Parametric dependence of the perpendicular velocity shear layer formation in TJ-II plasmas

In TJ-II plasmas, the perpendicular rotation velocity of the turbulence changes from positive to negative (from ion to electron diamagnetic direction) inside the LCMS when the line-averaged plasma density exceeds some critical value, this change being dominated by the inversion in the radial electric field. In this work we study the parameters that control the inversion in the perpendicular rotation. A parametric dependence of the critical density has been obtained studying plasmas confined in different magnetic configurations (different rotational transform and/or plasma volume) and heated with different ECH power levels. The studied data set shows positive exponential dependence on heating power and negative one on plasma radius, while the dependence on rotational transform has low statistical meaning. Besides, analysis of local plasma parameters points to plasma collisionality as the parameter that controls the inversion of the perpendicular rotation velocity of the turbulence. (author)