Microturbulence drive and suppression mechanisms in the DIII-D tokamak

Although it has long been suggested that microturbulence is responsible for anomalous transport, surprisingly little is known about the turbulence drive and suppression mechanisms which determine the observed levels of fluctuations. In DIII-D H-mode discharges, microturbulence has been observed to change in two regions. Collective FIR scattering has confirmed a rapid suppression in edge turbulence at the transition, as well as locally increased edge electric field shear, strongly supporting theoretical models of shear suppression of edge turbulence. During the ELM-free H-mode phase, interior turbulence is observed to reduce on a slower timescale, coincident with increased electric field shear and reduced ∇ne. The question then arises as to whether shear flow stabilization might also be responsible for the slower interior turbulence reduction. Experiments have been performed whereby the internal electric field is modified while maintaining similar pressure profiles. First, the long-term turbulence reduction observed in H-mode plasmas has been found to be much larger in boronized discharges when strong electric field shear is observed to penetrate deeper into the confinement region than in unboronized discharges. Second, comparison of co- and counter-injection discharges has revealed fluctuation levels increase in co- and decrease in counter-injection L-mode. Third, results have been obtained during experiments where the interior electric field was reduced through the use of open-quotes magnetic brakingclose quotes of the toroidal rotation. As expected, the rvec E x rvec B Doppler shift of the scattered fluctuation spectrum decreased as the electric field was reduced. Additionally, increases in turbulence levels were observed as the internal electric field and associated shear was reduced. In summary, interior fluctuation measurements indicate large reductions correlated with changes in the density and radial electric field profiles