The phenomenology of the L-H transition in the DIII-D tokamak

Detailed information on the changes in the edge plasma at the L-H transition has been obtained by using charge exchange recombination spectroscopy with high spatial resolution (6 mm) and high time resolution (0.5 ms). Measurements of the radial electric field, Er, show that Er decreases abruptly at the transition resulting in the formation of a negative well like radial profile just within the separatrix. There is a well developed region of gradient in Er which extends inwards 1 to 2 cm into the plasma from the magnetic separatrix. This region of E'r exhibits the greatest increase in impurity ion poloidal rotation at the L-H transition and the greatest reduction in plasma fluctuations. A transport barrier is formed in this same region as is signified by large increases in the observed gradients of ion temperature, carbon density, electron temperature and electron density. The development of the region of significant E'r, the increase in impurity ion poloidal rotation, the reduction of plasma turbulence and the formation of the transport barrier all occur simultaneously with the drop in the Dα signal which marks the transition. The well like structure of Er is located on and inside the separatrix as is clearly defined by the variation of the signal amplitudes of the spatial chords spanning the separatrix. The well like structure of Er moves with the separatrix as the separatrix location changes and is maintained throughout the H mode. (author). 38 refs, 6 figs