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We present the requirements, design and evaluation of the cryogeniccontinuously rotating half-wave plate (CHWP) for the Simons Observatory (SO).SO is a cosmic microwave background (CMB) polarization experiment at ParqueAstron\'{o}mico Atacama in northern Chile that covers a wide range of angularscales using both small (0.42 m) and large (6 m) aperture telescopes. Inparticular, the small aperture telescopes (SATs) focus on large angular scalesfor primordial B-mode polarization. To this end, the SATs employ a CHWP tomodulate the polarization of the incident light at 8~Hz, suppressingatmospheric $1/f$ noise and mitigating systematic uncertainties that wouldotherwise arise due to the differential response of detectors sensitive toorthogonal polarizations. The CHWP consists of a 505 mm diameter achromaticsapphire HWP and a cryogenic rotation mechanism, both of which are cooled downto $\sim$50 K to reduce detector thermal loading. Under normal operation theHWP is suspended by a superconducting magnetic bearing and rotates with aconstant 2 Hz frequency, controlled by an electromagnetic synchronous motor.The rotation angle is detected through an angular encoder with a noise level of0.07$\mu\mathrm{rad}\sqrt{\mathrm{s}}$. During a cooldown, the rotor is held inplace by a grip-and-release mechanism that serves as both an alignment deviceand a thermal path. In this paper we provide an overview of the SO SAT CHWP:its requirements, hardware design, and laboratory performance.

The Simons Observatory: Cryogenic Half Wave Plate Rotation Mechanism for the Small Aperture Telescopes