Studies of the pedestal structure and inter-ELM pedestal evolution in JET with the ITER-like wall

Abstract

The pedestal structure of type I ELMy H-modes has been analysed for JET with the ITERlike Wall (JET-ILW). The electron pressure pedestal width is independent of ρ* and increases proportionally to √βpol,PED. Additional broadening of the width is observed, at constant βpol, PED, with increasing ν* and/or neutral gas injection and the contribution of atomic physics effects in setting the pedestal width cannot as yet be ruled out. Neutral penetration alone does not determine the shape of the edge density profile in JET-ILW. The ratio of electron density to electron temperature scale lengths in the edge transport barrier region, ηe, is of order 2–3 within experimental uncertainties. Existing understanding, represented in the stationary linear peeling–ballooning mode stability and the EPED pedestal structure models, is extended to the dynamic evolution between ELM crashes in JET-ILW, in order to test the assumptions underlying these two models. The inter-ELM temporal evolution of the pedestal structure in JET-ILW is not unique, but depends on discharge conditions, such as heating power and gas injection levels. The strong reduction in pe,PED with increasing D2 gas injection at high power is primarily due to clamping of ∇Te half way through the ELM cycle and is suggestive of turbulence limiting the Te pedestal growth. The inter-ELM pedestal pressure evolution in JETILW is consistent with the EPED model assumptions at low gas rates and only at low beta at high gas rates. At higher beta and high gas rate the inter-ELM pedestal pressure evolution is qualitatively consistent with the kinetic ballooning mode (KBM) constraint but the peeling– ballooning (P–B) constraint is not satisfied and the ELM trigger mechanism remains as yet unexplained