A large-diameter cryogenic rotation stage for half-wave plate polarization modulation on the POLARBEAR-2 experiment

TL;DR

This paper presents the design and testing of a large-diameter cryogenic rotation stage for a polarization modulator in CMB experiments, enabling rapid polarization modulation while minimizing thermal noise.

Contribution

It introduces a novel 454-mm aperture cryogenic rotation stage with superconducting magnetic bearing for CMB polarization modulation.

Findings

Successful design and testing of the CRS at < 65 K

Achieved specifications for polarization modulation in CMB experiments

Plans for integration and future performance evaluation

Abstract

We describe the design of a cryogenic rotation stage (CRS) for use with the cryogenic half-wave plate (CHWP) polarization modulator on the POLARBEAR-2b and POLARBEAR-2c (PB2b/c) cosmic microwave background (CMB) experiments, the second and third installments of the Simons Array. Rapid modulation of the CMB polarization signal using a CHWP suppresses 1/f contamination due to atmospheric turbulence and allows a single polarimeter to measure both polarization states, mitigating systematic effects that arise when differencing orthogonal detectors. To modulate the full detector array while avoiding excess photon loading due to thermal emission, the CHWP must have a clear-aperture diameter of > 450 mm and be cooled to < 100 K. We have designed a 454-mm-clear-aperture, < 65 K CRS using a superconducting magnetic bearing driven by a synchronous magnetic motor. We present the specifications for the CRS, its interfacing to the PB2b/c receiver cryostat, its performance in a stand-alone test, and plans for future work.