Radiation pattern and impurity transport in argon seeded ELMy H-mode discharges in JET

This paper addresses the issues of impurity behaviour during the argon seeding experiments in JET, in which argon and D₂ have been simultaneously puffed in ELMy H-mode discharges to reach high density regimes, maintaining good confinement properties throughout the plasma discharge. The analysis is based mainly on a 1-D impurity diffusion model, which evaluates the impurity transport coefficients in three experimental scenarios. The available experimental data include, as a function of the time, the brightnesses of the soft x-rays and of several impurity lines, the effective charge Z_eff, the profiles of the radiated power and the fully stripped AR and C ion densities from charge-exchange recombination. From the simulation of these data it is possible to characterize the considered scenarios in terms of high-Z ion increase in the plasma centre and high radiation from the edge. In particular, it has been found that the configuration at high triangularity with continuous D₂ puffing, characterized by an outward impurity pinch velocity, features flat or slightly hollow impurity ion profiles and high radiation from a narrow region at the edge, possibly associated to the presence of a significant neutral density. In this case an edge radiative mantle is established without high radiation from the plasma centre.