QUBIC VI: cryogenic half wave plate rotator, design and performances

D'ALESSANDRO, G.; CLAUDIA G. SCOCCOLA; AND THE QUBIC COLLABORATION

JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS

IOP PUBLISHING LTD

Lugar: Londres; Año: 2021

1475-7516

Inflation Gravity Waves B-Modes polarization detection is the ultimate goalof modern large angular scale cosmic microwave background (CMB) experimentsaround the world. A big effort is undergoing with the deployment of manyground-based, balloon-borne and satellite experiments using different methodsto separate this faint polarized component from the incoming radiation. One ofthe largely used technique is the Stokes Polarimetry that uses a rotatinghalf-wave plate (HWP) and a linear polarizer to separate and modulate thepolarization components with low residual cross-polarization. This paperdescribes the QUBIC Stokes Polarimeter highlighting its design features and itsperformances. A common systematic with these devices is the generation of largespurious signals synchronous with the rotation and proportional to theemissivity of the optical elements. A key feature of the QUBIC StokesPolarimeter is to operate at cryogenic temperature in order to minimize thisunwanted component. Moving efficiently this large optical element at lowtemperature constitutes a big engineering challenge in order to reduce frictionpower dissipation. Big attention has been given during the designing phase tominimize the differential thermal contractions between parts. The rotation isdriven by a stepper motor placed outside the cryostat to avoid thermal loaddissipation at cryogenic temperature. The tests and the results presented inthis work show that the QUBIC polarimeter can easily achieve a precision below0.1° in positioning simply using the stepper motor precision and theoptical absolute encoder. The rotation induces only few mK of extra power loadon the second cryogenic stage (~ 8 K).