A large-diameter hollow-shaft cryogenic motor based on a superconducting magnetic bearing for millimeter-wave polarimetry

Abstract

In this paper, we present the design and measured performance of a novel cryogenic motor based on a superconducting magnetic bearing (SMB). The motor is tailored for use in millimeter-wave half-wave plate (HWP) polarimeters, where a HWP is rapidly rotated in front of a polarization analyzer or polarization-sensitive detector. This polarimetry technique is commonly used in cosmic microwave background polarization studies. The SMB we use is composed of fourteen yttrium barium copper oxide (YBCO) disks and a contiguous neodymium iron boron (NdFeB) ring magnet. The motor is a hollow-shaft motor because the HWP is ultimately installed in the rotor. The motor presented here has a 100 mm diameter rotor aperture. However, the design can be scaled up to rotor aperture diameters of approximately 500 mm. Our motor system is composed of four primary subsystems: (i) the rotor assembly, which includes the NdFeB ring magnet, (ii) the stator assembly, which includes the YBCO disks, (iii) an incremental encoder, and (iv) the drive electronics. While the YBCO is cooling through its superconducting transition, the rotor is held above the stator by a novel hold and release mechanism. The encoder subsystem consists of a custom-built encoder disk read out by two fiber optic readout sensors. For the demonstration described in this paper, we ran the motor at 50 K and tested rotation frequencies up to approximately 10 Hz. The feedback system was able to stabilize the rotation speed to approximately 0.4%, and the measured rotor orientation angle uncertainty is less than 0.15°. Lower temperature operation will require additional development activities, which we will discuss.

Critical Review of Flywheel Energy Storage System

“…Cryogenic cooling is needed to keep the bearings from failing. High-temperature superconductors (HTS) are in use with SMBs to combat this system requirement, and there have been recent attempts to incorporate cryogenic isolation for the SMBs to reduce costs and keep the operating temperatures low, but, at the moment, SMBs and PMBs need to be used at the same time [111,112]. The high-speed rotor and vacuum conditions mean that there is a high pressure placed on the system's suspension bearings [114].…”

Progress Report on the Large-Scale Polarization Explorer

“…The key unique properties of SWIPE are the use of a large aperture half-wave plate (HWP), acting as a broadband polarization modulator at the telescope input (see e.g. [5], [6]), and the optimization of throughput with the multi-moded coupling of the single detectors to the system optics [7]. The HWP rotation mechanism is based on a superconducting magnetic suspension system allowing continuous rotation at 60 rpm, corresponding to 4 Hz modulation of the polarized signal [6].…”

A cryogenic continuously rotating half-wave plate mechanism for the POLARBEAR-2b cosmic microwave background receiver

“…While stepped CHWPs have deployed on a variety of mm and sub-mm experiments, [26][27][28]32,33 the continuously rotating CHWP is an emerging technology. 24,[42][43][44] Operating a 500 mmdiameter, tens-of-kilogram, spinning instrument in a cryogenic vacuum space poses a menagerie of challenges, including differential thermal contraction, frictional dissipation, and cryomechanical durability. The PB-2b CHWP design addresses these challenges with several hardware advancements.…”