Discussion
What is the purpose of liquid helium in an NMR magnet?
Answer: D
The niobium alloy windings superconduct only below a few kelvin, so liquid helium holds them there and a surrounding jacket of liquid nitrogen reduces the helium boil-off. The sample itself sits at room temperature in a bore that is thermally isolated from the cryostat, so the helium does nothing for its relaxation. Neither inert blanketing nor vibration damping is its role. If the helium is allowed to run out the magnet quenches, dumping its stored energy as heat and boiling off the remaining cryogen, which is both expensive and hazardous.
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