Superconducting magnet

From NMR Wiki

Solenoid wound with superconducting wire provides the basis of every high-frequency NMR magnet.

Since superconductive state can be maintained at low temperatures, the coil is typically immersed in a cryo-coolant. All superconducting magnets currently used for NMR spectroscopy are cooled by liquid helium.

Strength of the magnetic field is determined by the current density contained in the superconducting coil. The higher is current density the higher is the magnetic field and the corresponding Larmor frequency.

Current density capacity can be increased in a given coil by improving joint and wire technology. When improving those technologies is impractical some increase in current capacity can be achieved by cooling the coil even further. The latter is the principle of operation of the "pumped" magnet systems (where temperature of liquid He is decreased by actively pumping evaporating He gas)