Joint testing of the 3 Tesla ST40 spherical tokamak toroidal field coil test assembly

Spherical Tokamaks used in magnetic fusion have a small centre-stack by design which causes a higher field on the conductor. ST40 [1] is a 3 Tesla spherical tokamak with a major radius of R = 40 cm and minor radius of a = 26 cm being built by Tokamak Energy Ltd. The high toroidal field (TF) requirement requires a wire current of 250 kA flowing in each of the 24 limbs totalling 6 MA in the centre-stack. Joint testing was used to investigate the interface resistance between the centre-stack and return limbs under a variety of contact pressures, using different shims, different coatings on the conductor and soldering. Initially a DC 400 A current source was used and later an air-core pulsed transformer delivering up to 16 kA of current was specifically designed and built for these measurements. Results from this paper can be used to predict the behaviour of critical joints in the region at the ends of the centre-stack where current is passed to the TF return limbs. A comparison with TF joint data from the National Spherical Torus Experiment (NSTX) is also given. This will aid in deciding which jointing method to use for ST40 and the required pressure to pre-load the interface.