QUBIC V: Cryogenic system design and performance

Abstract

Current experiments aimed at measuring the polarization of the Cosmic Microwave Background (CMB) use cryogenic detector arrays with cold optical systems to boost their mapping speed. For this reason, large volume cryogenic systems with large optical windows, working continuously for years, are needed. The cryogenic system of the QUBIC (Q & U Bolometric Interferometer for Cosmology) experiment solves a combination of simultaneous requirements: very large optical throughput (∼40 cm2sr), large volume (∼1 m3) and large mass (∼165 kg) of the cryogenic instrument. Here we describe its design, fabrication, experimental optimization and validation in the Technological Demonstrator configuration. The QUBIC cryogenic system is based on a large volume cryostat that uses two pulse-tube refrigerators to cool the instrument to ∼3 K. The instrument includes the cryogenic polarization modulator, the corrugated feedhorn array, and the lower temperature stages: a 4He evaporator cooling the interferometer beam combiner to ∼1 K and a 3He evaporator cooling the focal-plane detector arrays to ∼0.3 K. The cryogenic system has been tested and validated for more than 6 months of continuous operation. The detector arrays have reached a stable operating temperature of 0.33 K, while the polarization modulator has operated at a ∼10 K base temperature. The system has been tilted to cover the boresight elevation range 20°-90° without significant temperature variations. The instrument is now ready for deployment to the high Argentinean Andes.

QUBIC VI: Cryogenic half wave plate rotator, design and performance

“…Figure 2 shows a CAD 3D model (left) of the cryogenic HWP rotator and a picture (right) of the system installed in the QUBIC cryostat. It is placed at the top of the 4 K stage, after the IR blocker filters, as described in [25]. The most important components are the transmission system, the pulley system and the HWP support.…”

“…A problem related to the rotation is the thermal load dissipated by the rotation itself. A temperature gradient across the HWP arises due to a combination of the frictional power load dissipated during the rotation and the thermal conductivity between the HWP and its cryogenic stage (4 K for QUBIC [35]). This gradient, before and after the rotation, should be stable to a mK level.…”

“…• thermal load lower than 10 mW at the operating temperature; from experiment thermal budget, see section 2.3.2 in Masi et al [35] • 4 K stage settle-down time, after one HWP rotation, of <85 s.…”

QUBIC I: Overview and science program

“…This is therefore the optical equivalent of a (wide-band) correlator in classical interferometry. Being a bolometric device, the whole instrument operates at cryogenic temperatures thanks to a large cryostat described in Masi et al [36] from this series of articles.…”

“…The scientific overview and expected performance of the instrument are addressed here. The other papers in the special issue address the following topics: the ability of bolometric interferometry to do spectral imaging [33], the calibration and performance in the laboratory of the TD [34], the performance of the detector array and readout electronics [35], the cryogenic system performance [36], the Half Wave Plate rotator system [37], the back-to-back feedhorn-switch system [38], and the optical design and performance [39].…”