QUBIC VI: Cryogenic half wave plate rotator, design and performance
Authors: D'Alessandro G. , Mele L. , Columbro F. , Amico G. , Battistelli E.S. , de Bernardis P. , Coppolecchia A. , De Petris M. , Grandsire L. , Hamilton J.-Ch. , Lamagna L. , Marnieros S. , Masi S. , Mennella A. , O'Sullivan C. , Paiella A. , Piacentini F. , Piat M. , Pisano G. , Presta G. , Tartari A. , Torchinsky S.A. , Voisin F. , Zannoni M. , Ade P. , Alberro J.G. , Almela A. , Arnaldi L.H. , Auguste D. , Aumont J. , Azzoni S. , Banfi S. , Baù A. , Bélier B. , Bennett D. , Bergé L. , Bernard J.-Ph. , Bersanelli M. , Bigot-Sazy M.-A. , Bonaparte J. , Bonis J. , Bunn E. , Burke D. , Buzi D. , Cavaliere F. , Chanial P. , Chapron C. , Charlassier R. , Cobos Cerutti A.C. , De Gasperis G. , De Leo M. , Dheilly S. , Duca C. , Dumoulin L. , Etchegoyen A. , Fasciszewski A. , Ferreyro L.P. , Fracchia D. , Franceschet C. , Gamboa Lerena M.M. , Ganga K.M. , García B. , García Redondo M.E. , Gaspard M. , Gayer D. , Gervasi M. , Giard M. , Gilles V. , Giraud-Heraud Y. , Gómez Berisso M. , González M. , Gradziel M. , Hampel M.R. , Harari D. , Henrot-Versillé S. , Incardona F. , Jules E. , Kaplan J. , Kristukat C. , Loucatos S. , Louis T. , Maffei B. , Marty W. , Mattei A. , May A. , McCulloch M. , Melo D. , Montier L. , Mousset L. , Mundo L.M. , Murphy J.A. , Murphy J.D. , Nati F. , Olivieri E. , Oriol C. , Pajot F. , Passerini A. , Pastoriza H. , Pelosi A. , Perbost C. , Perciballi M. , Pezzotta F. , Piccirillo L. , Platino M. , Polenta G. , Prêle D. , Puddu R. , Rambaud D. , Rasztocky E. , Ringegni P. , Romero G.E. , Salum J.M. , Schillaci A. , Scóccola C.G. , Scully S. , Spinelli S. , Stankowiak G. , Stolpovskiy M. , Supanitsky A.D. , Thermeau J.-P. , Timbie P. , Tomasi M. , Tucker C. , Tucker G. , Viganò D. , Vittorio N. , Wicek F. , Wright M. , Zullo A. 
Published: April 2022

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 χPol
of 0.12%, with 71% of detectors showing a
χPol + 2σ upper limit <1%, measured using
selected detectors that had the best signal-to-noise ratio.