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.
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.
We have designed and tested a conduction-cooled high-Tc current lead for an AC 1kA or 1kArms (root mean square) fault current limiter, using tubular (Bi, Pb)2Sr2Ca2Cu3O10+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.