Magnex Scientific Limited

Andrew John Varney

Darren Houlden

A superconducting magnet system comprises a cryogenic vessel (), a superconducting magnet () contained in an inner reservoir () within the vessel () to be cooled by liquid helium within the inner reservoir (), and an outer reservoir () containing liquid helium and linked to the inner reservoir () by a feed tube () and a needle valve (). In operation of the system current is supplied to the magnet () by a removable current lead () extending through the wall of the vessel () by way of a supply passage () in order to initiate superconducting current flow in the magnet. The supply of current to the magnet () is then stopped and the lead () is removed whilst the superconducting current flow persists in the magnet. After an extended period of superconducting current flow in the magnet () and without stopping such superconducting current flow, liquid helium at a temperature of about 4.2 K may be supplied to an upper part of the inner reservoir () above the magnet () from the outer reservoir () such that the magnet is still surrounded by liquid helium at a temperature of below 4.2 K. After a period of time the supply of liquid helium to the inner reservoiris stopped, and the liquid helium in the inner reservoir () is cooled to a temperature below 4.2 K. Such a system is capable of being operated at very low temperatures down to below the lambda point, and of producing magnetic fields of high strengths and stability suitable for NMR spectroscopy.

Cryogenic cooling of superconducting magnet systems below temperature of 4.2 K

A superconducting electromagnet apparatus comprises a main coil assembly ( ) and a main current supply ( ) energising and de-energising the main coil assembly ( ), and for persisting the current flow in the main coil assembly ( ) 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 B shim coil assembly ( ) comprising superconducting shim coils connected within a closed loop and arranged to magnetically couple with the main coil assembly ( ) and an auxiliary current supply ( ) 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. A control circuit ( ) is provided for controlling the main and auxiliary current supplies ( ) and the main coil assembly ( ), the B shim coil assembly ( ) and the control circuit ( ) are adapted to at least partly compensate for the effect of variation of the magnetic field within the working volume with time. In this case the shim coil assembly performs both the function of a B shim and at the same time compensates for the effect of variation of the magnetic field within the working volume with time, thus avoiding the need to provide individual closed loop coil assemblies for performing these functions separately which would result in functional difficulties due to inductive coupling between these auxiliary coil assemblies.

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Oxford, United Kingdom

Andrew John Varney