  Current experiments aimed at measuring the polarization of the Cosmic
Microwave Background (CMB) use cryogenic detector arrays and cold optical
systems to boost the mapping speed of the sky survey. For these reasons, large
volume cryogenic systems, with large optical windows, working continuously for
years, are needed. Here we report on the cryogenic system of the QUBIC (Q and U
Bolometric Interferometer for Cosmology) experiment: 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, using two pulse-tube refrigerators to cool at ~3K a large (~1
m^3) volume, heavy (~165kg) instrument, including the cryogenic polarization
modulator, the corrugated feedhorns array, and the lower temperature stages; a
4He evaporator cooling at ~1K the interferometer beam combiner; a 3He
evaporator cooling at ~0.3K the focal-plane detector arrays. 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
0.33K, while the polarization modulator has been operated from a ~10K base
temperature. The system has been tilted to cover the boresight elevation range
20 deg -90 deg without significant temperature variations. The instrument is
now ready for deployment to the high Argentinean Andes.
