Research news from the division Plasma Edge and Wall
Evolution of the Edge Velocity Shear at the L-H-Transition
It is widely accepted that a velocity shear is responsible for the suppression of the edge turbulence, thus leading to the transition to the high confinement mode, the L-H transition. The flow is driven through a radial electric field. However, the origin and the evolution of it is still debated. It may be generated by turbulent stresses or neoclassical processes via the main ion pressure gradient. By spectroscopic means, the radial ExB velocity shear has been investigated at various magnetic field strengths, different electron densities, and in both hydrogen and deuterium plasmas. For all the cases, a threshold in the maximum flow velocity, a proxy for the shear, has been found at the H-mode onset. Moreover, the fast dynamics in the flow and the ion profiles were compared with a time resolution of 100 µs during different phases of the L–H transition. The ion pressure gradient is found to be the dominant contribution to the flow. Therefore, turbulence induced zonal flows can play a role only on timescales shorter than 100 µs. The experimental findings obtained in this work reveal the fundamental role of grad(pi) in the L–H transition physics.
For this work Marco Cavedon was awarded a PhD degree at the Technical University of Munich.