Development of superconducting power transmission cables operated at liquid helium temperature requires a cryogenic system with extremely low heat-leak characteristics. A thermal envelope, housing the superconducting cables and having the highest performance superinsidation systems, still exhibits high heat-leak losses in relation to the electrical losses generated by the superconductor. This paper describes a conceptual superconducting transmission line configuration using multiple-layer, vacuum-insulated pipe with intermediate thermal shielding to reduce the helium-level refrigeration load. The intermediate metallic shields, anchored at discrete temperature levels on a vent vapor tube, and sensible refrigeration of the vent vapor stream intercept heat leak to the lower temperature helium compartment, thus upgrading thermal performance of the insulated pipe and bringing heat-leak losses in line with superconducting losses.

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