A cryogenic continuously rotating half-wave plate for the POLARBEAR-2b cosmic microwave background receiver. (arXiv:2009.03972v2 [astro-ph.IM] UPDATED)

A cryogenic continuously rotating half-wave plate for the POLARBEAR-2b cosmic microwave background receiver. (arXiv:2009.03972v2 [astro-ph.IM] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Hill_C/0/1/0/all/0/1">C. A. Hill</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kusaka_A/0/1/0/all/0/1">A. Kusaka</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ashton_P/0/1/0/all/0/1">P. Ashton</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Barton_P/0/1/0/all/0/1">P. Barton</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Adkins_T/0/1/0/all/0/1">T. Adkins</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Arnold_K/0/1/0/all/0/1">K. Arnold</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bixler_B/0/1/0/all/0/1">B. Bixler</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ganjam_S/0/1/0/all/0/1">S. Ganjam</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lee_A/0/1/0/all/0/1">A. T. Lee</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Matsuda_F/0/1/0/all/0/1">F. Matsuda</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Matsumura_T/0/1/0/all/0/1">T. Matsumura</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sakurai_Y/0/1/0/all/0/1">Y. Sakurai</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tat_R/0/1/0/all/0/1">R. Tat</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhou_Y/0/1/0/all/0/1">Y. Zhou</a>

We present the design and laboratory evaluation of a cryogenic continuously
rotating half-wave plate (CHWP) for the POLARBEAR-2b (PB-2b) cosmic microwave
background (CMB) receiver, the second installment of the Simons Array. PB-2b
will observe at 5,200 m elevation in the Atacama Desert of Chile in two
frequency bands centered at 90 and 150 GHz. In order to suppress atmospheric
1/f noise and mitigate systematic effects that arise when differencing
orthogonal detectors, PB-2b modulates linear sky polarization using a CHWP
rotating at 2 Hz. The CHWP has a 440 mm clear aperture diameter and is cooled
to $approx$ 50 K in the PB-2b receiver cryostat. It consists of a low-friction
superconducting magnetic bearing (SMB) and a low-torque synchronous
electromagnetic motor, which together dissipate < 2 W. During cooldown, a
grip-and-release mechanism centers the rotor to < 0.5 mm, and during continuous
rotation, an incremental optical encoder measures the rotor angle with a noise
level of 0.1 $mathrm{mu rad / sqrt{Hz}}$. We discuss the experimental
requirements for the PB-2b CHWP, the designs of its various subsystems, and the
results of its evaluation in the laboratory. The presented CHWP has been
deployed to Chile and is expected to see first light on PB-2b in 2020 or 2021.

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