  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.
