A cryogenic continuously rotating half-wave plate for the POLARBEAR-2b cosmic microwave background receiver

TL;DR

This paper details the design, laboratory testing, and deployment of a cryogenic, continuously rotating half-wave plate for the POLARBEAR-2b CMB receiver, aimed at improving polarization measurements by reducing noise and systematic effects.

Contribution

It introduces a novel cryogenic CHWP with a superconducting magnetic bearing and optical encoder, optimized for CMB polarization observations in the POLARBEAR-2b experiment.

Findings

Successfully designed and tested the CHWP system in the laboratory.

Achieved rotor angle measurement noise of 0.1 μrad/√Hz.

Deployed the CHWP in Chile for upcoming CMB observations.

Abstract

We present the design and laboratory evaluation of a cryogenic continuously rotating half-wave plate (CHWP) for the POLARBEAR-2b (PB-2b) cosmic microwave background (CMB) receiver, the second installment of the Simons Array. PB-2b will observe at 5,200 m elevation in the Atacama Desert of Chile in two frequency bands centered at 90 and 150 GHz. In order to suppress atmospheric 1/f noise and mitigate systematic effects that arise when differencing orthogonal detectors, PB-2b modulates linear sky polarization using a CHWP rotating at 2 Hz. The CHWP has a 440 mm clear aperture diameter and is cooled to $\approx$ 50 K in the PB-2b receiver cryostat. It consists of a low-friction superconducting magnetic bearing (SMB) and a low-torque synchronous electromagnetic motor, which together dissipate < 2 W. During cooldown, a grip-and-release mechanism centers the rotor to < 0.5 mm, and during continuous rotation, an incremental optical encoder measures the rotor angle with a noise level of 0.1 $\mathrm{\mu rad / \sqrt{Hz}}$. We discuss the experimental requirements for the PB-2b CHWP, the designs of its various subsystems, and the results of its evaluation in the laboratory. The presented CHWP has been deployed to Chile and is expected to see first light on PB-2b in 2020 or 2021.