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The latest generation of cosmic microwave background (CMB) telescopes issearching for the undetected faint signature of gravitational waves frominflation in the polarized signal of the CMB. To achieve the unprecedentedlevels of sensitivity required, these experiments use arrays of superconductingTransition Edge Sensor (TES) bolometers that are cooled to sub-Kelvintemperatures for photon-noise limited performance. These TES detectors are readout using low- noise SQUID amplifiers. To rapidly test these detectors andsimilar devices in a laboratory setting, we constructed a cryogenicrefrigeration chain consisting of a commercial two-stage pulse-tube cooler,with a base temperature of 3 K, and a closed-cycle 3He/4He/3He sorption cooler,with a base temperature of 220 mK. A commercial dc SQUID system, with sensorscooled to 4 K, was used as a highly-sensitive cryogenic ammeter. Due to theextreme sensitivity of SQUIDs to changing magnetic fields, there are severalchallenges involving cooling them with pulse-tube coolers. Here we describe thesuccessful design and implementation of measures to reduce the vibration,electromagnetic interference, and other potential sources of noise associatedwith pulse-tube coolers.

Performance of a 4 Kelvin pulse-tube cooled cryostat with dc SQUID amplifiers for bolometric detector testing