Two phase reduction of microturbulence at the transition into H-mode measured on DIII-D

At the L-H transition in the DIII-D tokamak, a two phase reduction of microturbulence has been observed. A first, rapid (∼100 μs) suppression phase of microturbulence in a narrow region of the plasma edge is followed by a second, bulk turbulence reduction phase in the plasma interior, with a relative turbulence reduction (n/n) of ≥50% below L-mode level. This second, slow reduction phase (within 10's of ms) correlates with the results of recent transport studies on DIII-D, suggesting that transport coefficients decrease in a comparable time scale in the same region in the plasma. We have established a quantitative relationship between frequency shifts in far infrared scattering measurements and E x B plasma velocities measured with charge exchange recombination spectroscopy. The measured profiles correspond to a positive radial electric field, in the interior of the plasma, which predicts a frequency shift in the ion diamagnetic drift direction of the same magnitude as that actually observed in the scattering data. This analysis, together with results from reflectometer measurements, radially locate the density fluctuations and reflect their time evolution. (author) 8 refs., 4 figs