Turbulence Dynamics During Internal Transport Barrier Formation Via Beam Emission Spectroscopy on DIII-D,

The formation of localized internal transport barriers (ITBs) is observed at the appearance of low order values of $q_{min}$ surfaces in negative central shear L-mode discharges on DIII-D. Related GYRO simulations suggest that increased zonal flows may be responsible for such ITBs [1]. Newly expanded high-sensitivity 2D beam emission spectroscopy (BES) fluctuation and flow measurements will be utilized to quantitatively examine turbulence dynamics and test this prediction of a zonal flow- driven ITB trigger mechanism. Time delay estimation via dynamic programming is applied to the fluctuation data to measure high-frequency poloidal velocity fluctuations to search for increased zonal flow activity during $q$-triggered ITB formation. In addition, newly implemented rotation control capability via co- and counter-neutral beam injection on DIII-D will be exploited to study ITB formation with varying momentum input. \par\vskip3pt\noindent [1] R.E. Waltz, et al., Phys. Plasmas \bf{10}, 052301 (2006).