Plasma formation without a central solenoid in a Spherical Tokamak

Obtaining the plasma current in any ST is a challenge as there is limited space for the central solenoid; indeed this is absent in most proposed ST fusion devices due to the high neutron flux, there being no space for an effective shield. A range of start-up techniques will be described, with special emphasis on the Merging-Compression (M-C) method of plasma formation used on START and MAST, and a `Double Null Merging' scheme whereby two initial plasmas formed in low-order nulls between Poloidal Field coils are merged. Results from M-C confirm that high temperature plasmas are obtained from merging. Results and/or modelling indicate that both techniques can produce hot initial plasmas of 0.5MA in MAST without use of the central solenoid, in readiness for subsequent ramp-up using high power NBI and the associated input of flux (significant in an ST) from the increasing vertical field