Simulation of the time behaviour of impurities in JET Ar-seeded discharges and its relation with sawtoothing and RF heating

In JET, Ar seeding has proven to be a successful means to reach quasi-stationary regimes that feature, simultaneously, high density, high confinement and an edge radiation belt without significant contamination of the plasma. A detailed analysis shows that in the presence of centrally deposited ion cyclotron resonance heating (ICRH) Ar transport is modified, in the sense of a reduction of the inward pinch convection. Sawtooth (ST) crashes also hamper the accumulation of Ar in the core, though their contribution is less relevant compared to the effect of the ICRH. The analysis is carried out by means of a one-dimensional impurity transport code, which has been applied to several Ar-seeded discharges. The result about the role of ST crashes is independent of triangularity. In the high triangularity discharges, with continuous D refuelling, the convection of impurities in the core region is typically very small so that ICRH injection itself has little relevance in the control of the Ar behaviour.