Role of sawtooth in avoiding impurity accumulation and maintaining good confinement in JET radiative mantle discharges

Impurity injection in the JET ELMy H-mode regime has produced high-confinement, quasi-steady-state plasmas with densities close to the Greenwald density. However, at large Ar densities, a sudden loss of confinement is observed. A possible correlation between loss of confinement and the observed MHD phenomena, both in the core and in the edge of the plasma, was considered. The degradation in confinement coincided with impurity profile peaking following the disappearance of sawtooth activity. In addition, impurity density profile analysis confirmed that central MHD modes prevented impurity peaking. Experiments were designed to understand the role of sawtooth crashes in re-distributing impurities. Ion-cyclotron radio frequency heating was used to control the central q-profile and maintain sawtooth activity. This resulted in quasi-steady-state, high-performance plasmas with high Ar densities. At and high Ar injection rates, quasi-steady-states, which previously only lasted <1τE, were now maintained for the duration of the heating (Δ t ~ 9τ E). The increased central heating may have an additional beneficial effect in opposing impurity accumulation by changing the core power balance and modifying the impurity transport as predicted by neo-classical theory.