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

Abstract

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/LTc. The threshold value 1/LTcis 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, LTc (1/LT = |∇Te/Te|), in the electron temperature Te below which electron thermal diffusivity, χe, switches from low to high values. Plasma responses to steady state and MECH are modelled assuming the electron diffusivity as χPB = χ0 + αTe3/2(1/LT-1/LTc)1/2; here Te3/2 reflects the gyro-Bohm assumption, χ0 represents the heat transport for 1/LT<1/LTc and the term (1/LT-1/LTc)1/2, which sets in for 1/LT>1/LTc, mimics an extra transport possibly due to electron temperature gradient (ETG) modes. In agreement with ETG threshold 1/LTc is shown to be correlated with the magnetic shear s.