## 6 Future Instruments for Astrophysics and Fundamental Physics Research
### X-IFU Athena  
One of the most complex TES-based instruments currently under development is the X-ray Integral Field Units (X-IFU) of Athena [16], the L-class ESA's X-ray observatory mission, selected in June 2014 to address the Hot and Energetic Universe science theme [2] and scheduled to be launched on 2032 at L2 orbit. The Hot Universe refers to the baryons at temperatures above 1056 K, which are believed to account for half of the total baryonic content of the Universe. The best observational handle on the energetic universe is through X-ray observations of hot gas and accretion around black holes. Athena will be a large observatory offering an unprecedented combination of sensitive X-ray imaging, timing, and high-resolution spectroscopy. X-IFU [2] is a cryogenic imaging X-ray spectrometer, with spatially-resolved high spectra resolution over a 5\({}^{\prime}\) field-of-view (FoV) that will provide: (i) 3D integral filed spectroscopic mapping of hot cosmic plasma, (ii) weak spectroscopic line detection, (iii) physical characterization of the hot and energetic universe. X-IFU will host an array of more than 3000 TES pixels with a \(T_{c}\simeq 90\) mK, sensitive in the energy range of 0.2-12 keV, with an unprecedented instrument energy resolution of 2.5 eV at 7 keV. To achieve the X-IFU requirement, the intrinsic single pixel energy resolution has to be better than 2 eV, the quantum efficiency of the absorber \(QE>96.7,80\) and 63% at 1,7,9.5 keV, respectively, and pixel filling factor larger than 0.97. The prototype kilo pixel array of Mo/Au TESs, developed at NASA-GSFC and read out with the 8-column by 32-row TDM from NIST, has recently shown excellent results that meet the X-IFU requirement with margin. The detector consists of an array of \(50\times 50\) um\({}^{2}\), low resistance TESs with a single pixel resolution of \(<\)2 eV at 6 keV. The system has demonstrated a combined exquisite energy resolution, with more than 200 pixels, of: 1.95 eV for Ti-K\({}_{\alpha}\) (4.5 keV), 1.97 eV for Mn-K\({}_{\alpha}\) (5.9 keV), 2.16 eV for Co-K\({}_{\alpha}\) (6.9 keV), 2.33 eV for Cu-K\({}_{\alpha}\) (8 keV), 3.26 eV for Br-K\({}_{\alpha}\) (11.9 keV) [115]. The focus of the team will soon move on demonstrating the performance of a full scale \(>~{}3000\) pixels array. Thanks to its advanced readiness level, this technology is currently baselined for X-IFU for the mission adoption of Athena expected in 2022. The X-IFU Focal Plane Assembly (FPA) [160; 161; 162] and the detection chain shall face several implementation challenges related to the interaction between the different components of the read-out (i.e., the main sensor array, the old electronics stages and the warm electronics) and the thermal, mechanical, and electromagnetic environment. Moreover, the limited resources available on the space telescopes lead to strong constrain on the total allowed mass, volume, electrical power, and cooling power at the 50 mK and 2 K stages. The high-frequency EM-fields generated in the spacecraft call for a careful choice of the read-out technology that needs to be the less sensitive to electromagnetic interferences and capable of driving very long cable harness. The high susceptibility of the TES detectors and the focal plane assembly to DC and low frequency magnetic fields requires a proper magnetic shielding, by means of \(\mu\)-metal and Nb shields, to reduce the DC and the 50 Hz B-field to level \(<1\) uT with a stability \(\sim\)10 pT\({}_{rms}\). Faraday cages needs to be implemented at the FPA, warm electronics, and cryostat level and the harness needs to be heavily filtered given the high (\(\sim\)0.4 nV/\(\sqrt{\text{Hz}}\)) susceptibility in the DC-MHz range [161]. In Figure 13a, we show a rendered view of the FPA Demonstration Model of X-IFU, currently in the commissioning phase at SRON. On the right side of the figure, we show the pictures of a full scale X-IFU prototype array from NASA-GSFC with more than 3000 pixels on a pitch of 260 um.