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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. |
