Superconducting magnet

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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. Nearly all superconducting magnets currently used for NMR spectroscopy are cooled by liquid helium being at normal atmospheric pressure or at reduced pressure, but cryogen-free NMR magnets have now been developed. These are cooled by pulse tube of Gifford Mahon crycoolers, obviating the need for liquid helium or liquid nitrogen.

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. Most of produced SC (superconducting) magnets have/had 1H frequency 100, 180, 200, 220, 250, 270, 300, 360, 400, 500, 600, 700, 750, 800, 850, 900, 920, 950 and 1000 MHz

The room temperature bore of modern SC magnets is usually 54 or 89 mm and those magnets are called SB (standard bore) or WB (wide bore) magnets, correspondingly.

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) The pumpimg is usually applied for magnets up from >700 MHz

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