We describe a simple and reliable clamp/release system for the rotor of large superconducting magnetic bearings. The system was developed for a cryogenic half-wave plate spinner for cosmic microwave background polarimetry but can have other applications. The main features of this system are its large rotor mass compliance (∼10 kg); zero power dissipation while holding the rotor and zero power dissipation when the rotor is released; fast (∼40 ms), balanced release and clamp actions, and low power dissipation (∼30 J) on the cold stage during each operation; low cost; and high reliability over a very large number of operation cycles.

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