Issue
EPL
Volume 85, Number 2, January 2009
Article Number 25002
Number of page(s) 6
Section Physics of Gases, Plasmas and Electric Discharges
DOI http://dx.doi.org/10.1209/0295-5075/85/25002
Published online 22 January 2009
EPL, 85 (2009) 25002
DOI: 10.1209/0295-5075/85/25002

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

J. A. Alonso, C. Hidalgo, M. A. Pedrosa, J. L. De Pablos and the TJ-II team

Laboratorio Nacional de Fusión, EURATOM-CIEMAT Association - Av. Complutense 22, 28040 Madrid, Spain, EU

arturo.alonso@ciemat.es

received 16 July 2008; accepted in final form 15 December 2008; published January 2009
published online 22 January 2009

Abstract
The structure of turbulence and parallel and perpendicular E$\times$B flows have been investigated across the transition to improved confinement regimes induced by biasing. The radial electric field at the edge and the E$\times$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 $\Delta M_{\parallel}\approx 0.3$ in the plasma edge region which is in rough agreement with the expected change in the Pfirsch-Schlüter flow. In order to investigate the role of E$\times$B sheared flows and turbulence on parallel dynamics, the cross-correlation coefficient $\langle\tilde{v}_r\tilde{v}_\parallel\rangle/(\langle\tilde{v}_r^2\rangle\langle\tilde{v}_\parallel^2\rangle)^{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_r\langle\tilde{v}_r\tilde{v}_\parallel\rangle $ 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 modification.

PACS
52.35.Ra - Plasma turbulence.
52.30.-q - Plasma dynamics and flow.
52.55.Hc - Stellarators, torsatrons, heliacs, bumpy tori, and other toroidal confinement devices.

© EPLA 2009