Gradient length driven transport in EC heated FTU tokamak

Steady state electron heat transport of FTU high density plasmas (<ne>iin« 0.45-1 1020 m~3) during Electron Cyclotron Resonance Heating (ECRH) are compared to predictions of a model based on the assumption of a threshold gradient length, Lfc, in the electron temperature Te beyond which electron thermal conductivity, %e, switches from low to high values. The model accounts for the stiffness of Te slope observed in FTU plasmas heated with quite different radial shapes of the ECRH power deposition profile. These profiles are achieved by toroidal field tuning and steering two gaussian beams delivering up to 850 kW to the plasma in a radial position, p, ranging from p ~ 0.25 to p « 0.5. Plasma response is modelled by assuming the electron conductivity as %e = XeO + a Te^^2 (l/Lj - l/Lfc)^2') nere Te^ reflects the gyro-Bohm assumption, %EURQ represents the heat transport for Lf>Ljc while a, the so-called stiffness coefficient, is adjusted to cope with the observed experimental data. The term (1/L? - l/L^)1^ which sets in for LT<LTC mimics an extra transport possibly due to Electron Temperature Gradient (ETG) modes. All the analyzed discharges are well reproduced by the same critical gradient length, L-pc> the same stiffness coefficient, a, and the same 'low' conductiviy XeO.