Superconducting magnet

From NMR Wiki

Superconducting wire coiled around a solenoid cooled to 4K in a liquid helium bath provides the basis of every high frequency NMR magnet. Magnetic strength is determined by the current density applied to the superconducting coil. The higher the current density the higher the magnet frequency. Current density capacity can be increased in a given coil by improving joint and wire technology.