L-H transition and pedestal studies on MAST

摘要

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_φ) 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 At = 0.2 ms time resolution and the evolution of p_e and j_φ through an edge-localized mode (ELM) cycle. Measurements of the H-mode power threshold, P_(L-H) revealed that about 40% more power is required to access H-mode in 4He than in D and that a change in the Z-position of the X-point can change P_(L-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 (v) Vti ≈ 0 leading to T_i > T_e in the pedestal region with T_i ～ 0.3 keV close to the separatrix. A clear correlation of ▽T_i with v is observed. The measured jφ (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 jφ and ▽p_e evolution △t≈2 ms as well as profiles in discharges with counter current neutral beam injection raise questions with respect to this edge stability picture.