Shuntable low loss variable current vapor cooled leads for

Abstract

A shuntable low loss variable current vapor cooled lead (VCVCL) configuration delivers current to and from a superconductive load, such as a superconductive magnet, immersed in a cryogenic liquid in a way that minimizes the boil-off rate of the cryogenic liquid. The VCVCL configuration includes superconductive lead assemblies containing superconductive segments. The assemblies are connected in parallel between the superconductive load and an output current source or sink. Each assembly is controlled so that its superconducting segment is either superconducting or non-superconducting. By selectively controlling whether each lead assembly is superconducting or non-superconducting, by varying the cryogenic liquid level, the current flow to or from the superconducting load through the lead assemblies is shunted from those lead assemblies exhibiting a relatively high resistance to those having a relatively low resistance, and the current is selectively distributed between superconducting lead assemblies so that each lead assembly either carries near zero current or an optimum current. At near zero current, the lead assembly contributes very little to the helium boil-off rate because of negligible Joule heating, and because the path of thermal conduction to the liquid helium is significantly more resistive. At or near the optimum current, the helium boil-off rate approaches a theoretical minimum. By keeping the current in each lead assembly at near zero or near the optimum design current, the helium boil-off rate of the VCVCL configuration is minimized and is significantly less than that of conventionally designed lead arrays.