Analysis of the ExB flow shearing rate in JET ITB discharges

The role of turbulence stabilization by sheared ExB flow is analysed for different types of internal transport barrier (ITB) in JET discharges. After the ITB formation, the shearing rate ωs is found to consistently exceed a rather crude estimate for the ion temperature gradient driven turbulence. The roles played by toroidal plasma rotation and by density and temperature gradients in both the radial electric field Er and the shearing rate ωs are evaluated. It turns out that the major contribution in triggering the positive feedback leading to ITB onset is given by the toroidal rotation. (author)