Enhancement and suppression of turbulence by energetic-particle-driven geodesic acoustic modes.

We propose a novel mechanism of enhancement of turbulence by energetic-particle-driven geodesic acoustic modes (EGAMs). The dynamics of drift-wave-type turbulence in the phase space is investigated by wave-kinetic equation. Spatially inhomogeneous turbulence in the presence of a transport barrier is considered. We discovered that trapping of turbulence clumps by the EGAMs is the key parameter that determines either suppress or enhance turbulence. In regions where turbulence is unstable, EGAM suppresses the turbulence. In contrast, in the stable region, EGAM traps clumps of turbulence and carries them across the transport barrier, so that the turbulence can be enhanced. The turbulence trapped by EGAMs can propagate independent of the gradients of density and temperature, which leads to non-Fickian transport. Hence, there appear a new global characteristic velocity, the phase velocity of GAMs, for turbulence dynamics, in addition to the local group velocity and that of the turbulence spreading. With these effect, EGAMs can deteriorate transport barriers and affect turbulence substantially. This manuscript provides a basis to consider whether a coherent wave breaks or strengthen transport barriers.

Excitation of geodesic acoustic modes by external fields.

It is planned to use external magnetic perturbations at acoustic frequencies at the DIII-D tokamak to attempt to drive geodesic acoustic modes (GAM) to modify the turbulent transport. We show that this might not only be possible--despite the well-known electrostatic nature of the GAMs--but might be a viable and efficient method to generate GAMs in magnetically confined plasmas, by developing an elegant analytic method which allows us to couple numerical dynamic equilibrium calculations with massively parallel non-Boussinesq turbulence code runs and yields practical estimates of the effectivity of the method.

Giant electron tails and passing electron pinch effects in tokamak-core turbulence.

The anomalous particle transport in a tokamak core is believed to be linked to the advection of magnetically trapped electrons alone, owing to the passing electrons maintaining a thermal equilibrium along the field lines. Surprisingly, in nonlinear numerical studies, the radial flux of passing electrons rivals that of the trapped ones. The strong interaction of passing electrons and electric fluctuations is mediated by long tails of the modes along the magnetic field, which are generated by the passing electrons in the first place.

Thermodynamic potential in local turbulence simulations.

For the reduced local equations customary in, e.g., the computation of turbulence in tokamaks the energy is still a conserved quantity. However, kinetic and magnetic energy do not appear in the reduced energy functional and thus are not constrained by it, since they are ordered small compared to the fluctuations of internal energy in the reduction process. Constraints on velocity and field fluctuations can be derived using instead the generalized grand canonical potential, which is conserved in reversible processes. This is exemplified for the Boltzmann, gyrokinetic, and fluid equations.

Turbulent saturation of tokamak-core zonal flows.

Current theories of zonal flow dynamics focus on the transport of poloidal momentum. Different from a cylinder, stationary poloidal flows in a tokamak are accompanied by (possibly kinetic) flows along the magnetic field, which maintain incompressibility, and comprise the major part of the flow energy. In numerical turbulence studies, the flows saturate by the turbulent diffusion of the parallel flow, whereas the poloidal momentum transport continues to strongly drive the flows.

Scaling laws in two-dimensional turbulent convection.

Two-dimensional homogeneous turbulent convection is studied numerically. Though Bolgiano-Obukhov scaling is approximately valid, strong differences exist in the intermittency properties of velocity and temperature increments, where the latter are similar to those of a passive scalar. The main difference of the small-scale dynamics compared to a passive scalar arises from the Kelvin-Helmholtz instability, but this process does not affect the scaling properties. A condition for a scalar field to show the ramp-and-cliff structures of a passive scalar is discussed.

Transport control by coherent zonal flows in the core/edge transitional regime.

3D Braginskii turbulence simulations show that the energy flux in the core/edge transition region of a tokamak is strongly modulated-locally and on average-by radially propagating, nearly coherent sinusoidal or solitary zonal flows. Their primary drive is the anomalous transport together with the Stringer-Winsor term. The transport modulation and the flow excitation are due to wave-kinetic effects studied for the first time in turbulence simulations. The flow amplitudes and the transport sensitively depend on the magnetic curvature acting on the flows, which can be influenced, e.g., by shaping the plasma cross section.

Condensation of microturbulence-generated shear flows into global modes

In full flux-surface computer studies of tokamak edge turbulence, a spectrum of shear flows is found to control the turbulence level and not just the conventional (0,0)-mode flows. Flux tube domains too small for the large poloidal scale lengths of the continuous spectrum tend to overestimate the flows and thus underestimate the transport. It is shown analytically and numerically that under certain conditions dominant (0,0)-mode flows independent of the domain size develop, essentially through an analog of Bose-Einstein condensation for the shear flows.