Transport studies with the ECH system on FTU tokamak

Experiments performed on the FTU tokamak, aimed at validation of physics-based transport models, which yield the electron temperature profile stiffness by using electron cyclotron resonance heating (ECRH), and at the implementation of ECRH-based techniques for plasma real-time control, are presented. ECRH is used to probe transport features, both in the steady state and in response to time-varied heating. The experiments clearly show stiffness in the electron temperature profile response to localized ECRH. The low gradient plasma region near the axis is characterized by low stiffness, in the sense that the temperature gradient can change with increasing 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 significantly different heating profiles and intensity) and high diffusivity are found in the confinement region (0.15 < r/a < 0.5). Particular attention is given to the experimental investigation of the transition layer between low and high diffusivity (and low and high stiffness) regions, which is located at the EC deposition radius rdep when powerful ECRH is applied. A transition layer, 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. All observations fit well with a critical temperature gradient length modelling of local electron heat transport. The measurement of the rate of change of stored electron energy when ECRH is switched on (or off) is the basis of a technique for real-time and automatic detection of the absorption layer. Repetitive short pulses are used to increase the signal to noise ratio, with a very low duty cycle, so as not to waste ECRH power during the detection procedure. (author)