Experiments have been performed on MAST using internal (n=3) resonant magnetic perturbation coils. The application of the RMPs to L-mode discharges has shown a clear density pump out when the field line pitch angle at the low field side of the plasma is sufficiently well aligned with the applied field. The application of the RMPs before the L-H transition increases the power required to achieve H-mode by at least 30 %. In type I ELM-ing H-mode discharges, at a particular value of q 95 , the ELM frequency can be increased by a factor of 5 by the application of the RMPs. This effect on the ELMs and the L-mode density pump out is not correlated with the width of the region for which the Chirikov parameter, calculated using the vacuum field, is greater than 1 but may be correlated with the size of the resonant component of the applied field in the pedestal region or with the location of the peak plasma displacement when the plasma response is taken into account.

Access conditions for full suppression of edge localised modes (ELMs) by magnetic perturbations (MP) in low density high confinement mode (H-mode) plasmas are studied in the ASDEX Upgrade tokamak. The main empirical requirements for full ELM suppression in our experiments are: 1. The poloidal spectrum of the MP must be aligned for best plasma response from weakly stable kinkmodes, which amplify the perturbation, 2. The plasma edge density must be below a critical value, 3.3 × 10 19 m −3 . The edge collisionality is in the range ν * i = 0.15−0.42 (ions) and ν * e = 0.15−0.25 (electrons). However, our data does not show that the edge collisionality is the critical parameter that governs access to ELM suppression. 3. The pedestal pressure must be kept sufficiently low to avoid destabilisation of small ELMs. This requirement implies a systematic reduction of pedestal pressure of typically 30% compared to unmitigated ELMy H-mode in otherwise similar plasmas. 4. The edge safety factor q 95 lies within a certain window. Within the range probed so far, q 95 = 3.5−4.2, one such window, q 95 = 3.57−3.95 has been identified. Within the range of plasma rotation encountered so far, no apparent threshold of plasma rotation for ELM suppression is found. This includes cases with large cross field electron flow in the entire pedestal region.

The role of the pedestal position on the pedestal performance has been investigated in AUG, JET-ILW and TCV. When the pedestal is peeling-ballooning (PB) limited, the three machines show a similar behaviour. The outward shift of the pedestal density leads to the outward shift of the pedestal pressure which, in turns, reduces the PB stability, degrades the pedestal confinement and reduces the pedestal width. Once the experimental density position is considered, the EPED model is able to correctly predict the pedestal height. An estimate of the impact of the density position on a ITER baseline scenario shows that the maximum reduction in the pedestal height is 10% while the reduction in the fusion power is between 10% and 40% depending on the assumptions for the core transport model used.When the pedestal is not PB limited, a different behaviour is observed. The outward shift of the density is still empirically correlated with the pedestal degradation but no change in the pressure position is observed and the PB model is not able to correctly predict the pedestal height. On the other hand, the outward shift of the density leads to a significant increase of η e (where η e is the ratio of density to temperature scale lengths, η e = L ne /L Te ) which leads to the increase of the growth rate of microinstabilities (mainly ETG and ITG) by 50%. This suggests that, when the pedestal is not PB limited, the increase in the turbulent transport due to the outward shift of the density might play an important role in the decrease of the pedestal performance.

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