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 χe 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 χe with density in the range studied , while in the density gradient region, χe decreases with line density. In the rotational transform scan, there is an overall tendency of χe 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 χe. Finally, in the ECH power scan, χe is found to have an overall increment in 0.2 < ρ < 0.6 when QECH increases from 200 to 400 kW, although it is less significant in the density gradient region.