MHD ACTIVITY IN JET HOT ION H-MODE DISCHARGES

The MHD effects observed in the hot ion H modes in the pre-divertor co nfiguration of JET, including those generated in the preliminary triti um experiment, are described. Some observations were found to be simil ar to those in high beta regimes while others are new and appear to be pertinent to high performance discharges only. The high performance p hase is largely sawtooth free and dominated by fishbone activity, whic h increases in amplitude throughout this phase. During termination of the highperformance phase, the growth of a large variety of MHD activi ty with low mode numbers is observed. Also, edge instabilities possibl y associated with much larger mode numbers are seen in the D-alpha emi ssion. In some cases, the unusual structure of two central m = n = 1 i slands was found, and resistive MHD modelling indicates that this obse rvation is consistent with a nearly flat, non-monotonic q profile. In some discharges a sawtooth collapse immediately followed by an edge lo calized mode (ELM) is observed at, or shortly following, the terminati on of the high performance.