Development of an ultra-low Helium Loss Hi-Tc Magnet Lead System

Approaches for Hi-Tc superconducting magnet leads are available for high current applications and/or in combination with active cryocoolers. Incorporation of these leads into a typical research cryostat, however, faces difficulties related to their critical temperature, and perhaps surprisingly, their intrinsically low thermal conductivity. We have developed an approach for the use of Hi-Tc leads in such cryostats which utilize commercially available tape to carry currents of a hundred amperes, while maintaining very low liquid helium boiloff rates. This design relies on a parallel thermal path to tailor the temperature profile of the superconducting tape and is incorporated into our previously developed lead system, which couples the heat of the magnet leads to the exiting helium gas through the cryostat baffles. Computer modeling of the lead-cryostat system, including all relevant thermal paths, enables us to optimize the lead design, resulting in a net increase in helium boiloff for the incorporation of the leads into a cryostat of approximately one half liter per day without current, and of one liter per day when carrying a full hundred amperes. The design and implementation of the mechanical system necessary for this ultra-low helium loss magnet lead system will be presented.