What device is used to hold liquid helium in superconductive magnets?

The device that is used to hold liquid helium in superconductive magnets is a cryostat. A cryostat is specifically designed to maintain low temperatures and to keep cryogenic liquids like helium at the necessary conditions to achieve superconductivity in magnets.

In the context of superconductive magnets, maintaining the liquid helium at a stable temperature is crucial, as helium is used to cool the superconducting coils to temperatures near absolute zero, where they can exhibit superconductivity and allow for the efficient operation of the magnet.

While other options like a thermos, pressure vessel, and vacuum chamber serve specific functions, they are not tailored to the needs of holding and maintaining cryogenic liquids in a superconducting context. A thermos, for instance, is designed for general thermal insulation but not for the specialized temperature control required for superconductivity. A pressure vessel is used to contain gases or liquids at high pressure but does not address the need for cryogenic temperature maintenance. Similarly, while a vacuum chamber can create a vacuum environment to minimize heat transfer, it does not directly hold liquid helium or manage its temperature like a cryostat does.