Magnet assembly

A method for operating a magnet assembly for preventing a magnetic field externally of a working volume exceeding preset limits during a transient condition, said method comprising the steps of:

• generating a first magnetic field using a first superconducting coil;
• generating a second magnetic field using a second superconducting coil electrically connected in series with said first superconducting foil, the magnetic fields generated respectively by said first and second superconducting coils having corresponding components of substantially the same order of magnitude;
• said first and second magnetic fields, in combination, forming a resultant uniform magnetic field in a working volume with said second magnetic field substantially opposing said first magnetic field in a combined magnetic field externally of said working volume;
• maintaining said combined magnetic field externally of said working volume within preset limits during normal operation, by means of said first and second superconducting coils, in which the current in said first and second superconducting coils is substantially constant;
• and
• controlling currents induced in conductive parts surrounding said working volume during a transient condition, in which said current in said first and second superconducting coils is changing, by generating a magnetic field, using an electrically conductive ring, which opposes magnetic fields created by said currents induced in said conductive parts for maintaining said combined magnetic field externally of said working volume substantially within said present limits for normal operation during said transient condition.