Study of impurity transport in FTU ITB plasmas

Impurity transport during FTU electron internal transport barrier (ITB) discharges has been studied by means of a one-dimensional time dependent transport model to reproduce plasma line and continuum emissions. The impurity behaviour has been explored in two different experimental frames in which the formation and maintenance of an ITB is obtained in FTU. In the first scenario the lower hybrid and electron cyclotron resonance heating waves are launched during the current (Ip) flat-top phase, while in the second scenario the RF power is injected early during the Ip ramp-up phase. A diffusion coefficient enhanced inside the barrier, by a factor of 10, and an inward pinch velocity linearly increasing with the radius up to 3.5 m s-1, as found in the Ohmic case, are necessary to reproduce FTU plasma emission in the first scenario. A diffusion coefficient lowered inside the barrier (by a factor of 2) and about the same inward pinch velocity (v(a) = 5 m s-1 in this case) have to be assumed to interpret the impurity behaviour if the RF power is injected during the Ip ramp-up phase. These diffusion coefficient profiles are similar to the ion thermal diffusivity profiles predicted by the JETTO code: an improved electron thermal conductivity at the centre but degraded ion thermal conductivity is predicted for the flat-top ITB case, while improved electron thermal conductivity in the centre with a slightly improved ion diffusion inside the barrier results in the ramp-up ITB.