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The SAFARI instrument is a far-infrared imaging Fourier transform spectrometer for JAXA's SPICA mission. Taking advantage of the low emission of SPICA's 5 K telescope, SAFARI will provide sky background-limited, Nyquist-sampled spectroscopic imaging of a 2 2 field-of-view over 34-210 m, creating significant new possibilities for far-infrared astronomy. SAFARI's aggressive science goals drive the development of a unique detector system combining large-format Transition Edge Sensor arrays and frequency division multiplexed SQUID readout with a high 160 multiplexing factor. The detectors and their cold readout electronics are packaged into 3 focal plane arrays that will be integrated into SAFARI's focal plane unit. Here we present the preliminary system design and current development status of the SAFARI detector system.

We have constructed a compact dc SQUID electronics unit, in which the noise of the roomtemperature amplifier is measured by the SQUID and cancelled out. This makes it unnecessary to use a transformer and a modulation scheme to achieve noise matching between the SQUID and the amplifier. The electronics unit contains a P13~2-controller and it has demonstrated a bandwidth of 100 kHz, a 1.2 p @~/ & white noise level and a l/f noise corner of 0.3 Hz in a flux-locked loop mode. The design criteria for the noise-cancelled electronics are discussed, including the problem of exess noise.

Local negative feedback derived within the cryogenic stage from the output current of a voltage-biased SQUID series array is used to linearize the flux response and to simultaneously approach the noise matching resistance of the room-temperature readout amplifier. The flux noise level of the SQUID array was 0.5 µΦ 0 /Hz 1/2 in open loop and 0.8 µΦ 0 /Hz 1/2 in the feedback arrangement having a 2.2 Φ 0 peak-to-peak flux locking range. The noise level degraded to 2 µΦ 0 /Hz 1/2 in an arrangement with a 7 Φ 0 locking range. Very good linearity was observed in the feedback system regardless of the modest loop gain, owing to the open-loop SQUID characteristics which are more linear in voltage biased than current biased case. Upward and downward slew rates of 3.4 and 1.2 Φ 0 /µs were recorded which, however, do not represent ultimate limits of the approach. Local feedback schemes are reviewed and their effect on the linearity of a SQUID system is discussed. * This paper has been published as Supercond. Sci. Tech. 21 (2008) 045009, http://stacks.iop.org/0953-2048/21/045009 1 SQUIDs and SQUID arrays can be treated on equal footing by noting that a k-SQUID series array, whose each constituent SQUID has loop inductance L SQ , junction capacitance C J and is coupled to a m-turn input coil, has the same circuit parameters as a single SQUID with loop inductance L SQ /k, junction capacitance kC J , a km-turn input coil and a 1:k transformer at its output.

Frequency domain multiplexing (FDM) is the baseline readout system for the Xray Integral Field Unit (X-IFU) on board the Athena mission. Under the FDM scheme, TESs are coupled to a passive LC filter and biased with alternating current (AC bias) at MHz frequencies. Using high-quality factor LC filters and room temperature electronics developed at SRON and low-noise two-stage SQUID amplifiers provided by VTT, we have recently demonstrated good performance with the FDM readout of Mo/Au TES calorimeters with Au/Bi absorbers. We have achieved a performance requested for the demonstration model (DM) with the single pixel AC bias (∆ E =1.8 eV) and 9 pixel multiplexing (∆ E =2.6 eV) modes. We have also demonstrated 14-pixel multiplexing with an average energy resolution of 3.3 eV, which is limited by non-fundamental issues related to FDM readout in our lab setup.