A recently introduced tool for the analysis of turbulence, wavelet bicoherence [B. Ph. van Milligen, C. Hidalgo and E. Sánchez, Phys. Rev. Lett. 16 (1995) 395], is investigated. It is capable of detecting phase coupling-nonlinear interactions of the lowest (quadratic) orderwith 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.

We show that the modulational instability growth rate of zonal flows is determined directly from the quasilinear wave kinetic equation. We also demonstrate the relation between zonal-flow growth and the cross bispectrum of the high-frequency drift-wave-driven Reynolds stress and the low-frequency plasma potential by explicit calculation. Experimental measurements of the spatiotemporal evolution of the spectrum integrated bicoherence at the L-->H transition near the edge shear layer indicate a modification in the nonlinear phase coupling, which might be linked to the generation of sheared ExB flows.

Runaway of the reflectometer output phase can be produced by transversely propagating density fluctuations if the antenna system is operated in the tilt-mode, i.e. away from perpendicular incidence. The observed characteristics of the phase runaway are explained with an analytical model. The two-dimensional (2D) effects resulting from poloidally propagating density fluctuations are calculated using a time-independent full-wave 2D code. Numerical simulations for both monochromatic and turbulent density fluctuations are presented. Good agreement with the characteristics of phase runaway measured with reflectometry in the W7-AS stellarator is obtained. The potential of a reflectometer in the tilt-mode to measure the radially resolved poloidal propagation velocity of density fluctuations is analysed.

Parameter scans in density, heating power and isotope mass have been carried out in W7-AS. ECRH at a frequency of 140 GHz has allowed to study the density scaling of the energy confinement time of ECRH plasmas up to densities of 1020 m−3. In power scans it has been tried to relate the power degradation of the energy confinement to a local plasma parameter. Transport analyses using power balance an heat wave techniques indicate that the transport coefficient does not depend on the electron temperature or related parameters. This observation can be reconciled with power degradation if the transport coefficient is formally allowed to vary with changes in the heating power on a faster than the diffusive time scale. Such a transport process describes also the observations in the dynamic phases following large changes in the heating power.