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 T c . The threshold value 1/ L T c 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 T c (1/ L T = |∇ T e / T e |), in the electron temperature T e below which electron thermal diffusivity, χ e , switches from low to high values. Plasma responses to steady state and MECH are modelled assuming the electron diffusivity as χ PB = χ 0 + α T e 3/2 (1/ L T -1/ L T c ) 1/2 ; here T e 3/2 reflects the gyro-Bohm assumption, χ 0 represents the heat transport for 1/ L T <1/ L T c and the term (1/ L T -1/ L T c ) 1/2 , which sets in for 1/ L T >1/ L T c , mimics an extra transport possibly due to electron temperature gradient (ETG) modes. In agreement with ETG threshold 1/ L T c is shown to be correlated with the magnetic shear s .