Performance of a Low-Noise Test Facility for the SAFARI TES Bolometer 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 ( $mathit{NEP}sim 2times 10^{-19}~mathrm{W}/sqrt{mathrm{Hz}}$ ) 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.