Experiments on Electron Temperature Profile Resilience in FTU Tokamak with Continuous and Modulated ECRH

Experiments performed on FTU tokamak, aiming at validation of physics-based transport models of the electron temperature profile resilience, are presented. ECRH is used to probe transport features, both in steady state and in response to time-varying heating, while LHCD is used for current density profile shaping. The experiments clearly show resiliency in the electron temperature profile response to localized ECRH. Central, low gradient plasmas are characterized by low stiffness, in the sense that the temperature gradient can change with conducted heat flux, and low electron thermal diffusivity. Strong stiffness, in the sense that the temperature gradient length 1/LT=∇T/T does not change significantly even with largely different heating profiles and intensity, and high diffusivity are found in the confinement region (0.15<r/a<0.5). A particular attention is given to the experimental investigation of the transition layer between low-high diffusivity (and low-high stiffness) regions, which is found at the EC deposition radius rdep in case of powerful ECRH. A transition layer, well identified by heat waves launched by modulated ECH, is found also in plasmas with dominant ohmic heating, showing that it is a local plasma feature, and not merely a consequence of a step-wise increase in the conducted heat flux at rdep. A novel experimental technique providing localized modulation of the temperature gradient shows strong dependence of heat diffusivity on LT at the transition layer. All observations fit well with a critical temperature gradient length modelling of local electron heat transport.