Low Heat-Leak YBCO Leads for Satellite-Borne ADR Magnets

Future satellite missions carrying X-ray spectrometers will be cooled to milliKelvin temperatures by multi-stage Adiabatic Demagnetization Refrigerators (ADR), which, in turn will be precooled by closed-cycle cryocoolers. Each stage of the ADR will use a low temperature superconducting magnet, which will periodically be energized via high temperature superconducting (HTS) leads. The present design of leads contributes a large fraction of the total heat load on the cryogenics system and so we propose to fabricate Yttria Barium Copper Oxide (YBCO) lead assemblies with a predicted heat load of about one order of magnitude lower than the established design that uses brittle filaments supported on a glass-fiber tube. Our suggested approach is to fabricate inherently tough and flexible leads in the 2A to 10A range from commercially available YBCO tape rated to over 200A. The substrates of such tapes are tough, strong low thermal conductivity metals which can be cut longitudinally into thin strips approximately 300 micrometers in width using a precision dicing saw. In principle, leads of any length or width can be made this way and tailored to a variety of specifications. In the region of the thermal gradient, the protective silver coating is removed and replaced by a non-conducting encapsulant; the as-manufactured silver coating is left at the ends to make electrical joints. This is a process which should lend itself well to small-scale manufacture. The strength and toughness of the leads means that they can be incorporated into a loom supported by taught Kevlar threads, which is a very low heat-leak method used extensively in space cryogenics. We expect that the overall result of this project will be to reduce significantly the size, weight and power requirement of the satellite-borne cryosystem and at the same time produce a lighter more robust lead structure.

The current leads proposed are of direct use to NASA X-ray telescopes; in particular the XRS-type cryostat. The techniques proposed may be applicable to other low-temperature detectors used by NASA and even in cryogenic rocket fuel instrumentation and management.

All superconducting magnets operating at 4 degrees Kelvin e.g. NMR, MRI, magnetic separation, use current leads running from room temperature to 4K. HTS leads are of potential benefit in reduction of helium consumption in all these applications. Helium is becoming an increasingly scarce commodity, and the financial benefits of saving helium boil-off are increasing for many commercial users of these superconducting magnet technologies. Superconducting electronics is being proposed as the foundation of energy-efficient supercomputers operating at 4 degrees Kelvin. Such systems would need HTS leads similar to those proposed here to maintain energy efficiency.

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