1 EX / P 5 - 17 Confinement and fuelling in MAST

The dependencies of energy confinement on the main engineering parameters have been investigated in the Mega Ampere Spherical Tokamak (MAST) in H-mode by expanding operational space towards higher plasma currents (up to 1.2MA) and heating powers (up to 3.5MW). Multivariate fits show that the dependence of energy confinement time on plasma current Ip is weaker than linear while the dependence on toroidal magnetic field BT is stronger than linear, in contrast to conventional energy confinement scalings. These Ip and BT dependencies have also been confirmed by single parameter scans. Transport analysis indicates that the strong BT scaling of energy confinement could possibly be explained by weaker q and stronger ν* dependence of heat diffusivity. Particle confinement has been studied using shallow high field side pellets. The pellet deposition can be explained only by invoking the grad-B drift of the pellet ablatant. The pellet creates a distinct zone with sharp positive density gradient and doubled temperature gradient. Simulations using the linear GS2 and CUTIE codes show that these changes could modify the character of micro-turbulence in the pellet deposition zone. The pellet retention time scales with energy confinement time and increases with pellet deposition depth. Energy and particle confinement scalings derived from MAST data are used for prediction towards a Component Test Facility based on the spherical tokamak (ST-CTF). The energy confinement time scaling gives about ~ 1.6 more favourable prediction than the IPB98(y,2) scaling, however, the strong BT dependence in STs has yet to be tested towards the low ν* of a ST-CTF. The scaling for particle confinement predicts that for ST-CTF pellet particle throughput should be about 20 Pa m/s, which is 1/4 of the value predicted by the same scaling for ITER.