Edge issues in ITB plasmas in JET

Abstract

This paper documents critical issues in internal transport barrier (ITB) plasmas in JET, namely the transition from type III to type I edge localized modes (ELMs), and the subsequent impact of large amplitude ELMs on the ITB. Benign type III ELMs are observed in ITB plasmas at input powers much larger (up to a factor 3) than the empirical threshold for type III/I transition derived from standard H-modes. Various measurements indirectly suggest a larger fraction of plasma current at the edge of ITB plasmas. Experimental results look consistent with a type III ELM regime controlled by a large fraction of edge current. Especially, the transition to type I ELMs does not occur for a broad current profile (ℓi≈0.78) characterized by low edge magnetic shear (s95≈2.6), and a back transition from type I to type III has been found to well correlate with an increase of the edge current. When large ELMs occur, strong perturbations δTe on electron temperature are generated, and propagate inwards on a ballistic timescales, at vburst≈160 m s−1. It is of the order of one third (respectively one ninth) of the curvature (respectively diamagnetic) drift. This propagation looks reminiscent of non-local transport experiments. The perturbation induced by large ELMs can reach the ITB. In this case, δTe increases in the vicinity of the ITB before being strongly damped further inside. Such ELMs also lead to a transient increase of Te gradient at the ITB, which then moves inwards on a diffusive timescale (χ≈3×10−2 m2 s−1) while degrading.