Design and development of an ambient-temperature continuously-rotating achromatic half-wave plate for CMB polarization modulation on the POLARBEAR-2 experiment

TL;DR

This paper details the design, construction, and testing of an ambient-temperature, continuously-rotating achromatic half-wave plate for polarization modulation in the POLARBEAR-2 CMB experiment, enhancing measurement sensitivity.

Contribution

It introduces a novel ambient-temperature sapphire achromatic HWP with a 2 Hz rotation suitable for CMB polarization studies, including design and performance analysis.

Findings

Successful implementation of a 500-mm sapphire HWP rotating at 2 Hz

Effective suppression of 1/f noise in polarization measurements

Improved polarization measurement accuracy for CMB studies

Abstract

We describe the development of an ambient-temperature continuously-rotating half-wave plate (HWP) for study of the Cosmic Microwave Background (CMB) polarization by the POLARBEAR-2 (PB2) experiment. Rapid polarization modulation suppresses 1/f noise due to unpolarized atmospheric turbulence and improves sensitivity to degree-angular-scale CMB fluctuations where the inflationary gravitational wave signal is thought to exist. A HWP modulator rotates the input polarization signal and therefore allows a single polarimeter to measure both linear polarization states, eliminating systematic errors associated with differencing of orthogonal detectors. PB2 projects a 365-mm-diameter focal plane of 7,588 dichroic, 95/150 GHz transition-edge-sensor bolometers onto a 4-degree field of view that scans the sky at $\sim$ 1 degree per second. We find that a 500-mm-diameter ambient-temperature sapphire achromatic HWP rotating at 2 Hz is a suitable polarization modulator for PB2. We present the design considerations for the PB2 HWP, the construction of the HWP optical stack and rotation mechanism, and the performance of the fully-assembled HWP instrument. We conclude with a discussion of HWP polarization modulation for future Simons Array receivers.