Tungsten Screening and Impurity Control in JET

The impact of tungsten on stability and confinement of plasma discharges is studied in JET featuring the ITER-like wall. A W-diagnostic combining the soft X-ray cameras and a VUV spectrometer was used to analyze the balance between heating and radiative cooling in the core plasma. In a typical example discharge featuring impurity accumulation, the core W radiation is quantified. It is strong enough to affect the Te-profiles. The flattening of the Te-profile is thought to cause a further reduction of turbulent transport including a turbulent outward pinch. In this situation the neoclassical inward pinch is provided with more weight, which leads to further accumulation of W. Smaller deuterium gas puff levels which lead to low ELM frequencies allow for a more efficient penetration of eroded W into the main plasma. For low gas puff conditions, also W-erosion and electron density peaking foster impurity accumulation which ought to be mitigated by a medium sized gas puff in the range of 1-2×1022 electrons/s.