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The POLARBEAR-2 Cryogenic Receiver for Cosmic Microwave Background Polarization Science

  • Author(s): Howe, Logan
  • Advisor(s): Keating, Brian
  • et al.
Abstract

Measurements of the Cosmic Microwave Background (CMB) have proved pivotal

over the last few decades in the development of ΛCDM, the standard model of cosmology.

Coupled with the standard model of particle physics, these two theories describe a majority

of our observations of the Universe’s structure, dynamics, and evolution. Beyond discovering

the specifics of how our Universe was formed, remaining open questions regarding our

Universe include the masses of neutrino species, the exact nature of dark matter, and the

equation of state of dark energy – to name a few. The CMB is imprinted with information

that can help answer all these questions, making measurements of the temperature and

polarization field of the CMB at high precision an effective path to increasing our understanding

of fundamental physics. The polarization field especially, composed of parity even

E-mode and parity odd B-mode patterns, possesses untapped constraining power, at both

very large and very small angular scales.

This dissertation describes the design and characterization of cryogenic receivers for

the Simons Array CMB polarization experiment. The Simons Array is located at 5200 m

elevation in the Atacama desert, Chile and consists of three off-axis Gregorian-Dragone

telescopes, each coupled to a POLARBEAR-2 cryogenic receiver. Each receiver’s focal plane

is comprised of 7,588 transition edge sensor (TES) bolometers cooled to 250 mK and

read out using 4 K superconducting quantum interference devices (SQUIDs) using digital

frequency division multiplexing (DfMUX). The POLARBEAR-2 receiver cryostat consists of

an optics tube and backend cryostat, which are built and tested separately, then integrated

for final testing before deployment to the Chilean site. Here we describe fabrication

and cryogenic validation of two POLARBEAR-2 backends, and of the complete second

POLARBEAR-2 receiver: POLARBEAR-2b. Additionally, we discuss readout and detector

integration, including detailed SQUID characterization and TES array measurements, and

demonstration of deployment readiness of all selected devices and subcomponents. Finally,

we describe efforts and progress towards final lab validation of the POLARBEAR-2b receiver

and final demonstrations of deployment readiness.

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