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