Empirical electron heat diffusivity (χ_e) models based on electron temperature (T_e) and/or its gradient (∇T_e) dependence are tested against the results of cold pulse propagation and ECH modulation experiments performed on LHD, using time dependent transport simulations.
The electron heat diffusivity estimated from transient analysis is slightly larger (about a factor of 1-2 depending on the discharges) than that estimated from static (power balance) analysis. In addition the transient analysis also indicates that the heat pinch term is required to represent the time evolutions of electron temperature perturbations. One of the candidates to explain such experimental results is the dependence of electron heat diffusivity on T_e and ∇T_e. Then various parametric dependences of χ_e(T_e,∇T_e), e.g. χ_e=ξT_e^α∇T_e^β, are tested (ξ, α and β are chosen so that the best fit between experimental results and time dependent transport simulation is obtained). The electrostatic gyro-Bohm assumption (α=3/2) is consistent with experimental results on LHD. The relevance of the critical gradient length model, which is consistent with Tokamak MECH experimental results, will be also discussed.