| Description: |
Comparative studies of the structure of turbulence have been carried out in stellarator plasmas with low (TJ-IU) and medium (L2-M) magnetic shear. In both devices, the value of the radial coherence of fluctuations is bursty. In the plasma edge region, the radial coherence is dominated by low frequency fluctuations ( 100 kHz). Numerical calculations of resistive ballooning turbulence show that in the non-linear phase, the correlation length is of the order of the width of the linear mode poloidal components, whereas in the linear phase, the correlation length is related to the width of the envelope. Toroidal coupling effects appear to be relevant in the nonlinear phase in the proximity of instability thresholds. The electrostatic Reynolds stress shows a radial gradient close to the velocity shear location in the TJ-IU torsatron, thus suggesting that this mechanism can drive poloidal flows. Keywords: stellarator, anomalous transport, flows, L-H transition " Corresponding author's e-mail: carlos.hidalg @ ciemat.es 1. lntroduction |