Effect of the Divertor Geometry on the Pedestal Confinement in JET-ILW

The baseline type I ELMy H-mode scenario has been re-established in JET with the new tungsten divertor and beryllium-main wall (JET-ILW) in 2011. In comparison with carbon wall (JET-C) discharges in similar conditions, a degradation of the confinement has been observed, with the reduction mainly driven by a lower pedestal pressure. The causes of the pedestal degradation are not clear yet and are currently under investigations. Some of the main possibilities are a difference in the pedestal stability (possibly driven by the different impurity content between JET-C and JETILW) and/or a difference in the recycling flux in the divertor area. This work investigates the effect of the magnetic configuration in the divertor region on the pedestal structure, pedestal confinement and ELMs. A set low triangularity H-mode discharges have been performed using the same NBI power (two power levels, 14MW and 22MW) and a similar core shape but changing the strike position relative to the pump throat. Three configurations have been tested: with the outer strike point on the horizontal target (on tile 5), in the corner (tile 6) and on the vertical target (tile 7).
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