Sub-Kelvin Cooling for the BICEP Array Project

Kavli Affiliate: James Bock

| First 5 Authors: Lionel Duband, Thomas Prouve, James Bock, Lorenzo Moncelsi, Alessandro Schillaci

| Summary:

In the field of astrophysics, the faint signal from distant galaxies and
other dim cosmological sources at millimeter and submillimeter wavelengths
require the use of high-sensitivity experiments. Cryogenics and the use of
low-temperature detectors are essential to the accomplishment of the scientific
objectives, allowing lower detector noise levels and improved instrument
stability. Bolometric detectors are usually cooled to temperatures below 1K,
and the constraints on the instrument are stringent, whether the experiment is
a space-based platform or a ground-based telescope. The latter are usually
deployed in remote and harsh environments such as the South Pole, where
maintenance needs to be kept minimal. CEA-SBT has acquired a strong heritage in
the development of vibration-free multistage helium-sorption coolers, which can
provide cooling down to 200 mK when mounted on a cold stage at temperatures
<5K. In this paper, we focus on the development of a three-stage cooler
dedicated to the BICEP Array project led by Caltech/JPL, which aims to study
the birth of the Universe and specifically the unique B-mode pattern imprinted
by primordial gravitational waves on the polarization of the Cosmic Microwave
Background. Several cryogenic receivers are being developed, each featuring one
such helium-sorption cooler operated from a 4K stage cooled by a Cryomech
pulse-tube with heat lifts of >1.35W at 4.2K and >36W at 45K. The major
challenge of this project is the large masses to be cooled to sub-kelvin
temperatures (26 kg at 250mK) and the resulting long cool-down time, which in
this novel cooler design is kept to a minimum with the implementation of
passive and active thermal links between different temperature stages. A first
unit has been sized to provide 230, 70 and 2{mu}W of net heat lifts at the
maximum temperatures of 2.8K, 340 and 250mK, respectively, for a minimum
duration of 48 hours.

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