Investigation of turbulent transport and shear flows in the edge of toroidal plasmas

Intense Langmuir-probe measurements were carried out in the toroidal low-temperature plasma of the torsatron TJ-K in order to investigate the origin and dynamics of intermittent transport events, so-called blobs, at the transition from closed to open field lines. The statistical properties of the fluctuations at the plasma boundary agree with observations made in fusion edge plasmas. Blobs were found to be generated locally through a change in turbulence drive across the separatrix. The non-linear spectral energy transfer from small-scale fluctuations into large-scale flows was measured with a 128-probe array. The results point to the transfer being a key loss channel for turbulence energy leading to a reduction in turbulent transport. Earlier observations [M.A. Pedrosa et al., Phys. Rev. Lett. 100, 215003 (2008)] of enhanced long-range correlations in the plasma potential through externally induced shear flows in TJ-II stellarator were verified. The newly measured correlation of zonal vorticity and Reynolds stress at induced flow shear indicates an enhancement of zonal-flow drive.