Patent ID: 7629868

Claim:
A method of maintaining cryogenic cooling of a superconducting magnet surrounded by liquid helium at a temperature of below 4.2 K within an inner chamber of a cryogenic vessel, the method comprising: supplying current to the magnet by way of a removable current lead extending through a supply passage passing through the wall of the vessel in order to initiate superconducting current flow in the magnet; stopping the supply of current to the magnet whilst the superconducting current flow persists in the magnet; withdrawing the current lead from the supply passage; after an extended period of superconducting current flow in the magnet without stopping such superconducting current flow, supplying by way of a valve, liquid helium at a temperature of about 4.2 K to an upper part of the inner chamber above the magnet such that the magnet is still surrounded by liquid helium at a temperature of below 4.2 K; stopping the supply of liquid helium to the inner chamber by closing the valve; and cooling the liquid helium in the upper part of the inner chamber to a temperature of below 4.2 K by reducing the pressure in the upper part of the inner chamber by pumping whilst the valve remains closed, whereby the flow of liquid helium from the outer chamber to the upper part of the inner chamber occurs as a result of the reduced pressure upon opening of said valve.