ELMs behaviour and edge plasma stability in JET

Abstract

Recent experimental results and plasma edge stability studies of ELMs are presented for both H-mode and optimized shear (OS) plasmas in JET. High triangularity (δ~0.5) ELMy \\mbox{H-mode} experiments in JET achieved simultaneously high confinement (HIPB(y,2) = 1) and high density (~0.9 nGR). At high pedestal density (~1.1nGR), an anomalous decrease of type I ELMs frequency and energy losses per ELM was observed. A specific for high triangularity and high density (δ~0.5 and ne~1.1nGR) MHD activity with broad-band frequency spectrum (<30 kHz) between type I ELMs was associated with type II-like ELMs which were observed in JET at rather low q95~3, compared to other machines. The coupled JETTO-MISHKA codes transport and MHD stability analysis for these scenarios confirmed a possible access to the second stability zone for the ballooning modes and stability for peeling modes at the same time. The second part of the paper is devoted to the experimental comparative study of the transition from type I to type III ELMs , carried out in the same plasma configurations for standard ELMy \\mbox{H-modes} and OS plasmas. The type III to type I ELM transition in OS discharges is found to deviate strongly from the known power scaling for this transition, derived for standard H-mode plasmas. In fact, type III ELMs are still observed at much higher power in OS scenarios, and this experimental observation suggests that larger current fraction at the plasma edge in OS discharges, compared to standard H-mode could prevent the type III to I ELMs transition.