Experimental studies of multi-scale structures and their interactions …

Recent researches of plasma turbulence have made a great progress in its understanding by the outstanding development of the experimental apparatus and data analysis techniques. Nowadays the plasma turbulence can be regarded as a system of micro-scale fluctuations (such as drift waves) and meso-scale structures, such as zonal flows and streamers generated from the background micro-scale fluctuations [Phys. Rev. Lett. 93 165002 (2004)]. The view of plasma turbulence, moreover, has been extended to include the macro-scale fluctuating structures excited by the background turbulence, very recently. The modern view of the plasma turbulence, i.e., a system of micro-, meso- and macro-scale fluctuating structures interacting with each other, has contributed to fundamental understanding of structural formation in turbulent plasma and to more exact prediction of ITER performance. On the other hand, numerical analysis tools have been developed to elucidate the nonlinear couplings and energy transfer direction between the elementary waves in turbulence. These tools have been used to prove that the meso-scale structures, such as zonal flows and streamers, are really driven by the background turbulence. In this colloquium, the recent experiments and the modern view of plasma turbulence are presented [A. Fujisawa, Nucl. Fusion 49 013001 (2009) . G. R. Tynan et al., Plasma Phys. Control. Fusion 51 (2009) 113001], with emphasis on the results of CHS experiments in National Institute for Fusion Science (Japan), including the identification of the zonal magnetic field [A. Fujisawa et al., Phys. Rev. Lett. 98 165001 (2007)]. This is a decisive experimental progress toward the verification of the hypothesis for the historical enigma of dynamo problems.