Development of a high field solenoid magnet for the MAST Spherical Tokamak

The Mega Amp Spherical Tokamak (MAST) is an experimental fusion device currently operating at the Culham Science Centre. The low aspect ratio of the spherical tokamak combined with the need to develop a high flux swing from its central solenoid leads to high forces on the conductors that form the centre column. This column consists of a compact solenoid wound around the inner limbs of the toroidal field coils. In order to achieve the nominal plasma parameters, the dimensions of the centre column were optimised to make best possible use of the space available. The most critical region of the solenoid magnet design is the end turn and tail section where complex 3D magnetic fields interact with the current in the end turn conductors giving an asymmetric stress distribution. The hoop load in this end turn is particularly troublesome since it must be reacted across a bonded joint, which inevitably generates tensile and shear stresses within it. This paper describes how this problem has been solved for the new MAST centre column by the use of compact quadrupole tails formed close to the main coil and connected to it by tight bends formed in the water cooled copper conductor. A test coil employing these features and fitted with engineering diagnostics has been manufactured and tested at Culham to the maximum design current. This coil not only demonstrated the manufacturing processes involved but also provided test data, which compares well with engineering analysis.