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.