ELECTRON HEAT DIFFUSIVITY IN THE SAWTOOTHING TOKAMAK CORE

The electron heat diffusivity has been measured in the sawtoothing cor e of the TEXT-U tokamak plasma during electron cyclotron heating (ECH) . ECH creates a non-steady-state electron temperature (T-e) gradient o n a time scale faster than a single sawtooth ramp, whose relaxation is measured with two ECE systems. The measured diffusivity is considerab ly smaller than that measured in the steady state during the ohmic pha se of the discharge outside the sawtooth inversion radius. In these co nditions, high-resolution ECE has also shown the presence of small sca le, non-symmetric structures characterized by very large temperature g radients. These spatially localized, temporally cyclical structures, p reviously described as plasma filamentation, have been identified as p lasma magnetic islands. If these small-scale gradients are taken into account then the reported measurements imply an electron heat diffusiv ity which approaches the neoclassical prediction.