Cryogenic instruments based on imaging arrays of superconducting Transition Edge Sensor (TES) detectors operating at very-low temperature are proposed for future high- and low-energy astrophysics space missions, including the X-IFU instrument for Athena and the SAFARI instrument for SPICA. 2-D arrays of 1000's of TES detectors at 50 mK enable high-sensitivity imaging with high optical efficiency and fast response. The detectors are operated using frequency division multiplexed SQUID amplifiers that offer the low input noise and input impedance required for TES readout, with a low power dissipation to fit within the limited cooling powers available at low temperatures. The TES detectors and their SQUID readout electronics are integrated inside Focal Plane Assemblies (FPAs) that isolate the detectors and readout electronics from their environment. This project has developed three key technologies for a TES FPA: a) magnetic shields to suppress spurious detector response to background magnetic fields; b) high-density flexible interconnects to connect large-format TES arrays with their FDMmultiplexed SQUID readout; and c) a first demonstration of the mechanical integration of a large-format TES array and its 50 mK readout electronics.

The final activity was to develop a mechanical prototype of the 50 mK detector and readout assembly, incorporating the reworkable interconnect concept (Fig. 4-1). The TES and LC filter chips are mounted on symmetric arrays of leaf-springs that absorb the CTE difference between Si and Cu and maintain the detector alignment during cooldown. Structural analyses were used to optimize stiffness and strength, in particular of the mounted TES wafer. This design reflects the current concept for the X-IFU FPA (eg. TES and LC filter dimensions and with a dummy anti-coincidence detector assembly). A prototype assembly was built and tested to verify its manufacturability and robustness vs. thermal cycling and vibration. Twenty cycles to 80 K caused no visible damage. Low-level sine sweep vibration tests verified the assembly's stiffness and its integrity before and after high-level vibration tests. Random vibration tests were performed using spectra scaled in amplitude from estimated qualification loads, starting at a low level and ramping up to the nominal level (0dB). No damage was observed after the 0dB test, so testing continued to characterize the design margins. The assembly survived a +3dB test, but glue spots fixing the TES wafer to its support failed at +6dB. Post-test disassembly and inspection confirmed an adhesion failure between the glue and the Cu support, and analysis of this joint has started to understand and optimize this in the future. Local damage was also observed on the TES wafer under the corners of the interconnects, indicating contact between the interconnect and TES chips during vibration. Future design optimizations are considered.