Phase randomization of three-wave interactions in capillary waves.

We present new experimental results on the transition from coherent-phase to random-phase three-wave interactions in capillary waves under parametric excitation. Above the excitation threshold, coherent wave harmonics spectrally broaden. An increase in the pumping amplitude increases spectral widths of wave harmonics and eventually causes a strong decrease in the degree of the three-wave phase coupling. The results point to the modulation instability of capillary waves, which leads to breaking of continuous waves into ensembles of short-lived wavelets or envelope solitons, as the reason for the phase randomization of three-wave interactions.

Turbulence-condensate interaction in two dimensions.

We present experimental results on turbulence generated in thin fluid layers in the presence of a large-scale coherent flow, or a spectral condensate. It is shown that the condensate modifies the third-order velocity moment in a much wider interval of scales than the second one. The modification may include the change of sign of the third moment in the inverse cascade. This observation may help resolve a controversy on the energy flux in mesoscale atmospheric turbulence (10-500 km): to recover a correct energy flux from the third velocity moment one needs first to subtract the coherent flow. We find that the condensate also increases the velocity flatness.

Suppression of turbulence by self-generated and imposed mean flows.

The first direct experimental evidence of the suppression of quasi-two-dimensional turbulence by mean flows is presented. The flow either is induced externally or appears in the process of spectral condensation due to an inverse cascade in bounded turbulence. The observed suppression of large scales is consistent with an expected reduction in the correlation time of turbulent eddies due to shearing. At high flow velocities, sweeping of the forcing-scale vortices reduces the energy input, leading to a reduction in the turbulence level.

Strong ExB shear flows in the transport-barrier region in H-mode plasma.

We report the first experimental observation of stationary zonal flow in the transport-barrier region of the H-mode plasma. Strong peaks in Er shear mark the width of this region. A strong m = n = 0 low-frequency (f < 0.6 kHz) zonal flow is observed in regions of increased Er, suggesting a substantial contribution of zonal flow to the spatial modulation of Er radial profiles. Radial localization of the zonal flow is correlated with a region of zero magnetic shear and low-order (7/5) rational surfaces.

Spectral condensation of turbulence in plasmas and fluids and its role in low-to-high phase transitions in toroidal plasma.

Transitions from turbulence to order are studied experimentally in thin fluid layers and in magnetically confined toroidal plasma. It is shown that turbulence self-organizes through the mechanism of spectral condensation in both systems. The spectral redistribution of the turbulent energy leads to the reduction in the turbulence level, generation of coherent flow, reduction in the particle diffusion, and increase in the system's energy. The higher-order state in the plasma is sustained via the non-local spectral coupling of the linearly unstable spectral range to the large-scale mean flow. Spectral condensation of turbulence is discussed in terms of its role in the low-to-high confinement transitions in toroidal plasma which show similarity with phase transitions.

Formation and structure of transport barriers during confinement transitions in toroidal plasma.

Density pedestal formation is studied experimentally during spontaneous low-to-high confinement transitions in the H-1 heliac. Poloidally extended potential structures, or zonal flows, seem to play the major role both in the spatial structure and in the temporal evolution of the pedestal formation. Zonal flows transiently generate radially localized maxima in the radial electric-field shear in L mode which coincides with the radial location of the pedestal in H mode.

Inverse energy cascade correlated with turbulent-structure generation in toroidal plasma.

We report the first experimental observation of the inverse energy cascade correlated with the generation of large turbulent structures. Spectral energy is nonlinearly transferred from the unstable region of the spectrum into large coherent structures and into broadband turbulence in agreement with theoretical expectations. These results are obtained by producing plasma in the H-1 heliac whose parameters allow a single-field, Hasegawa-Mima-type model to be used for the spectral energy transfer analysis.

Turbulent transport reduction and randomization of coherent fluctuations by zonal flows in toroidal plasma.

Fluctuation-driven particle flux is greatly reduced in the plasma radial region where zonal flows are present in the H-1 toroidal heliac. This occurs without reduction in the fluctuation level. Statistical properties of fluctuations are significantly modified in this region. It is shown that the randomization of phases of coherent structures by zonal flows is responsible for the observed effect. This mechanism of transport reduction complements theoretically predicted random shearing of turbulence by zonal flows and does not require the fluctuations suppression.

Experimental evidence of self-regulation of fluctuations by time-varying flows.

We report the first extended experimental results indicating that radially localized time-varying potential structures, which possess many of the characteristics of zonal flows, are generated by strong fluctuations. Experiments performed in the H-1 heliac show that these poloidally symmetric flows are nonlinearly coupled to other fluctuations and are responsible for significant modifications in fluctuations and in the fluctuation-driven transport.

Nonambipolarity of fluctuation-driven fluxes and its effect on the radial electric field.

The nonambipolarity of the fluctuation-driven particle transport is demonstrated experimentally. Convective radial transport of electrons by fluctuations is found to be significantly stronger than that of ions, leading to a mean fluctuation-driven radial current balanced in steady state by other bipolar particle losses. Fluctuation suppression leads to a sudden disappearance of this current and results in significant modification to the radial electric field. The observed change in the electric field is in good agreement with the measured fluctuation-driven flux.

Inward turbulent transport produced by positively sheared radial electric field in stellarators.

Inward turbulent particle transport observed in the rf heated plasma of the H-1 toroidal heliac is reproduced in the CHS heliotron/torsatron by generating a region of positive radial electric field shear (E'(r)>0) using electron cyclotron resonance heating of the plasma edge. Empirical condition of the radial reversal of the turbulent flux derived from two experiments indicates that the shear electric field might be universally responsible for the recorrelation of the density and plasma potential fluctuations leading to the inward transport.