L-H transition and pedestal studies on MAST

On MAST studies of the profile evolution of the electron temperature (T-e), electron density (n(e)), radial electric field (E-r) as well as novel measurements of the ion temperature (T-i) and toroidal current density (j(phi)) in the pedestal region allow further insight into the processes forming and defining the pedestal such as the H-mode access conditions and MHD stability. This includes studies of fast evolution of T-e, n(e) and E-r with Delta t = 0.2ms time resolution and the evolution of p(e) and j(phi) through an edge-localized mode (ELM) cycle. Measurements of the H-mode power threshold, PL-H revealed that about 40% more power is required to access H-mode in He-4 than in D and that a change in the Z-position of the X-point can change PL-H significantly in single and double null configurations. The profile measurements in the L-mode phase prior to H-mode suggest that neither the gradient nor the value of the mean T-e or E-r at the plasma edge play a major role in triggering the L-H transition. After the transitions, first the fluctuations are suppressed, then the E-r shear layer and the n(e) pedestal develops followed by the T-e pedestal. In the banana regime at low collisionality (nu(*)) del T-i approximate to 0 leading to T-i > T-e in the pedestal region with T-i similar to 0.3 keV close to the separatrix. A clear correlation of del T-i with nu(*) is observed. The measured j(phi) (using the motional Stark effect) T-e and n(e) are in broad agreement with the common peeling-ballooning stability picture for ELMs and neoclassical calculations of the bootstrap current. The j(phi) and del p(e) evolution Delta t approximate to 2ms as well as profiles in discharges with counter current neutral beam injection raise questions with respect to this edge stability picture.