Radial Correlation Length of Turbulent Density Fluctuations in DIII-D Plasmas,

The radial correlation length (L$_r$) of turbulent density fluctuations is an important quantity for understanding turbulent transport in tokamak plasmas. In DIII-D, a correlation reflectometer and a tunable multi-channel reflectometer system allow L$_r$ measurement with both high time and spatial resolutions. In this presentation, results will be reported from two recent areas of study: (1) Measurements of L$_r$ in Ohmic, ECH, and NBI heated L-mode plasmas, and comparisons to predictions from nonlinear gyrokinetic codes; and (2) Measurements of fast changes in L$_r$ during the L- to H-mode transition. Preliminary results show that: (1) in general, L$_r$ increases from the edge to core and scales as (5-10) $\rho_s$ ($\rho_s$ is the ion gyroradius using $T_e$); (2) L$_r$ decreases with ECH in otherwise Ohmic plasmas; and (3) at the L-H transition, cross-correlations between reflectometer channels close to separatrix increase simultaneously as divertor D$_\alpha$ signal starts to decrease, while in the bulk plasma, they begin decreasing after a propagation delay.