Studies of the pedestal characteristics and quantities determining ELM energy losses in MAST are presented. Progress is reported on the attempts to determine the quantities that affect the pedestal height and understanding ELM losses. High temperature pedestal plasmas have been achieved which have collisionalities one order of magnitude lower than previous results. The pedestal widths obtained in these low collisionality plasmas are in better agreement with banana orbit scalings than previous high collisionality plasmas, suggesting that banana orbits can only play a role in determining the minimum width when the collisionality is sufficiently low. A stability analysis performed on these plasmas shows them to be near the ballooning limit and to have broad mode structures which would predict large ELM energy losses. These ELM energy losses have been observed at the target resulting in peak power densities in excess of ~20 MWm -2 . The fraction of pedestal energy released by an ELM as a function of collisionality has been compared with data from other devices. A model for ELM energy losses has been proposed and compared to data from MAST and JET.* ped ν ) is typically large (1-3). In order to make the results more comparable to other devices and more relevant for extrapolation to

Neo-classical tokamak plasma theory predicts poloidal rotation driven by the temperature gradient of order ~ few km/s. In conventional aspect ratio tokamak plasmas, e.g. on JET and DIII-D, poloidal velocities considerably in excess of the neo-classical values have been measured, particularly in the presence of internal transport barriers (ITBs), by means of charge-exchange recombination spectroscopy (CXRS) on the fully ionised C 6+ impurity ions. Comparison between such measurements and theoretical predictions requires careful corrections to be made for apparent 'pseudo' velocities, which can arise from the finite lifetime of the excited atoms in the magnetised plasma and the energy dependence of the charge-exchange excitation process. In present day spherical tokamak (ST) plasmas this correction is an order of magnitude smaller than on large conventional tokamaks, which operate at higher temperature and magnetic field, hence reducing any associated systematic uncertainties. On MAST measurements of toroidal and poloidal flows of the C 6+ impurities are available from high-resolution Doppler CXRS measurements, where the appropriate corrections for the pseudo-velocities are made. Comparison of the measured C 6+ velocities with neo-classical theory requires calculation of the impurity flow, which differs from that of the bulk ions due to the respective diamagnetic contributions for each species and inter-species friction forces. Comparisons are made with the predictions of a recent neo-classical theory [1, 2], which calculates the full neo-classical transport matrix for bulk ions and a single impurity species for a strongly rotating plasma, as well as a simpler neo-classical theory [3] for an impure plasma. Initial results for both Land H-mode plasmas show that, within the measurement uncertainties, the measured poloidal rotation of the core plasma is consistent with the neo-classical predictions.

A combination of recently installed state-of-the-art imaging and profile diagnostics, together with established plasma simulation codes, are providing for the first time on Mega Ampère Spherical Tokamak (MAST) the tools required for studying confinement and transport, from the core through to the plasma edge and scrape-off-layer (SOL). The H-mode edge transport barrier is now routinely turned on and off using a combination of poloidally localized fuelling and fine balancing of the X-points. Theory, supported by experiment, indicates that the edge radial electric field and toroidal flow velocity (thought to play an important role in H-mode access) are largest if gas fuelling is concentrated at the inboard side. H-mode plasmas show predominantly type III ELM characteristics, with confinement H H factor (w.r.t. scaling law IPB98[y, 2]) around ∼1.0. Combining MAST H-mode data with the International Tokamak Physics Activities (ITPA) analyses, results in an L-H power threshold scaling proportional to plasma surface area (rather than P LH ∼ R 2 ). In addition, MAST favours an inverse aspect ratio