On the link between parallel flows, turbulence and electric fields in the edge of the TJ-II stellarator

The structure of turbulence and parallel and perpendicular E x B flows have been investigated across the transition to improved confinement regimes induced by biasing. The radial electricfield at the edge and the E x B shear increase, while the level of edge fluctuations is reduced significantly during such improved confinement regimes in TJ-II. In addition, the parallel flow Mach number changes by Delta M(parallel to) approximate to 0.3 in the plasma edge region which is in rough agreement with the expected change in the P firsch-Schluter flow. In order to investigate the role of E x B sheared flows and turbulence on parallel dynamics, the cross-correlation coefficient <(v) over tilde (r)(v) over tilde (parallel to)>/(<(v) over tilde (2)(r)> <(v) over tilde (2)(parallel to)>)(1/2) was computed in the presence of perpendicular flows induced by biasing. Although the level of turbulence decreases, the phase coherence increases and the resulting turbulent forces partial derivative(r) <(v) over tilde (r) (v) over tilde (parallel to)> are shown to be high enough to affect parallel dynamics. These findings represent the first experimental evidence of the dual role of electric fields as a stabilizing mechanism of plasma turbulence and as an agent affecting the parallel momentum balance via turbulence modi. cation. Copyright (C) EPLA, 2009