Microturbulence damping mechanisms in the DIII-D tokamak[ATOTHER]@f|[/ATOTHER].

Although it has long been suggested that microturbulence is responsible for anomalous transport, relatively little is known about turbulence drive and suppression mechanisms which determine the observed fluctuation levels. In the DIII-D (DIII-D) tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] during H-mode discharges, microturbulence has been observed to change in two stages. Collective far-infrared scattering has confirmed and quantified a rapid (... 100 µsec) suppression of edge microturbulence at the L→H transition, as well as locally increased edge electric field shear, consistent with theoretical models which predict microturbulence suppression due to E×B sheared flow. Additionally, during the subsequent H-mode phase, a slower (10-100 msec) reduction of interior microturbulence is observed, coincident with increased electric field shear and reduced Δn[sub e] in the same region. This reduction is much larger in boronized discharges which also generally have larger rotation velocity. Experiments have been performed whereby the interior radial electric field is reduced while maintaining similar pressure profiles through the use of "magnetic braking" of toroidal rotation. As expected, the E×B Doppler frequency shift of the scattered fluctuation spectrum decreased as the electric field was reduced. The microturbulence levels were observed to increase when the interior electric field and the associated shear were reduced. [ABSTRACT FROM AUTHOR]