Observation of Confined Current Structures in JET High Temperature Pedestals and Transient ELM Suppression

Full text: In an effort to approach the pedestal conditions to be expected in ITER studies of high temperature pedestal (Te,ped > 2.5 keV) plasmas were carried out at JET. In such plasmas the MHD phenomenon known at JET as an Outer Mode (OM) was observed to appear spontaneously when fuelling and recycling were low and NBI heating relatively high. While present, the OM can clamp the pedestal pressure and substantially delay ELMs, producing good confinement in a quasi-stationary state, possibly related to the QH-mode. The longest-lived OM was observed in a Hot Ion H-mode. This OM begins after the L to H transition and delays the appearance of the first ELM up to 1 s (confinement time is of order 400 ms, both during OM and during ELMy steady phase). The OM leads to a reduction in the rate of rise of density in core and pedestal, pedestal top pressure and rotation become stationary, the power outflux is regularised. Simultaneously with OM appearance, the Hot Ion H-mode regime is lost and confinement reduces to H98y ∼ 1. After an initial transient drop Wdia stays approximately constant during OM and ELMy phases. The OM appears to be terminated by the slowly rising pedestal density, or by the reduction in edge rotation shear. OMs can also be terminated by ELMs. When well established, the OM is a stable feature of the plasma, surviving sawtooth crashes, but not large ELMs. A detailed investigation of the OM leads us to conclude that it is a confined closed co-current ribbon, located at the flat-top of the pressure pedestal, co-rotating toroidally with the plasma. The time-averaged heat flux to the divertor target during the OM and ELMy phases is very similar, it can be as high as 10-30 MW/m2. A continuous OM, if it can be sustained, could become a useful operating regime for ELM elimination. (author)