Identification of Zonal Flows in a Toroidal Plasma

Abstract

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...

Extension of geodesic acoustic mode theory to helical systems

“…Similar oscillations of potential were found also in the helical device CHS͑Compact Helical System͒, 25 in which the mode structure has been clearly elucidated by use of a dual HIBP. In the H-1͑Heliac device-1͒ 26 and HT-7 ͑Hefei Tokamak͒ devices, 27 where the electron temperature is low in the edge region, electrostatic probes were utilized in the experiment.…”

Long-distance correlation and zonal flow structures induced by mean E×B shear flows in the biasing H-mode at TEXTOR

“…10,11 Comparing the frequency spectra of the ZF in our case with those of residual ZFs or finite-frequency ZFs seen in other machines, 11,13,19,20 it seems that the ZF structure observed by us is similar to the residual ZFs. In addition, we have calculated the autobispectrum of potential fluctuations using techniques introduced in Ref.…”

On the bicoherence analysis of plasma turbulence

“…The identification of mesoscale structures ͓such as zonal flow 4 and geodesic acoustic modes 7 ͑GAMs͔͒ and their interaction with ambient turbulence is a highlight of the experimental study of plasma turbulence. The identification of a mesoscale zonal flow has been in progress, 8 and the efforts in the measurement of the nonlinear interactions are also ongoing. One routine method in measuring the nonlinear interactions among the fluctuating quantities is the bicoherence method.…”