Experimental study of growth rate scaling and quasicoherent states in plasma turbulence

Abstract

Experimental results of the scaling of linear growth rate with the electrostatic fluctuation levels are presented. A quadratic scaling was obtained for the ion temperature gradient-driven instability (ITG), characteristic of weak turbulence. For the rotationally driven E×B mode, a linear scaling was obtained, characteristic of strong turbulence. Moreover, the linear growth rate scaling for the E×B mode exhibited two distinct behaviors, corresponding to a broadening and a narrowing of the spectral width as a function of increasing fluctuation level. The spectral broadening is associated with the usual turbulent broadening, while the spectral narrowing is attributed to the presence of coherent structures via self-organization. Radial harmonics were detected and shown to contribute to the spread in the spectral width. These structures were also studied via a bispectral analysis. The evolution to the quasicoherent state was also found to result in the presence of a fewer number of radial harmonics in the saturated state. It revealed the presence of three-wave coupling involving the radial and azimuthal harmonics, suggesting a possible mechanism for the nonlinear saturation of the mode and anomalous transport.