Abstract
We describe the design of a cryogenic rotation stage (CRS) for use with the cryogenic half-wave plate (CHWP) polarization modulator on the POLARBEAR-2b and POLARBEAR-2c (PB2b/c) cosmic microwave background (CMB) experiments, the second and third installments of the Simons Array. Rapid modulation of the CMB polarization signal using a CHWP suppresses 1/f contamination due to atmospheric turbulence and allows a single polarimeter to measure both polarization states, mitigating systematic effects that arise when differencing orthogonal detectors. To modulate the full detector array while avoiding excess photon loading due to thermal emission, the CHWP must have a clear-aperture diameter of > 450 mm and be cooled to < 100 K. We have designed a 454 mm clear-aperture, < 65 K CRS using a superconducting magnetic bearing driven by a synchronous magnetic motor. We present the specifications for the CRS, its interfacing to the PB2b/c receiver cryostat, its performance in a stand-alone test, and plans for future work.
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Acknowledgements
We gratefully acknowledge support for PB2b/c CRS development from the National Science Foundation through MSIP Grant #1440338 as well as the Laboratory Directed Research and Development Program at Lawrence Berkeley National Laboratory.
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Hill, C.A., Kusaka, A., Barton, P. et al. A Large-Diameter Cryogenic Rotation Stage for Half-Wave Plate Polarization Modulation on the POLARBEAR-2 Experiment. J Low Temp Phys 193, 851–859 (2018). https://doi.org/10.1007/s10909-018-1980-6
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DOI: https://doi.org/10.1007/s10909-018-1980-6