Comparison of turbulence measurements from DIII-D low-mode and high-performance plasmas to turbulence simulations and models

Measured turbulence characteristics (correlation lengths, spectra, etc.) in low-confinement (L-mode) and high-performance plasmas in the DIII-D tokamak [Luxon et al., Proceedings Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] show many similarities with the characteristics determined from turbulence simulations. Radial correlation lengths Δr of density fluctuations from L-mode discharges are found to be numerically similar to the ion poloidal gyroradius ρθ,s, or 5–10 times the ion gyroradius ρs over the radial region 0.2<r/a<1.0. Comparison of these correlation lengths to ion temperature gradient gyrokinetic simulations (the UCLA-University of Alberta, Canada UCAN code [Sydora et al., Plasma Phys. Controlled Fusion 38, A281 (1996)]) shows that without zonal flows simulation values of Δr are very long, spanning much of the 65 cm minor radius. With zonal flows included, these decrease to near the measured values in both magnitude and radial behavior. In order to determine if Δr scaled as ρθ,s or 5–10 times ρs, an experiment was performed which modified ρθs while keeping other plasma parameters approximately fixed. It was found that the experimental Δr did not scale as ρθ,s, which was similar to low-resolution UCAN simulations. Finally, both experimental measurements and gyrokinetic simulations indicate a significant reduction in the radial correlation length from high-performance quiescent double barrier discharges, as compared to normal L-mode, consistent with reduced transport in these high-performance plasmas.