The Astro-H high temperature superconductor lead assemblies

Abstract

The Soft X-ray Spectrometer (SXS) instrument, one of several instruments on JAXA’s Astro-H mission, will observe diffuse X-ray sources with unparalleled spectral resolution using a microcalorimeter array operating at 50mK. The array is cooled with a multi-stage Adiabatic Demagnetization Refrigerator mounted on a 40l helium tank. The tank is at the center of a typical “shell in shell” cryostat, with the innermost shield cooled by a JT cryocooler, and successive outer shields cooled by stirling-cycle cryocoolers. To achieve a multi-year liquid helium lifetime and to avoid exceeding the limited capacity of the JT cooler, very strict requirements are placed on every source of heat leak into these surfaces from the higher temperature shields. However, each ADR stage draws a maximum of 2A, and the Wiedemann–Franz Law precludes even an optimized set of normal-metal leads capable of such high current from achieving the required low thermal conductance. Instead, a set of lead assemblies have been developed based on narrow high temperature superconductor (HTS) tapes derived from commercially available coated conductors.Although the HTS tapes are flexible and have high tensile strength, they are extremely sensitive to damage through a number of mechanisms. A robust set of assemblies have been developed that provide mechanical support to the tapes, provide appropriate interfaces at either end, and yet still meet the challenging thermal requirements. An Engineering Model (EM) set of HTS lead assemblies have survived environmental testing, both as individual units and as part of the EM cryostat, and have performed without problem in recent operation of the EM instrument. The Flight Model (FM) HTS lead assemblies are currently nearing completion.