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This work describes the redesign performed on the X ray Integral Field Unit Focal Plane Assembly (X-IFU FPA) Development Model (DM) with a focus on the mechanical modifications of the T0 detector stage. In addition the development progress of several critical interconnection technologies used within the DM 1.1 is described. These include fixation of the main TES array to the metal support, a dedicated wire bonding process of the main TES array to the side panel MUX carrier chips and interconnection of the super-conducting Nb flex cables to the connecting Printed Circuit Boards (PCB).
The X-IFU focal plane assembly development model: evaluation of the global magnetic shielding factor
The required magnetic shielding attenuation is obtained combining a cryogenic mu-metal (CMM) shield and a superconducting Niobium (Nb) shield. From a finite element model (FEM), the CMM shield alone should suppress the static field (for example Earth magnetic field) at the detector location with a factor 50; while the combination of the two has the goal of suppressing variable fields with a shielding factor of ∼106, defined as the ratio of the magnitude of the external field relative to the maximum field perpendicular to the sensor array. These numbers are referring to the component normal to the plane of the sensor array. In plane with sensor array the shielding factor can be a 104 times larger.
We use the TES detectors as very sensitive magnetometers. The TES critical current Ic exhibits oscillatory Fraunhofer-like behaviour, when magnetic field perpendicular is applied to it. Whether the variable external magnetic field perpendicular to the detectors penetrates the shields, the operating current ITES through the TES, which is stabilised in temperature and biased in a sensitive point in transition, may change and this variation can be used as witness for the shielding factor evaluation.
In this work we discuss the difficulties in performing such a measurement, where a Helmholtz coil outside the cryostat were used to apply an external magnetic field (< 200 μH) in order to avoid any permanent magnetisation of the CMM shield. Both AC and DC external magnetic field have been applied with different orientation respect to the detector plane. This improves the statistic of a very sensitive measurement, but also serves to maximise the signal measured by a number of TESs spread over the south-quadrant (closer to the Nb shield) of the array, where the field lines show different intensity. Preliminary results set a lower limit in the evaluation of the global FPA shielding factor of ∼105. Improvement in the measurement setup and in the analysis will be discussed in view of the future models of the FPA.
The development test program, including operation at 100 K has been completed. The verification test programme is currently being carried out and will include functional testing at 40 K.
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