Experimental studies of mesoscale structure and its interactions with microscale waves in plasma turbulence

Abstract

The modern view of plasma turbulence has been established due to the discovery of zonal flows and other structures drift-waves generate, and contribute to exploring new manners of understanding of transport and structural formation in magnetized plasmas and astronomic objects. This article presents recent development of laboratory experiments for plasma turbulence to have advanced its understanding and have made the paradigm shift. The topics include the discoveries of mesoscale structures, such as zonal flows and streamers, the recent development of analyzing techniques to quantify the couplings between different scale structures and methods to elucidate energy transfer direction, and turbulence transport and barrier formation. Finally, future experiments are suggested for establishing the first-principle laws of turbulence transport and structural formation.

Symmetry-Breaking of Turbulence Structure and Position Identification in Toroidal Plasmas

“…The pressuregradient-driven turbulence can generate a global flow [24], and such a flow forms a doubly-connected flows in torus. Experimental identification of flow generation has been in progress [45,58,74], and a lot of fundamental mechanisms are known and wait experimental confirmations. For instance, the in-out and/or up-down asymmetry of turbulence has been known to play essential roles in the generation of poloidal and toroidal flows (axial vector field) from the turbulence which are driven by the density and/or temperature (scalar quantity).…”

“…Among various progresses, one may point out that the essential elements in these developments are the physics understanding of, such as, (i) the multiple-scale nonlinear interactions (including nonlinear instabilities) , (ii) bifurcation of the combined structure of turbulence and global inhomogeneity [26][27][28][29][30][31][32][33][34][35][36][37][38], and (iii) new aspect of the far-nonequilibrium feature of confined plasmas (i.e., the idea of 'heating heat turbulence', or 'fuelling fuels turbulence') [39,40]. Another decisive progress is the advancement of the data-analysis method, which has enabled one to measure the magnitude of nonlinear interactions quantitatively [41][42][43][44][45], with examples of successful identifications of nonlinear excitations [46][47][48][49][50][51] and the identification of hysteresis in gradient-flux relation [52] (see reviews [53,54]). Figure 1 author's e-mail: takuma@artsci.kyushu-u.ac.jp illustrates the experimental confirmations of meso-and macroscopic perturbations in toroidal plasmas; spatiotemporal correlations of zonal flow (a), global fluctuation (b) and streamer (c), respectively.…”

Nonclassical Transport and Particle-Field Coupling: from Laboratory Plasmas to the Solar Wind

“…In well diagnosed laboratory plasmas, coexistence and interaction (Fujisawa 2011;Diamond et al 2011) between small scale drift turbulence (Tynan et al 2009) and large scale coherent nonlinear structures-zonal flows (Diamond et al 2005), streamers (Yamada et al 2008), and other objects that exhibit long range correlation (Inagaki et al 2011)-is an established feature which is central to the phenomenology of the global system (Wagner 2007). Approaches to modelling this span the zero-dimensional Lotka-Volterra predatorprey paradigm (Malkov and Diamond 2009), nonlinear few-wave coupling (Manfredi et al 2001), and large scale numerical simulations, which however are challenged by the need to incorporate a wide range of physically relevant lengthscales.…”

Robustness of predator-prey models for confinement regime transitions in fusion plasmas

“…Whether zonal flows or streamers are preferentially formed under specific plasma conditions, and how they compete, has been addressed from various perspectives [13][14][15], and remains an open question. For a recent review of experimental observations of the interaction between mesoscale structures (such as zonal flows and streamers) and microscale structures (such as drift turbulence), see [16]; of drift turbulence, particularly in relation to transitions in global confinement, see [17]; and of the L-H transition, see [18]. A recent review of these physics issues in a broad context is provided by [19].…”