Strongly driven fusion-grade plasmas often develop spontaneous (radial) narrow zones with steep gradients. This spontaneous local improvement of confinement is of fundamental importance for the operation of fusion plasmas. In fact, the important H-mode depends on it. This mechanism may also underlie the formation of internal transport barriers.
While this phenomenon is still not completely understood, much progress has been made in recent years. It is believed that turbulent fluctuations drive sheared or zonal flows via the Reynolds Stress Mechanism. <ref>S.B. Korsholm et al, Reynolds stress and shear flow generation, Plasma Phys. Control. Fusion 43 (2001) 1377-1395</ref> This mechanism transfers energy of high-frequency drift type turbulence to low wavelength modes. The zonal flow itself is radially localized and has a very long (infinite) toroidal and poloidal wavelength. This flow then shears the turbulent eddies apart, leading to local turbulence suppression at specific radial locations, and a concomitant local reduction of transport. <ref>P.H. Diamond et al, Zonal flows in plasma - a review, Plasma Phys. Control. Fusion 47 (2005) R35-R161</ref>
At TJ-II, much effort has been invested in the experimental measurement and theoretical understanding of this phenomenon. <ref>P.H. Diamond et al, In search of the elusive zonal flow using cross-bicoherence analysis, Phys. Rev. Lett. 84, 12 (2000) 4842</ref> <ref>C. Hidalgo et al, Experimental Investigation of Dynamical Coupling between Turbulent Transport and Parallel Flows in the JET Plasma-Boundary Region, Phys. Rev. Lett. 91 (2003) 065001</ref> <ref>C. Hidalgo et al., Experimental evidence of coupling between sheared-flow development and an increase in the level of turbulence in the TJ-II stellarator, Phys. Rev. E 70 (2004) 067402</ref> <ref>C. Hidalgo et al, Improved confinement regimes induced by limiter biasing in the TJ-II stellarator, Plasma Phys. Control. Fusion 46 (2004) 287-297</ref> <ref>M.A. Pedrosa et al., Threshold for sheared flow and turbulence development in the TJ-II stellarator, Plasma Phys. Control. Fusion 47 (2005) 777-788</ref> <ref>E. Sánchez et al., On the energy transfer between flows and turbulence in the plasma boundary of fusion devices, J. of Nuclear Materials 337 (2005) 296</ref> <ref>B.A. Carreras et al, Critical transition for the edge shear layer formation: Comparison of model and experiment, Phys. Plasmas 13 (2006) 122509</ref> <ref>B. Gonçalves et al., Role of Turbulence on Edge Momentum Redistribution in the TJ-II Stellarator, Phys. Rev. Lett. 96 (2006) 145001</ref> <ref>B.Ph. van Milligen et al, Bicoherence during confinement transitions in the TJ-II stellarator, Nucl. Fusion 48 (2008) 115003</ref> <ref>M.A. Pedrosa et al, Evidence of Long-Distance Correlation of Fluctuations during Edge Transitions to Improved-Confinement Regimes in the TJ-II Stellarator, Phys. Rev. Lett. 100 (2008) 215003</ref> <ref>I. Calvo et al, Zonal flows and long-distance correlations during the formation of the edge shear layer in the TJ-II stellarator, Plasma Phys. Control. Fusion 51 (2009) 065007</ref> <ref>C. Hidalgo et al, Multi-scale physics mechanisms and spontaneous edge transport bifurcations in fusion plasmas, Europhys. Lett. 87 (2009) 55002</ref> <ref>T. Estrada et al, Sheared flows and transition to improved confinement regime in the TJ-II stellarator, Plasma Phys. Control. Fusion 51 (2009) 124015</ref> <ref>B.Ph. van Milligen et al, The dynamics of the formation of the edge particle transport barrier at TJ-II, Nucl. Fusion 51 (2011) 113002</ref>