Fluctuation-Induced Turbulent Transport and Sheared Flows close to Instability Threshold in the TJ-II Stellarator M. A. Pedrosa, C. Hidalgo, T. Estrada, B. Gonçalves * , E. Ascasíbar, I. Pastor, J. A. Jiménez, A. López-Fraguas, and the TJ-II team Laboratorio Nacional de Fusión por Confinamiento Magnético, Asociación EURATOM-CIEMAT, 28040-Madrid, SPAIN *Euratom-IST, 1049-001 Lisbon, Portugal Magnetic well and instability thresholds in the TJ-II stellarator TJ-II is a low magnetic shear stellarator of the Heliac type [1] (average major radius of 1.5 m and average minor radius of < 0.22 m) that provides plasma stability through the existence of a magnetic well in the whole plasma radius (magnetic well depth is defined as W=100x[U( )- U(0)]/U(0), where U( ) is the specific volume at a given effective radius ; 0 refers to the magnetic axis). Previous magneto hydrodynamic (MHD) studies [2, 3] have examined the stability properties of the device and shown the characteristics of the magnetic well term for ideal and resistive interchange modes, finding that the presence of magnetic well is the main stabilising mechanism in TJ-II. Experimental evidence of the influence of the magnetic well in regulating the fluctuations in the plasma edge region of the TJ-II stellarator has been recently reported [4]. Making use of the flexibility of the TJ-II magnetic configuration, the magnetic well depth may be modified over a broad range of values, from 0% to 6%. Three different magnetic configurations with very similar iota profiles and magnetic well 2.4, 0.6 and 0.2 % have been investigated. Figure 1 shows the vacuum configurations under study (configurations are labelled by means of the current flowing in three of the four sets of coils of TJ-II). Magnetic well profiles are shown in figure 2. In the magnetic configuration with magnetic well 0.2 %, a -0.4 -0.2 0 0.2 0.4 -0.4 -0.2 0 0.2 0.4 100_40_63 Z (m) R-R0 (m) -0.4 -0.2 0 0.2 0.4 100_40_81 R-R0 (m) -0.4 -0.2 0 0.2 0.4 100_40_90 R-R0 (m) Fig. 1.- Vacuum calculations of the plasma magnetic surfaces in configurations with magnetic well 2.4, 0.6 and 0.2% respectively and similar rotational transform profile. Lines show the approximated position of the Langmuir probe. 29th EPS Conference on Plasma Phys. and Contr. Fusion Montreux, 17-21 June 2002 ECA Vol. 26B, P-3.207 (2002)