The role of MHD in causing impurity peaking in JET hybrid plasmas

In hybrid plasma operation in JET with its ITER-like wall (JET-ILW) it is found that n > 1 tearing activity can significantly enhance the rate of on-axis peaking of high- Z impurities, which in turn significantly degrades discharge performance. Core n = 1 instabilities can be beneficial in removing impurities from the plasma core (e.g. sawteeth or fishbones), but can conversely also degrade core confinement (particularly in combination with simultaneous n = 3 activity). The nature of magnetohydrodynamic instabilities in JET hybrid discharges, with both its previous carbon wall and subsequent JET-ILW, is surveyed statistically and the character of the instabilities is examined. Possible qualitative models for how the n > 1 islands can enhance the on-axis impurity transport accumulation processes are presented.