QUBIC VI: cryogenic half wave plate rotator, design and performances

TL;DR

This paper presents the design and performance of the QUBIC VI cryogenic half-wave plate rotator, a device used in CMB polarization experiments to accurately modulate polarization signals while operating at cryogenic temperatures.

Contribution

It introduces a cryogenic HWP rotator with minimized systematic errors and demonstrates its effective operation with high positional accuracy and low thermal load.

Findings

Achieves below 0.1° positioning precision.

Induces only a few millikelvin of additional thermal load.

Operates effectively at cryogenic temperatures (~8 K).

Abstract

Inflation Gravity Waves B-Modes polarization detection is the ultimate goal of modern large angular scale cosmic microwave background (CMB) experiments around the world. A big effort is undergoing with the deployment of many ground-based, balloon-borne and satellite experiments using different methods to separate this faint polarized component from the incoming radiation. One of the largely used technique is the Stokes Polarimetry that uses a rotating half-wave plate (HWP) and a linear polarizer to separate and modulate the polarization components with low residual cross-polarization. This paper describes the QUBIC Stokes Polarimeter highlighting its design features and its performances. A common systematic with these devices is the generation of large spurious signals synchronous with the rotation and proportional to the emissivity of the optical elements. A key feature of the QUBIC Stokes Polarimeter is to operate at cryogenic temperature in order to minimize this unwanted component. Moving efficiently this large optical element at low temperature constitutes a big engineering challenge in order to reduce friction power dissipation. Big attention has been given during the designing phase to minimize the differential thermal contractions between parts. The rotation is driven by a stepper motor placed outside the cryostat to avoid thermal load dissipation at cryogenic temperature. The tests and the results presented in this work show that the QUBIC polarimeter can easily achieve a precision below 0.1{\deg} in positioning simply using the stepper motor precision and the optical absolute encoder. The rotation induces only few mK of extra power load on the second cryogenic stage (~ 8 K).