The influence of plasma density and edge gradients on the development of perpendicular sheared flow has been investigated in the plasma edge region of the TJ-II stellarator. The development of the naturally occurring velocity shear layer requires a minimum plasma density. Experimental findings have shown that there is a coupling between the onset of sheared flow development and an increase in the level of plasma edge turbulence; once sheared flow is fully developed the level of fluctuations and turbulent transport slightly decreases whereas edge gradients and plasma density increases. Electron density profiles show a broadening evolution as density increases above the critical value where sheared flow is developed, while the temperature profile remains similar, reflecting the strong impact of plasma density in the global confinement scaling. Furthermore, the shearing rate of the spontaneous sheared flow turns out to be close to the one needed to trigger a transition to improved confinement regimes. Density ramp experiments show, within the experimental uncertainty, no evidence of hysteresis during the spontaneous shear development. Power modulation, in the proximity of the critical plasma density, allows the characterization of plasma potential and electric field relaxation during the transition. The present results have a direct impact on the understanding of the physics mechanisms underlying the generation of critical sheared flow, pointing to the important role of turbulent driven flow.

The link between edge sheared flows and turbulence is investigated in the plasma edge region of the TJ-II stellarator and the results are compared with results in other devices like JET tokamak. In the TJ-II stellarator there is a threshold density to trigger the development of edge shear flows. During sheared flow development the degree of turbulence anisotropy ( ṽ ṽr ) is modified. The fact that different quadratic terms in fluctuating velocities ( ṽ ṽr and ṽ⊥ ṽr ) change during edge sheared flow generation means that shear flow physics involves 3D physics phenomena in which both perpendicular and parallel dynamics are involved. A new strategy has been recently applied to plasma physics to quantify the local energy transfer between flows and turbulence by computing the production term. Experimental results show that turbulence can act as an energy sink and energy source for the mean flow near the shear layer. Measurements of the turbulence production show the importance of 3D effects on the energy transfer between flows and turbulence.

The influence of limiter biasing on plasma confinement, turbulence and plasma flows is investigated in the TJ-II stellarator. Experimental results show that it is possible to modify global confinement and edge plasma parameters with both positive and negative biasing. Significant and minor modifications in the structure of plasma fluctuations have been observed during the transition to improved confinement regimes induced by limiter biasing. These results show evidence of electric field induced improved confinement via multiple mechanisms. The investigation of the relaxation of plasma potential and electric fields shows evidence of two different characteristic decay times.

This paper presents an overview of experimental results and progress made in investigating the link between magnetic topology, electric fields and transport in the TJ-II stellarator. The smooth change from positive to negative electric field observed in the core region as the density is raised is correlated with global and local transport data. A statistical description of transport is emerging as a new way to describe the coupling between profiles, plasma flows and turbulence. TJ-II experiments show that the location of rational surfaces inside the plasma can, in some circumstances, provide a trigger for the development of core transitions, providing a critical test for the various models that have been proposed to explain the appearance of transport barriers in relation to magnetic topology. In the plasma core, perpendicular rotation is strongly coupled to plasma density, showing a reversal consistent with neoclassical expectations. In contrast, spontaneous sheared flows in the plasma edge appear to be coupled strongly to plasma turbulence, consistent with the expectation for turbulent driven flows. The local injection of hydrocarbons through a mobile limiter and the erosion produced by plasmas with well-known edge parameters opens the possibility of performing carbon transport studies, relevant for understanding co-deposit formation in fusion devices.

Edge polarization experiments were carried out on TJ-II using an electrode. It has been found that the plasma response is different at densities below and above the threshold value to trigger the spontaneously development of E×B sheared flows. At low densities, the edge plasma potential is fully controlled by external biasing, while at higher densities it is determined not only by external biasing but also by the electric fields spontaneous developed. Although an improvement in particle confinement is observed for both polarities, a larger increase is observed for negative electrode bias. 52.55.Hc PACS

In tokamak machines with low toroidal magnetic fields and high plasma densities, the accessibility conditions impose restrictions to the detection of the Electron Cyclotron Emission ( ECE ). In these machines, the righthand cutoff condition can be used as an independent method to determine the local electron density from the ECE data in thermal discharges. In this paper is shown the results obatined from the detection of ECE radiation in the TCABR tokamak, in operation at the Institute of Physics of University of São Paulo. The effect of the ECE radiation cutoff was observed for different radial positions of the plasma column. To reach the ECE cutoff condition, the electron density was increased monotonically by the use of an external gas puffing system. For sufficient high densities, the emission at some frequencies is cutoff and the first and the last frequency to be cutoff depends on the shape of the density profile. These measurements do not require the plasma to be optically thick. It was observed that, for a toroidal field BO = 1.14 T , the first cutoff of ECE occurs for a radial position r ∼ = 5 cm . From these measurements the radial electron density was determined. For a symmetric parabolic profile n e = n eo [1 − (r/a) 2 ] α , values of α between 0.86 and 0.97 were experimentally obtained. A good agreement of these values with those obtained from the microwave interferometer measurements ( α ≈ 0.85 ) was found. Therefore, the ECE right-hand cutoff constitutes an independent method to obtain information about the electron density profile.

Radial profiles of the poloidal velocity of fluctuations and parallel flows have been simultaneously investigated in the proximity of the last closed flux surface in the TJ-II stellarator. It has been observed that sheared perpendicular and parallel flows are linked. The parallel flow is affected by the degree of instability in the plasma edge and its radial gradient is near the threshold of Kelvin-Helmholtz instabilities. Furthermore, parallel flows and edge instabilities are dynamically coupled. These results show that, at least partially, parallel flows are controlled by plasma turbulence. Plasma profiles, parallel flows and fluctuations have also been investigated in the proximity of low order rational surfaces in the plasma boundary region. Evidence of quasi-coherent modes and a modification in the level of fluctuations have been observed near the 4/2 rational surface located in the plasma boundary region. Parallel flows show differences in configurations with and without low order rationals: a minimum appears at the outer side of the rational location, whereas in the configurations free of rationals, this minimum disappears. Plasma edge biasing is an effective tool to induce changes in parallel and perpendicular flows.

The influence of plasma density and edge gradients in the development of perpendicular sheared flow has been investigated in the plasma edge region of the TJ-II stellarator. It has been experimentally observed that the generation of spontaneous perpendicular sheared flow (i.e. the naturally occurring shear layer) requires a minimum plasma density or gradient. It has been found that there is a coupling between the onset of sheared flow development and an increase in the level of plasma edge turbulence; once sheared flow is fully developed the level of fluctuations and turbulent transport slightly decreases whereas edge gradients and plasma density increase. The resulting shearing rate is close to the one required to trigger a transition to improved confinement regimes with reduction of edge turbulence, showing that spontaneous sheared flows and fluctuations keep themselves near marginal stability. : 52.55.Fi, 52.55.Gj, 52.35.Ra, 52.55.Hc PACS