Fig. 2-1 Conceptual view of zonal flow in torus plasma (the red and the blue zones) Plasma flows having opposite directions are excited spontaneously. For GAM, which is a kind of zonal flow, the flow directions vary temporally.

Fig. 2-2 The temporal behavior of the GAM and the density fluctuation (a) and (c): The electrostatic potential driving the plasma flow. The oscillation is evidence of the oscillating flow, GAM. (b): Power spectrum of the density fluctuation. (d): The density fluctuation and its envelope. Power and envelope of the fluctuation oscillate in sync with the GAM.

Confining the higher-pressure plasma for an extended time is an essential task for the development of the fusion reactor. In magnetically confined plasmas, the confinement performance is limited by excitation of plasma fluctuations inducing transport of the plasma. Therefore, understanding and control of the fluctuations are important. To date, a flow shearing the plasma has demonstrated suppression of the fluctuation and improvement of the plasma confinement. Zonal flow has recently received much attention as an excitation mechanism of shear flow. A conceptual view is shown in Fig. 2-1, and such a shear flow is excited spontaneously through nonlinear coupling of the plasma turbulence. The study of zonal flow can contribute substantially to the prediction of the confinement performance of future machines and to the progress of the physics of nonequilibrium and nonlinear systems. In the JFT-2M tokamak, a heavy ion beam probe (HIBP) detected a form of zonal flow. This flow is called a Geodesic-acoustic mode (GAM), and the flow direction varies temporally. Fig. 2-2(a) shows the electrostatic potential driving plasma flow. The periodic oscillation means the flow direction varies periodically. An advantage of the HIBP is the simultaneous measurement of the electrostatic potential and density fluctuation at the same location. The plasma density fluctuation and the GAM were observed with the HIBP on JFT-2M, and the close correlation between them was demonstrated experimentally (Fig. 2-2). The power of the density fluctuation (Figs. 2-2(b) and (d)) is modulated in sync with the GAM oscillation (Figs. 2-2(a) and (c)), and a statistical analysis indicates the modulation strongly correlates with the GAM. Moreover, we discovered for the first time that the plasma transport induced by the fluctuation was also modulated. The interaction between the GAM and the fluctuation was explained theoretically. Such phenomena are universal in magnetically confined plasmas, so the study of them in collaboration with experiment, theory, and simulation will lead to an understanding of the physics in huge devices such as ITER, as well as in medium and small devices.

Reference T. Ido et al., Electrostatic Fluctuation and Fluctuation-Induced Particle Flux During Formation of the Edge Transport Barrier in the JFT-2M Tokamak, Proceedings of 20th IAEA Fusion Energy Conference, Nov. 1-6, 2004, Vilamoura, Portugal, IAEA-CN-116/EX/4-6Rb (2005) (submitted to Nucl. Fusion).

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