Status and perspectives of MAST start-up in the absence of solenoid flux

Spherical Tokamak (ST) based Component Test Facility (CTF) or ST Power Plant requires high values of plasma current I p : nevertheless the I p start-up and ramp-up have to be obtained without a central solenoid, due to the high neutron flux (there being no space for an effective shield). The use of the poloidal field coils (PF) is one of the most promising techniques to achieve this goal. In fact, the peculiar characteristics of the ST equilibria allow for using the PF coil magnetic flux alone in order to obtain an initial current and for ramping it up to the flat-top value. Among the present large ST devices, MAST is particularly suitable to test those new techniques, due to the large space inside its vacuum vessel that contains all the PF coils. Two different methods has been investigated on MAST: Merging Compression (M/C) and Double Null Merging (DNM).