted to increase SMES capacity, and constitute a high reliability system. However, one set of current leads is adapted to one module of SMES. According to the capacity of the SMES system, the number of current leads increase, and the heat load of current leads increases. Therefore, it is necessary to decrease the heat-in-leak of current leads. This paper describes the design to optimize the HTS current lead for the SMES system. It is important for HTS current leads adapted to SMES systems to improve reliability. The HTS current leads have been designed to minimize the heat load and to maintain high reliability.

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