This Letter presents experimental confirmation of the presence of zonal flows in magnetically confined toroidal plasma using an advanced diagnostic system -dual heavy ion beam probes. The simultaneous observation of an electric field at two distant toroidal locations ( 1:5 m apart) in the high temperature ( 1 keV) plasma provides a fluctuation spectrum of electric field (or flow), a spatiotemporal structure of the zonal flows (characteristic radial length of 1:5 cm and lifetime of 1:5 ms), their long-range correlation with toroidal symmetry n 0 , and the difference in the zonal flow amplitude with and without a transport barrier. These constitute essential elements of turbulence-zonal flow systems, and illustrate one of the fundamental processes of structure formation in nature. Zonal flows-azimuthally symmetric bandlike shear flows-are ubiquitous phenomena in the Universe [1][2][3]; examples include Jovian belts and zones, the terrestrial atmospheric jet stream, the super-rotation of the Venusian atmosphere, and the rotation profile of the solar tachocline. Zonal flows have been expected to be present in magnetically confined toroidal plasmas [4] since the characteristics of drift wave turbulence in the plasmas are analogous to Rossby wave turbulence to cause the phenomena in the rotating planets. Recently, their crucial role in determining the turbulent level and resultant transport has been widely recognized, and the identification of the zonal flows becomes an urgent issue in the fusion research to enhance the prospect of plasma burning in the International Thermonuclear Experimental Reactor [5][6][7].In toroidal plasmas, the zonal flows emerge in electric field fluctuation symmetric m n 0 on magnetic flux surface with finite radial wave numbers (see for review, e.g., [8,9]). Two major branches of zonal flows are expected in magnetic confined toroidal plasmas, i.e., a residual flow of nearly zero frequency, and an oscillatory flow termed geodesic acoustic modes (GAMs) [10,11]. These zonal flows are driven exclusively by nonlinear interactions (or inverse cascade) through energy transfer from the microscopic drift waves. Inversely, the zonal flows regulate the drift wave turbulence and resultant transports. The time-varying E B shearing of zonal flows, similar to the mean flows [12], has a significant effect on plasma turbulence and transport.Direct nonlinear simulations have, in fact, confirmed the appearance of and generation processes for zonal flows [13][14][15][16][17][18][19][20], and their essential role in turbulence and transport of toroidal plasmas. In experiments, however, only indirect signs have been obtained for zonal flows and their role in confinement. Coherent oscillations presumed to be GAMs were detected in measurements with a heavy ion beam probe (HIBP) [21,22], with traditional probes [23,24], and with beam emission spectroscopy using a modified time-delayed-estimation analysis technique [25]. Bicoherence analysis showed an increase in nonlinear interaction between zonal flows and turbule...

We report observations of the dynamic response of micro-fluctuations and turbulent flux to a low-frequency heating power modulation in the Large Helical Device. The responses of heat flux and micro-fluctuation intensity differ from that of the change in temperature gradient. This result violates the local transport model, where turbulence is determined by the local temperature gradient. A new relationship between flux, gradient and turbulence is found. In addition to the temperature gradient, the heating rate is proposed as a new, direct controlling parameter of turbulence to explain the fast response of turbulence against periodic modulation of heating power.

The three-dimensional magnetic configuration of a stellarator offers two specific mechanisms for a transition to improved particle and energy confinement. One route goes through the so-called electron-root confinement regime, which leads to a reduction of neoclassical transport via strong radial electric fields. In this Letter evidence for a second route is presented. It opens due to the layer of a strongly varying radial electric field which is present in the transitional region from neoclassical electron to ion-root confinement. This type of improvement acts on turbulent transport.

Excitation of the turbulence in the range of drift wave frequency and zonal flow in magnetized plasmas is analyzed. Nonlinear stabilization effect on zonal flow drive is introduced, and the steady state solution is obtained. The condition for the onset of turbulent transport is obtained and partition ratio of fluctuation energy into turbulence and zonal flows is derived. The turbulent transport coefficient, which includes the effect of zonal flow, is also obtained. Analytic result and direct numerical simulation show a good agreement.

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