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Abstract

Recent observations of MHD activity in type-I ELMy H-mode discharges on JET have revealed two phenomena: (i) the so-called palm tree mode, a new, snake-like MHD mode at the q=3 surface which is excited by type-I ELMs, and (ii) coherent MHD mode activity as a precursor to the ELM collapse. Both modes are detected by magnetic pick up coils and can also be seen on the edge ECE and SXR measurements. They are located a few cm inside the separatrix. Palm tree modes have been identified in a wide range of plasma conditions, which comprise standard ELMy H-modes, ITER-like plasma shapes, pellet fuelling, and even pure helium plasmas. The mode frequency increases in time and starts to saturate until the mode finally decays. A likely explanation of the palm tree mode is, that it is the remnant of an (3,1)-island created due to edge ergodisation by the ELM perturbation. Refined methods of data evaluation allowed to identified low frequency coherent type-I ELM precursor modes with toroidal mode numbers in the range 2 to 14. They seem to have a kink-like structure, and appear normally 0.5-1ms before the ELM, but in many cases even much earlier.