Comparison of Velocity Shear with Turbulence Reduction Driven by Biasing in a Simple Cylindrical Slab Plasma

In an experimental realization of the sheared cylindrical slab, the level of plasma turbulence is strongly reduced by application of a sufficient bias potential difference in the radial direction. Density fluctuation levels △nrms/n decrease by more than a factor of five. The ion flow velocity profile is measured spectroscopically from the Doppler shift of an argon ion line. Comparison of the shearing rates with the turbulent amplitudes as a function of bias show no relation between the shearing rate and turbulence reduction, contrary to expectations.

Understanding L-H transition in tokamak fusion plasmas

This paper reviews the current state of understanding of the L-H transition phenomenon in tokamak plasmas with a focus on two central issues： （a） the mechanism for turbulence quick suppression at the L-H transition; （b） the mechanism for subsequent generation of sheared flow. We briefly review recent advances in the understanding of the fast suppression of edge turbulence across the L-H transition. We uncover a comprehensive physical picture of the L-H transition by piecing together a number of recent experimental observations and insights obtained from 1D and 2D simulation models. Different roles played by diamagnetic mean flow, neoclassical-driven mean flow, turbulence-driven mean flow, and turbulence-driven zonal flows are discussed and clarified. It is found that the L-H transition occurs spontaneously mediated by a shift in the radial wavenumber spectrum of edge turbulence, which provides a critical evidence for the theory of turbulence quench by the flow shear. Remaining questions and some key directions for future investigations are proposed.

A new model of the L–H transition and H-mode power threshold

In order to understand the mechanism of the confinement bifurcation and H-mode power threshold in magnetically confined plasma,a new dynamical model of the L-H transition based on edge instability phase transition（EIPT） has been developed.With the typical plasma parameters of the EAST tokamak,the self-consistent turbulence growth rate is analyzed using the simplest case of pressure-driven ballooning-type instability,which indicates that the L-H transition can be caused by the stabilization of the edge instability through EIPT.The weak E？×？B flow shear in L-mode is able to increase the ion inertia of the electrostatic motion by increasing the radial wave number of the tilted turbulence structures,which play an important role for accelerating the trigger process of EIPT rather than directly to suppress the turbulent transport.With the acceleration mechanism of E？×？B flow shear,fast L-H and H-L transitions are demonstrated under the control of the input heating power.Due to the simplified scrape-offlayer boundary condition applied,the ratio between the heating powers at the H-L and L-H transition respectively differs from the ratio by Nusselt number.The results of the modeling reveal a scaling of the power threshold of the L-H transition,P_（L-H）？∝？n^（0.76） B^（0.8） for deuterium plasma.It is found finite Larmor radius induces an isotope effect of the H-mode power threshold.

Texas Helimak

Helimak is an experimental approximation to the ideal cylindrical slab, a onedimensional magnetized plasma with magnetic curvature and shear. The Texas Helimak realizes this approximation to a large degree; the finite size of the device can be neglected for many phenomena. Specifically, the drift-wave turbulence characteristic of a slab is observed with scale lengths small compared with the device size. The device and the general features of its behavior are described here. The device is capable of studying drift-wave turbulence, scrape-off layer （SOL） turbulence, and the stabilization of turbulence by imposing velocity shear.

Response of turbulent enstrophy to sudden implementation of spanwise wall oscillation in channel flow

The response of turbulent enstrophy to a sudden implementation of spanwise wall oscillation（SWO） is studied in a turbulent channel flow via direct numerical simulation. In the beginning of the application of SWO, a significant correlation is formed between ω′yand ω′z. A transient growth of turbulent enstrophy occurs, which directly enhances turbulent dissipation and drifts the turbulent flow towards a new lower-drag condition. Afterwards, the terms related to the stretching of vorticity（ωx, ω′y, and ωz）,the inclination of ω′yby ？w/？y, the turn of z by ？v′/？z, and the horizontal shear of z by ？w′/？x are suppressed due to the presence of SWO, leading to attenuation of the turbulent enstrophy.

Drag reduction in turbulent channel flow using bidirectional wavy Lorentz force

Turbulent control and drag reduction in a channel flow via a bidirectional traveling wave induced by spanwise oscillating Lorentz force have been investigated in the paper.The results based on the direct numerical simulation(DNS)indicate that the bidirectional wavy Lorentz force with appropriate control parameters can result in a regular decline of near-wall streaks and vortex structures with respect to the flow direction,leading to the effective suppression of turbulence generation and significant reduction in skin-friction drag.In addition,experiments are carried out in a water tunnel via electro-magnetic(EM)actuators designed to produce the bidirectional traveling wave excitation as described in calculations.As a result,the actual substantial drag reduction is realized successfully in these experiments.