Gradient length driven electron heat transport study in modulated electron cyclotron heating FTU tokamak

Perturbative and steady-state heat transport of FTU tokamak in current ramp-up discharges are investigated by means of modulated electron cyclotron heating (MECH). Perturbative and steady-state transport experiments are coherent with an electron heat transport which switches from low to high values when electron temperature gradient length reaches a threshold value 1/L-Tc. The threshold value 1/L-Tc, is shown to be proportional to the ratio s/q. The experimental findings are compared to predictions of an empirical model based on the assumption of a threshold gradient length, L-Tc (1/L-T = /delT(e)/T-e/), in the electron temperature T-e below which electron thermal diffusivity, chi(e) switches from low to high values. Plasma responses to steady state and MECH are modelled assuming the electron diffusivity as chi(PB) = chi(0) + alphaT(e)(3/2) (1/L-T - 1/L-Tc)(1/2); here T-e(3/2) reflects the gyro-Bohm assumption,chi(0) represents the heat transport for 1/L-T < 1/L-Tc and the term (1/L-T - 1/L-Tc)(1/2) which sets in for 1/L-T > 1/L-Tc, mimics an extra transport possibly due to electron temperature gradient (ETG) modes. In agreement with ETG threshold 1/L-Tc is shown to be correlated with the magnetic shear s.