Ultra-Thin Large-Aperture Vacuum Windows for Millimeter Wavelengths Receivers

TL;DR

This paper explores the use of ultra-thin, high-strength polyethylene vacuum windows to improve the sensitivity of large-aperture millimeter-wave telescopes for cosmic microwave background observations.

Contribution

It introduces high-modulus polyethylene as a new material for vacuum windows, enabling thinner, more transparent windows for large-aperture ground-based CMB experiments.

Findings

High-modulus polyethylene has tensile strength two orders of magnitude greater than traditional materials.

Ultra-thin polyethylene windows show promising optical transmission properties.

Mechanical testing indicates suitability for atmospheric pressure conditions.

Abstract

Targeting faint polarization patterns arising from Primordial Gravitational Waves in the Cosmic Microwave Background requires excellent observational sensitivity. Optical elements in small aperture experiments such as Bicep3 and Keck Array are designed to optimize throughput and minimize losses from transmission, reflection and scattering at millimeter wavelengths. As aperture size increases, cryostat vacuum windows must withstand larger forces from atmospheric pressure and the solution has often led to a thicker window at the expense of larger transmission loss. We have identified a new candidate material for the fabrication of vacuum windows: with a tensile strength two orders of magnitude larger than previously used materials, woven high-modulus polyethylene could allow for dramatically thinner windows, and therefore significantly reduced losses and higher sensitivity. In these proceedings we investigate the suitability of high-modulus polyethylene windows for ground-based CMB experiments, such as current and future receivers in the Bicep/Keck Array program. This includes characterizing their optical transmission as well as their mechanical behavior under atmospheric pressure. We find that such ultra-thin materials are promising candidates to improve the performance of large-aperture instruments at millimeter wavelengths, and outline a plan for further tests ahead of a possible upcoming field deployment of such a science-grade window.