Abstract: A superconducting magnet system comprises an annular cryogenic vessel (1) having an outer vacuum container (2) and containing a superconducting magnet (3) within an annular reservoir (4) containing liquid helium. A cryocooler (5) has a first stage (7) linked by a thermal link (9) to a thermal shield (6) within the vacuum space surrounding the reservoir (4) and a second stage (8) that serves to recondense evaporating helium gas from the reservoir (4). In the absence of special measures, such a cryocooled shield (6) would warm up quickly in the event of a power failure and would radiate heat onto the reservoir (4) causing all of the liquid helium to evaporate. However an inertial shield (11) is provided between the reservoir (4) and the thermal shield (6) in such a position that the outgoing helium gas from the reservoir (4) carries away the heat being transferred to the inertial shield (11) from the thermal shield (6) and thus slows down the rate at which the thermal inertial shield (11) warms up.

Abstract: A superconducting magnet system comprises a cryogenic vessel (1), a superconducting magnet (11) contained in an inner reservoir (16) within the vessel (1) to be cooled by liquid helium within the inner reservoir (16), and an outer reservoir (7) containing liquid helium and linked to the inner reservoir (16) by a feed tube (10) and a needle valve (9). In operation of the system current is supplied to the magnet (11) by a removable current lead (19) extending through the wall of the vessel (1) by way of a supply passage (17) in order to initiate superconducting current flow in the magnet. The supply of current to the magnet (11) is then stopped and the lead (19) is removed whilst the superconducting current flow persists in the magnet. After an extended period of superconducting current flow in the magnet (11) and without stopping such superconducting current flow, liquid helium at a temperature of about 4.

Abstract: A superconducting electromagnet apparatus comprises a main coil assembly (1) and a main current supply (5) energising and de-energising the main coil assembly (1), and for persisting the current flow in the main coil assembly (1) when a desired constant current level has been reached, in order to generate a central magnetic field of high homogeneity in a working volume. The apparatus further comprises a B0 shim coil assembly (2) comprising superconducting shim coils connected within a closed loop and arranged to magnetically couple with the main coil assembly (1) and an auxiliary current supply (6) for supplying current to the closed loop, and for persisting the current flow in the closed loop when a desired constant current level has been reached, in order to provide fine adjustment of the central magnetic field within the working volume without significantly degrading the homogeneity of the central magnetic field.