MHD activity in JET hot ion H mode discharges

The MHD effects observed in the hot ion H modes in the pre-divertor configuration of JET, including those generated in the preliminary tritium experiment, are described. Some observations were found to be similar to those in high beta regimes while others are new and appear to be pertinent to high performance discharges only. The high performance phase is largely sawtooth free and dominated by fishbone activity, which increases in amplitude throughout this phase. During termination of the high performance phase, the growth of a large variety of MHD activity with low mode numbers is observed. Also, edge instabilities possibly associated with much larger mode numbers are seen in the D_alpha emission. In some cases, the unusual structure of two central m=n=1 islands was found, and resistive MHD modelling indicates that this observation is consistent with a nearly flat, non-monotonic q profile. In some discharges a sawtooth collapse immediately followed by an edge localized mode (ELM) is observed at, or shortly following, the termination of the high performance.