General Atomics, Inc.

Edward E Bowles

Michael Heiberger

Dean G Morris

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.

Shuntable low loss variable current vapor cooled leads for

High-voltage, solid-state switches that are capable of interrupting current, offer relatively low on-state losses (low forward voltage drop) and provide for high current densities. Low-voltage transistors and silicon-controlled rectifiers are employed to achieve low power losses at high current densities. One embodiment of the present invention relates to a solid-state switch for controlling a source of direct current. The switch includes four switching elements. Each element has a first terminal, a second terminal and a control signal input terminal. The switching elements provide a high resistance to the flow of electrical current in response to a first control signal. A second control signal causes the switching elements to offer a low resistance to the flow of electrical current. The first and second switching elements are connected in series, as are the third and fourth switching elements. The series combination of the first and second switching elements is then connected in parallel with the series combination of the third and fourth switching elements. A second embodiment of the invention includes three switching elements similar to the elements of the first embodiment, but with the first and the second switching elements wired in series, and with the third switching element being connected in parallel with the series combination of the first and second switching elements. The combination of switching elements allow a silicon-controlled rectifier to be used as a turn-off switch.

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San Diego, CA, United States of America

Edward E Bowles