A recently introduced tool for the analysis of turbulence, wavelet bicoherence [van Milligen, Hidalgo, and Sánchez, Phys. Rev. Lett. 16, 395 (1995)], is investigated. It is capable of detecting phase coupling—nonlinear interactions of the lowest (quadratic) order—with time resolution. To demonstrate its potential, it is applied to numerical models of chaos and turbulence and to real measurements. It detected the coupling interaction between two coupled van der Pol oscillators. When applied to a model of drift wave turbulence relevant to plasma physics, it detected a highly localized coherent structure. Analyzing reflectometry measurements made in fusion plasmas, it detected temporal intermittency and a strong increase in nonlinear phase coupling coinciding with the L/H (low-to-high confinement mode) transition.

The TJ-II device is a medium-size (R0 = 1.5 m, 〈ap〉 = 0.2 to 0.25 m, B0 = 1 T) helical-axis stellarator to be built at the CIEMAT site in Madrid. Its main characteristics are (a) potential ...

The nonlinear evolution of resistive pressure-gradient-driven turbulence with diamagnetic effects included generates a dc electric field (poloidal velocity) through the convective nonlinearity in the momentum balance equation. This radial electric field has a strong shear and contributes to the saturation of the turbulence; its effect on the saturation level of turbulence is more important than the change of the time and length scales of the modes by the direct ω* effects.

A simple, paradigmatic model of long‐wavelength drift wave turbulence in the presence of a sheared poloidal flow is analyzed in detail. Linear theory predicts that velocity shear induces a strong stabilizing effect by shifting the eigenmode away from the k⋅B=0 resonant surface, thereby enhancing ion damping. However, multiple‐helicity numerical calculations indicate that velocity shear has little or no effect on saturated fluctuation levels. Analysis suggests that this result is related to the incidence of a spiky, radially intermittent profile of the turbulent fluctuation levels, induced by low‐q mode rational surfaces, and occurs when the turbulent diffusivity exceeds the product of diamagnetic frequency and gyroradius squared. An analytical theory that explains the observed suppression of velocity and magnetic shear damping is presented.