Impurity injection in RFX-mod Reversed Field Pinch for impurity transport studies

In RFX-mod RFP device, strong evidences that impurity core penetration is prevented, as the result of a strong outward impurity convection, have been found. Strong convection has not been found for the main gas [1] and a favorable situation with peaked or flat density profiles and hollow impurity profiles is produced. Analysis of impurity transport relies on best reconstruction of impurity emission pattern with a 1-dim Collisional-Radiative code in which the radial impurity flux is schematized as a sum of a convective and a diffusive terms [2,3]. The diffusion coefficient D and the velocity V input to the simulation are varied until the experimental emission is reproduced. While the steady-state impurity profile is determined by the ratio between the convection V to the diffusion D parameters V/D ( peaking factor) , to discriminate D and V transient perturbative experiments are needed. Outward convection has been found for Ni and Ne in various experimental conditions [4] including the improved confinement self-organized helical regimes occurring at high plasma current (I>1.2 MA). The impurity flux convective term is positive (outward) over the whole plasma radius. The outward pinch is higher in a localized radial region, where a poloidal flow shear is also observed, forming a barrier which is wider and stronger in the helical regime. Such a barrier opposes the impurity penetration preventing core plasma contamination. The experimental evidence of impurity outward convection in RFX-mod found in Ni LBO and Ne gas puffing experiments (W LBO didn't show accumulation effects too), has been then confirmed for C and Li solid room temperature pellets experiments [5], and by recent Ne doped D2 cryogenic pellet injection. Similar D and V have been found for all the considered impurity species, without strong dependence on mass/charge. A coherent impurity transport theoretical frame is still missing: with the present knowledge of the RFX-mod ion temperature profile, the found impurity outward convection cannot be ascribed to a classical 'temperature screening' effect.