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 Zeff, 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.