Nonlinear pertubative electron heat transport study in the ASDEX upgrade tokamak

Electron heat transport properties have been investigated by means of localized steady-state and modulated electron cyclotron heating in the ASDEX Upgrade tokamak. The experimental findings have been successfully compared with the model assumption of a threshold gradient length, L/sub Tc/, (1/L/sub T/ = - nabla T/sub e0//T/sub e0/), in the electron temperature T /sub e0/ below which the electron thermal conductivity measured under steady-state conditions, chi /sub ePB/, shows a sudden increase and the heat conduction `seen' by a temperature perturbation, chi /sub eHP/, switches from low to high values. In this work electron cyclotron heating (ECH) power is used both to modify the steady-state gradient length and to induce a nonlinear transition from low to high electron heat transport at the position where the temperature gradient crosses its critical (or threshold) value. Steady-state ECH has been injected at different radial positions and the changes induced in electron heat transport have been studied by means of cold pulses launched from the plasma boundary. Cold pulses, obtained by injecting radiating impurities by means of laser blow off of silicon, have proved to be a flexible tool for investigating both the radial position of the step in chi /sub eHP/ and the dependence on local plasma parameters of the underlying physical mechanism.