MHD stability analysis of diagnostic optimized configuration shots in JET

The plasma edge MHD stability is analysed for several JET discharges in the diagnostic optimized configuration. The stability analysis of Type I ELMy plasmas shows how after an edge localized mode (ELM) crash the plasma edge is deep in the stable region against low- to intermediate-n peeling?ballooning modes. As the pressure gradient steepens and the edge current builds up, the plasma reaches the low- to intermediate-n peeling?ballooning mode stability boundary just before the ELM crash. Increasing the plasma fuelling by gas puffing makes the second stability access against high-n ballooning modes narrower until it closes completely and the ELMs change from Type I to Type III. Reducing the plasma heating has a similar effect. Increasing the safety factor at the plasma edge improves the stability against low- to intermediate-n modes allowing steeper pressure gradients to develop before an ELM crash.