Patent ID: 7840244

Claim:
A superconducting cable, comprising: (a) a heat-insulated pipe; (b) a cable core that is housed in the heat-insulated pipe and the cable core comprises: (b1) a superconducting conductor made of a superconducting material; (b2) a poorly heat-conductive pipe placed at the outer side of the outer circumference of the superconducting conductor: (b3) an electrically insulating layer placed on the outer circumference of the poorly heat-conductive pipe; and (b4) an outer superconducting layer that is placed on the outer circumference of the electrically insulating layer and the outer superconducting layer is made of a superconducting material; (c) a conductor-use coolant that is circulated in the poorly heat-conductive pipe to cool the superconducting conductor to make it achieve a superconducting state; (d) an insulation-use coolant that fills the heat-insulated pipe to perform electric insulation of the superconducting conductor; wherein the electrically insulating layer has a ρ (resistivity) grading along a radial direction such that the resistivity of the electrically insulating layer decreases in the radial direction toward an innermost portion of the electrically insulating layer, and the resistivity of the electrically insulating layer increases in the radial direction toward an outermost portion of the electrically insulating layer; wherein the electrically insulating layer with ρ (resistivity) grading reduces a radial distribution of a direct current electric field in the electrically insulating layer; wherein the electrically insulating layer includes portions that include, in the vicinity of the superconducting conductor, a high dielectric constant layer that has a dielectric constant ∈ higher than the dielectric constant of other portions of the electrically insulating layer; and wherein the electrically insulating layer is formed such that the dielectric constant ∈ increases toward the innermost portion of the electrically insulating layer, and the dielectric constant ∈, decreases toward the outermost portion of the electrically insulating layer.