Plasma turbulence and associated transport in TFTR D-D and D-T discharges

The bulk of the low-amplitude turbulent density fluctuations are found in the range approx. 0.1 - 0.3 for supershot and L-mode confinement regimes in TFTR. The wavenumber spectrum in the radial direction peaks at kr = 0, while the poloidal spectrum peaks near kθρi approx. 0.1 - 0.3, in general agreement with expectations from nonlinear ion drift wave turbulence models. A nonlinear wave-wave coupling analysis indicates a positive linear growth rate at very low kθρi < 0.06, and a cascade of energy to the higher-k, shorter-wavelength modes at the peak of the power spectrum. In contrast, the edge region shows additional unstable modes at high k which cascade to the lower k peak. The observed density fluctuation level and corresponding decorrelation lengths and times are sufficient to account for the observed local thermal transport in the plasma confinement region and generally indicate scaling consistent with the local transport, but direct evidence of causality between these fluctuations and transport has not been shown. The relative scalings of fluctuation amplitude versus correlation length do not agree with a simple mixing length scaling in the confinement region. Initial comparisons of fluctuation data for D-D and D-T plasmas indicate that changes in n-tilde/n for r/a ≥ 0.5 are less than or comparable to statistical variations in the data, and do not appear to proportionally scale with the decreases in χi observed at smaller radii in the plasma. (author). 16 refs, 4 figs