Implementation and testing of a 100 mK ADR backed by a 4He/3He sorption fridge for CMB-S4 project

Abstract CMB-S4 is a ground-based experiment that will use large-format bolometer arrays to map Cosmic Microwave Background (CMB) polarization with unprecedented sensitivity, including the search for a B-mode polarization pattern associated with cosmic inflation. The focal planes on which the superconducting detectors are mounted weigh about 20 kg each and must be cooled at 100 mK. To reach this temperature for a first receiver fielding prototype hardware, we have developed an Adiabatic Demagnetization Refrigerator (ADR) used in conjunction with an existing 4He/3He sorption refrigerator. The ADR unit described here is based on a Chromium Potassium Alum (CPA) salt pill. It is backed by a 3-stage (4He/3He/3He) sorption fridge, coupled through a 3He gas-gap heat switch. This heat switch is connected to a 3He stage of the sorption fridge to dissipate the heat generated by magnetization. The ADR is cycled with a 1.3 T magnetic field. The parasitic field at the detectors’ location during observations is kept below 2.5 µT thanks to a ferromagnetic shield. The pill is supported by Kevlar lines, which are doubly intercepted by the 4He and 3He stages of the sorption fridge to reduce conductive heat losses. With a total recycling time of 12 hours, this cryogenic assembly provides 2 µW of cooling power during 48 hours at 100 mK. The ADR has been designed by CEA/DSBT in France and is now implemented in a cryostat at Caltech.