Effect of Fuelling Location on Pedestal and ELMs in JET

Since the introduction of the Tungsten (W) divertor in JET, as part of the ITER-Like Wall (ILW) project, we confirm the ASDEX-Upgrade finding that low fuelling, with associated infrequent large ELMs, can cause W sputtering and entrain W into the plasma, eventually cooling the plasma core. To avoid core W accumulation, typically large fuelling is used at JET, producing high ELM frequencies and smaller ELMs. On the other hand, large fuelling or low pumping are known to have a detrimental effect on confinement: H98 ~ 0.8 has been typical in heavily fuelled 2MA baseline plasmas, both with C wall and ILW at JET. It is possible that heavy fuelling localised in the divertor area drives the plasma towards detachment and cools the X-point, thereby affecting pedestal stability, and the speed at which the separatrix recovers from the ELM. To study this we varied the poloidal fuelling location in ILW plasmas with 2MA, 2.3T, ne/ne,Greenwald ~ 0.65, 12­14MW of NBI heating and medium fuelling levels. Two plasma shapes were studied: one with good pumping due to strikes near the pump duct, as shown in blue in Figure 1, and a more conventional one with reduced pumping, in red in Figure 1. Da viewing lines of inner strike, X-point and outer strike are shown for blue shape. Fuelling scans were carried out from various locations: inner strike divertor area, (iDIV, circles in next plot), upper low field side (uLFS, squares) and plasma top (Top, triangles). The divertor fuelling is toroidally distributed at source, uLFS is available at 1 toroidal location, top fuelling used 2 toroidal locations simultaneously.
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