No abstract

Experimental results from a magnetized argon plasma column demonstrate a controlled transition to a turbulent state as the magnetic field (B) strength is increased. At lower B there is an onset of fluctuations in density and potential. These are shown to be due to drift waves that have been modified by flow shear. As B is increased the character of the fluctuations undergoes several changes. These changes include a general decrease of coherence, an increase in the phase lag (between density and potential), and a straightening of the observed dispersion relation. Concomitantly, the intensifying and broadening fluctuation spectra lead to significant cross-field radial particle transport. Other nonlinear dynamical activity is inferred during the transition, e.g., three-wave interactions, the formation of localized structures (that do not significantly contribute to the net particle transport), and energy transfer to the largest available scales.

The nonlinear coupling between small scale high-frequency turbulence and larger scale lower-frequency fluctuations increases transiently in transitions to improved confinement in the DIII-D tokamak. This increase starts before the rapid turbulence suppression and E x B shear-flow development in the region that becomes the H-mode transport barrier/shear flow region. After the transition, the coupling returns to L-mode levels. These results are consistent with expectations for spontaneous transitions to improved confinement triggered by a turbulence-driven sheared flow.