Experimental electron heat diffusion in ECH plasmas of the TJ-II stellarator

Interpretative transport has been used to revisit the global scalings of TJ-II plasmas with electron cyclotron resonance heating (ECH), from a local perspective. Density, rotational transform and ECH power scans were analysed based mainly upon Thomson scattering data (electron density and temperature) in steady state discharges. The thermal diffusivity χ_e is estimated assuming pure diffusion and negligible convective heat fluxes in a set of 201 discharges. The density scan indicates that for normalized minor radii ρ ≲ 0.4 there is no significant change of χ_e with density in the range studied , while in the density gradient region, χ_e decreases with line density. In the rotational transform scan, there is an overall tendency of χ_e to decrease with increasing rotational transform, but we also find that the presence of a low order rational value of the rotational transform is accompanied by a local lowering of χ_e. Finally, in the ECH power scan, χ_e is found to have an overall increment in 0.2 < ρ < 0.6 when Q_ECH increases from 200 to 400 kW, although it is less significant in the density gradient region.