The Simons Observatory: Large diameter and large load-capacity superconducting magnetic bearing for a millimeter-wave polarization modulator

We present the design methodology and characterization of a superconducting magnetic bearing (SMB) system for the polarization modulator in the small aperture telescope-low-frequency (SAT-LF), one of the SATs of the Simons Observatory (SO) that is sensitive at 30/40 GHz. SO is a cosmic microwave background (CMB) polarization experiment, with SATs targeting the search for primordial parity-odd polarization anisotropies at degree scales. Each SAT uses a cryogenic, rotating half-wave plate (HWP) as a polarization modulator to reduce atmospheric 1/f noise and instrumental systematics. The HWP system employs an SMB, composed of a magnet and a superconductor, to provide a 550 mm clear aperture and stable 2 Hz rotation at 50 K. A challenge in the SAT-LF HWP system is the heavy 35 kg load on the SMB due to a thicker HWP than used in previous telescopes. Since the SMB stiffness is critical for maintaining HWP alignment in the telescope, we developed a quantitative prediction method using finite element simulations with H-formulation. We evaluated stiffness for various magnet and superconductor geometries, refining their dimensions. The predictions are within 9% of the measurements of the SMB system. We also showed that the SMB produces sufficiently low friction-induced heat dissipation, measured at 0.26 W during 2 Hz rotation. The design methodology and implementation of the SMB here not only provides an enabling technology for SO SAT-LF but also is a stepping stone for future CMB experiments that make use of HWP polarization modulators.