In:
Journal of Cosmology and Astroparticle Physics, IOP Publishing, Vol. 2022, No. 04 ( 2022-04-01), p. 039-
Abstract:
Setting an upper limit or detection of B-mode polarization
imprinted by gravitational waves from Inflation is one goal of modern large angular scale cosmic microwave background (CMB)
experiments around the world. A great effort is being made in the deployment of many ground-based, balloon-borne and satellite
experiments, using different methods to separate this faint polarized component from the incoming radiation. QUBIC exploits one
of the most widely-used techniques to extract the input Stokes parameters, consisting in a rotating half-wave plate (HWP) and a
linear polarizer to separate and modulate polarization components. QUBIC uses a step-by-step rotating HWP, with 15°
steps, combined with a 0.4°s -1 azimuth sky scan
speed. The rotation is driven by a stepper motor mounted on the cryostat outer shell to avoid heat load at internal cryogenic
stages. The design of this optical element is an engineering challenge due to its large 370 mm diameter and the
8 K operation temperature that are unique features of the QUBIC experiment. We present the design for a modulator
mechanism for up to 370 mm, and the first optical tests by using the prototype of QUBIC HWP (180 mm
diameter). The tests and results presented in this work show that the QUBIC HWP rotator can achieve a precision of 0.15°
in position by using the stepper motor and custom-made optical encoder. The rotation induces 〈 5.0 mW (95% C.L) of
power load on the 4 K stage, resulting in no thermal issues on this stage during measurements. We measure a temperature
settle-down characteristic time of 28 s after a rotation through a 15° step, compatible with the scanning
strategy, and we estimate a maximum temperature gradient within the HWP of ≤ 10 mK. This was calculated by setting
up finite element thermal simulations that include the temperature profiles measured during the rotator operations. We report
polarization modulation measurements performed at 150 GHz, showing a polarization efficiency 〉 99%
(68% C.L.) and a median cross-polarization χ Pol
of 0.12%, with 71% of detectors showing a χ Pol + 2 σ upper limit 〈 1%, measured using
selected detectors that had the best signal-to-noise ratio.
Type of Medium:
Online Resource
ISSN:
1475-7516
DOI:
10.1088/1475-7516/2022/04/039
Language:
Unknown
Publisher:
IOP Publishing
Publication Date:
2022
detail.hit.zdb_id:
2104147-7
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