Esolano

Specific experiments were made at JET to study high temperature pedestals, in the range Te_ped ~2.5-5 keV. The idea was to see if the [[Edge Localized Modes|ELMs]] are qualitatively different when resistivity and collisionality are low. The surprising result of the experiment was a new insight on pedestal stability: with low gas fuelling and low recycling conditions (required to achieve high temperature pedestals) an Outer Mode often appears, delaying ELMs. The outer mode had been assumed to be an ideal kink, unstable in the steep gradient region of the pedestal, which would eventually grow into an ELM. We have now found that the outer mode is a fairly stable current ribbon, lasting as much as 1.5 s, located at the top of the pedestal. There are tantalising similarities with the EHO observed in DIII-D. This could provide a new route to ELM-free operation.