The Athena mission entered a redefinition phase in July 2022, driven by the imperative to reduce the mission cost at completion for the European Space Agency below an acceptable target, while maintaining the flagship nature of its science return. This notably called for a complete redesign of the X-ray Integral Field Unit (X-IFU) cryogenic architecture towards a simpler active cooling chain. Passive cooling via successive radiative panels at spacecraft level is now used to provide a 50 K thermal environment to an X-IFU owned cryostat. 4.5 K cooling is achieved via a single remote active cryocooler unit, while a multi-stage Adiabatic Demagnetization Refrigerator ensures heat lift down to the 50 mK required by the detectors. Amidst these changes, the core concept of the readout chain remains robust, employing Transition Edge Sensor microcalorimeters and a SQUID-based Time-Division Multiplexing scheme. Noteworthy is the introduction of a slower pixel. This enables an increase in the multiplexing factor (from 34 to 48) without compromising the instrument energy resolution, hence keeping significant system margins to the new 4 eV resolution requirement. This allows reducing the number of channels by more than a factor two, and thus the resource demands on the system, while keeping a 4' field of view (compared to 5' before). In this article, we will give an overview of this new architecture, before detailing its anticipated performances. Finally, we will present the new X-IFU schedule, with its short term focus on demonstration activities towards a mission adoption in early 2027.

With applications in cosmology, infrared astronomy and CMB survey, frequency-division multiplexing (FDM) proved to be a viable readout for transition-edge sensors (TESs). We investigate the occurrence of out-of-band resonances (OBR) which could constrain the bandwidth of the FDM readout of TES bolometers. The study includes SPICE modeling of the entire setup including the cryogenic harness, LC filters, Superconducting Quantum Interference Device (SQUID) and room-temperature amplifier. Simulation results show that the long harness (for flight model) could cause multiple reflections that generate repetitive spikes in the spectrum. Peaks of the OBR are mainly due to the parasitic capacitances at the input of SQUID. Implementing a low-pass RC circuit (snubber) at the input of the SQUID dampened the OBR. As a result, the first peak only appears around 20 MHz which is a safe margin for the 1 MHz - 3.8 MHz FDM in use in the prototype readout. Using a spectrum analyzer and broadband LNAs, we also measured the OBR for the prototype FDM readout in the laboratory up to 500 MHz. The measurement was conducted at temperatures of 50 mK and 4 K and for various biasing of the DC SQUID. It turns out that OBRs are more intense at 50 mK and are caused by the harness impedance mismatch rather than the SQUID. Simulation codes and supporting materials are available at https://github.com/githubamin/LT-Spice-Simulation-of-FDM-readout .

We demonstrate multiplexed readout of 43 transition edge sensor (TES) bolometers operating at 90 mK using a frequency division multiplexing (FDM) readout chain with bias frequencies ranging from 1 to 3.5 MHz and a typical frequency spacing of 32 kHz. We improve the previously reported performance of our FDM system by two important steps. First, we replace the coplanar wires with microstrip wires, which minimize the cross talk from mutual inductance. From the measured electrical cross talk (ECT) map, the ECT of all pixels is carrier leakage dominated. Only five pixels show an ECT level higher than 1%. Second, we reduce the thermal response speed of the TES detectors by a factor of 20 by increasing the heat capacity of the TES, which allows us to bias all TES detectors below 50% in transition without oscillations. We compare the current-voltage curves and noise spectra of the TESs measured in single-pixel mode and multiplexing mode. We also compare the noise equivalent power (NEP) and the saturation power of the bolometers in both modes, where 38 pixels show less than 10% difference in NEP and 5% difference in saturation power when measured in the two different modes. The measured noise spectrum is in good agreement with the simulated noise based on measured parameters from an impedance measurement, confirming that our TES is dominated by phonon noise.

We demonstrate multiplexing readout of 60 transition edge sensor (TES) bolometers operating at 90 mK using a frequency division multiplexing readout chain with bias frequencies ranging from 1 to 3.5 MHz and with a typical frequency spacing of 32 kHz. The readout chain starts with a two-stage SQUID amplifier and has a noise level of 9.5 pA/Hz. We compare current–voltage curves and noise spectra of TESs measured in a single-pixel mode and in a multiplexing mode. We also map the noise equivalent power (NEP) and the saturation power of the bolometers in both modes, where there are 43 pixels that do not show more than 10% difference in NEP and 5% in saturation power when measured in single pixel and multiplex modes. We have read out a TES with an NEP of 0.45 aW/Hz in the multiplexing-mode, which demonstrates the capability of reading out ultra-low noise TES bolometer arrays for space applications.

We have characterized and mapped the electrical cross talk (ECT) of a frequency division multiplexing (FDM) system with a transition edge sensor (TES) bolometer array, which is intended for space applications. By adding a small modulation at 120 Hz to the AC bias voltage of one bolometer and measuring the cross talk response in the current noise spectra of the others simultaneously, we have for the first time mapped the ECT level of 61 pixels with a nominal frequency spacing of 32 kHz in a 61 × 61 matrix and a carrier frequency ranging from 1 MHz to 4 MHz. We find that about 94% of the pixels show an ECT level of less than 0.4%. Only the adjacent pixels reach this level, and the ECT for the rest of the pixels is less than 0.1%. We also observe higher ECT levels, up to 10%, between some of the pixels, which have bundled long, parallel coplanar wires connecting TES bolometers to inductor-capacitor filters. In this case, the high mutual inductances dominate. To mitigate this source of ECT, the coplanar wires should be replaced by microstrip wires in the array. Our study suggests that an FDM system can have a relatively low ECT level, e.g., around 0.4% if the frequency spacing is 30 kHz. Our results successfully demonstrate a low electrical cross talk for a space FDM technology.

AbstractMeasurements in the infrared wavelength domain allow direct assessment of the physical state and energy balance of cool matter in space, enabling the detailed study of the processes that govern the formation and evolution of stars and planetary systems in galaxies over cosmic time. Previous infrared missions revealed a great deal about the obscured Universe, but were hampered by limited sensitivity.SPICA takes the next step in infrared observational capability by combining a large 2.5-meter diameter telescope, cooled to below 8 K, with instruments employing ultra-sensitive detectors. A combination of passive cooling and mechanical coolers will be used to cool both the telescope and the instruments. With mechanical coolers the mission lifetime is not limited by the supply of cryogen. With the combination of low telescope background and instruments with state-of-the-art detectors SPICA provides a huge advance on the capabilities of previous missions.SPICA instruments offer spectral resolving power ranging from R ~50 through 11 000 in the 17–230 μm domain and R ~28.000 spectroscopy between 12 and 18 μm. SPICA will provide efficient 30–37 μm broad band mapping, and small field spectroscopic and polarimetric imaging at 100, 200 and 350 μm. SPICA will provide infrared spectroscopy with an unprecedented sensitivity of ~5 × 10−20 W m−2 (5σ/1 h)—over two orders of magnitude improvement over what earlier missions. This exceptional performance leap, will open entirely new domains in infrared astronomy; galaxy evolution and metal production over cosmic time, dust formation and evolution from very early epochs onwards, the formation history of planetary systems.

SAFARI is one of the focal-plane instruments for the European/ Japanese far-IR SPICA mission proposed for the ESA M5 selection. It is based on three arrays with in total 3550 TES-based bolometers with noise-equivalent powers (NEP) of 2∙10-19 W/Hz. The arrays are operated in three wavelength bands: S-band for 30-60 µm, M-band for 60-110 µm and L-band for 110-210 µm, and have high optical efficiency. SRON is developing Frequency Domain Multiplexing (FDM) for readout of large AC biased TES arrays for both the SAFARI instrument, and the XIFU instrument on the X-ray Athena mission. In FDM for SAFARI, the TES bolometers are AC biased and read out using 24 channels. Each channel contains 160 pixels of which the resonance frequencies are defined by in-house developed cryogenic lithographic LC filters. FDM is based on the amplitude modulation of a carrier signal, which also provides the AC voltage bias, with the signal detected by the TES. To overcome the dynamic range limitations of the SQUID pre-amplifier, baseband feedback (BBFB) is applied. BBFB attempts to cancel the error signal in the sum-point, at the input coil of the SQUID, by feeding back a remodulated signal to the sum-point, and therefore improving the dynamic range of the SQUID pre-amplifier. Previously we have reported on a detailed study of the effects of electrical crosstalk using our first iteration of a prototype of the full 160 pixel FDM experiment and the successful readout of 132 pixels using our 176 pixel FDM system. After the demonstration it is important to perform more detailed measurements to consolidate the system. For instance, one of the important next steps is to expose the FDM system to an optical infrared source. The cold part of the FDM system consists of a detector chip with 176 pixels with a designed NEP of 7∙10-19 W/Hz and two matching LC filter chips, each of which contains 88 carefully placed high-Q resonators, with a total of 176 different resonance frequencies, and a single-stage SQUID. The warm electronics consist of a low-noise amplifier (LNA) and a digital board on which the generation of the bias carriers, the demodulation of the signal and remodulation of the feedback signal are performed. The optical experiment will be conducted in a Leiden Cryogenics dilution refrigerator with a cooling power of about 200µW at 100 mK. This system contains multiple optical sources. These include a conical black body radiator which can be operated in the range of 3-34K and a light pipe through which the experiment can be illuminated from outside the cryostat. Dark measurements are conducted in a Janis ADR system with a base temperature of 50mK. In this paper we describe the experimental tests and results of the more detailed testing of our 176 pixel TES bolometer system.

In this paper we present a new cavity backshort design for TES (transition edge sensor) detectors which will provide increased coupling of the incoming astronomical signal to the detectors. The increased coupling results from the improved geometry of the cavities, where the geometry is a consequence of the proposed chemical etching manufacturing technique. Using a number of modelling techniques, predicted results of the performance of the cavities for frequencies of 4.3-10 THz are presented and compared to more standard cavity designs. Excellent optical efficiency is demonstrated, with improved response flatness across the band. In order to verify the simulated results, a scaled model cavity was built for testing at the lower W-band frequencies (75-100 GHz) with a VNA system. Further testing of the scale model at THz frequencies was carried out using a globar and bolometer via an FTS measurement set-up. The experimental results are presented, and compared to the simulations. Although there is relatively poor comparison between simulation and measurement at some frequencies, the discrepancies are explained by means of higher-mode excitation in the measured cavity which are not accounted for in the single-mode simulations. To verify this assumption, a better behaved cylindrical cavity is simulated and measured, where excellent agreement is demonstrated in those results. It can be concluded that both the simulations and the supporting measurements give confidence that this novel cavity design will indeed provide much-improved optical coupling for TES detectors in the far-infrared/THz band. (C) 2018 Elsevier B.V. All rights reserved.

We have measured the optical response of prototype detectors for SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. SAFARI’s three bolometer arrays, coupled with a Fourier transform spectrometer, will provide images of a 2′×2′ field of view with spectral information over the wavelength range 34–210 μm. Each horn-coupled bolometer consists of a transition edge sensor (TES), with a transition temperature close to 100 mK, and a thin-film Ta absorber on a thermally-isolated silicon nitride membrane. SAFARI requires extremely sensitive detectors (NEP∼2×10−19 W/\(\sqrt{\mathrm{Hz}}\)), with correspondingly low saturation powers (∼5 fW), to take advantage of SPICA’s cooled optics. To meet the challenge of testing such sensitive detectors we have constructed an ultra-low background test facility based on a cryogen-free high-capacity dilution refrigerator, paying careful attention to stray-light exclusion, shielding, and vibration isolation. For optical measurements the system contains internal cold (3–30 K) and hot (∼300 K) black-body calibration sources, as well as a light pipe for external illumination. We discuss our measurements of high optical efficiency in prototype SAFARI detectors and describe recent improvements to the test facility that will enable us to test the full SAFARI focal-plane arrays.

We have constructed a test facility for characterizing the focal plane arrays of SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. SAFARI’s three bolometer arrays are populated with extremely sensitive (\(\mathit{NEP}\sim 2\times 10^{-19}~\mathrm{W}/\sqrt{\mathrm{Hz}}\)) transition edge sensors with a transition temperature close to 100 mK. The extreme sensitivity and low saturation power (∼4 fW) of SAFARI’s detectors present challenges to characterizing them. In optimizing the SAFARI Detector System Test Facility we have paid careful attention to stray-light exclusion as well as electrical, magnetic, and mechanical isolation. We present measurements verifying the facility’s performance and analyze them in terms of a two-fluid model of the TES current on the transition to investigate the background power level. We have measured a detector NEP of (5.1±0.4)×10−19 W Hz−1/2, showing that the facility is ready to test the SAFARI prototype arrays and is approaching the performance needed for testing the flight arrays.

SRON is developing an electronic read-out system for an array of transition edge sensors (TES) based on the techniques of frequency domain multiplexing (FDM) and base-band feedback (BBFB). The astronomical applications of our system are the read-out of soft X-ray micro-calorimeters in a potential instrument on the European X-ray mission-under-study Athena and far-IR bolometers for the SAFARI instrument on the Japanese mission SPICA. In this paper we demonstrate the simultaneous locking of up to 51 BBFB loops. While locked, the in-band read-out noise of the loops is shown to reach the \(\mbox{10~pA/}\sqrt{\mathrm{Hz}}\) level required for these missions.

SPICA is an infra-red (IR) telescope with a cryogenically cooled mirror (~5K) with three instruments on board, one of which is SAFARI that is an imaging Fourier Transform Spectrometer (FTS) with three bands covering the wavelength of 34-210 μm. We develop transition edge sensors (TES) array for short wavelength band (34-60 μm) of SAFARI. These are based on superconducting Ti/Au bilayer as TES bolometers with a Tc of about 105 mK and thin Ta film as IR absorbers on suspended silicon nitride (SiN) membranes. These membranes are supported by long and narrow SiN legs that act as weak thermal links between the TES and the bath. Previously an electrical noise equivalent power (NEP) of 4×10-19 W/√Hz was achieved for a single pixel of such detectors. As an intermediate step toward a full-size SAFARI array (43×43), we fabricated several 8×9 detector arrays. Here we describe the design and the outcome of the dark and optical tests of several of these devices. We achieved high yield (<93%) and high uniformity in terms of critical temperature (<5%) and normal resistance (7%) across the arrays. The measured dark NEPs are as low as 5×10-19 W/√Hz with a response time of about 1.4 ms at preferred operating bias point. The optical coupling is implemented using pyramidal horns array on the top and hemispherical cavity behind the chip that gives a measured total optical coupling efficiency of 30±7%.

COVER is an experiment which aims to detect the signature of gravitational waves from inflation by measuring the B-mode polarization of the cosmic microwave background. CLOVER consists of three telescopes operating at 97, 150, and 220 GHz. The 97-GHz telescope has 160 feedhorns in its focal plane while the 150 and 220-GHz telescopes have 256 horns each. The horns are arranged in a hexagonal array and feed a polarimeter which uses finfine-coupled TES bolometers as detectors. To detect the two polarizations the 97-GHz telescope has 320 detectors while the 150 and 220-GHz telescopes have 512 detectors each. To achieve the target NEPs (1.5, 2.5, and 4.5 x 10 -17 \V/ 071-Z) the detectors are cooled to 100 mK for the 97 and 150-GHz polarimeters and 230 mK for the 220-GHz polarimeter. Each detector is fabricated as a single chip to ensure a 100% operational focal plane. The detectors are contained in linear modules made of copper which form split-block waveguides. The detector modules contain 16 or 20 detectors each for compatibility with the hexagonal arrays of horns in the telescopes' focal planes. Each detector module contains a time-division SQUID multiplexer to read out the detectors. Further amplification of the multiplexed signals is provided by SQUID series arrays. The first prototype detectors for COVER operate with a bath temperature of 230 mK and are used to validate the detector design as well as the polarimeter technology. We describe the design of the CAWER detectors, detector blocks, and readout, and present preliminary measurements of the prototype detectors' performance.

Detectors based on transition edge sensors (TESs) must achieve theoretically predicted noise levels if they are to be suitable for the next generation of space-borne astronomical telescopes. The noise of an ideal detector is determined by the sum of three contributions: (i) thermal-fluctuation noise in the heat link to the bath, (ii) Johnson noise in the sensor itself, and (iii) noise in the electrical read-out circuit. Many groups have reported TESs with noise levels significantly above the theoretical predictions. We use two well-defined experimental configurations to measure the read-out noise spectra of Mo–Cu TESs with transition temperatures of 370 and 200mK. The TESs are geometrically simple, comprising superconducting and normal metal films on a silicon nitride (SiNx) membrane. The measurements are compared with a multiparameter noise model, which is based on a physical model of the thin-film devices. Taking into consideration separate, accurate measurements of the heat capacity of identical SiNx membranes, we are able to provide a good account of both the magnitude and frequency dependences of the measured current-noise spectra. We find that an important excess noise mechanism involves the random exchange of heat between the heat capacity of the bilayer and the heat capacity of the nitride membrane, with either the thermal conductance of the membrane, or in some cases the thermal conductance of the bilayer, being the mediating path. Clear design recommendations are given to achieve the best possible noise performance.