MHD Stability Analysis of Small ELM Regimes in JET

We have analysed the edge stability of JET discharges with small ELMs using the high resolution Thomson scattering system for accurate edge profiles in the equilibrium reconstruction. For the reference plasmas with large Type I ELMs we confirm the results from earlier analyses that the edge stability is limited by intermediate-n peeling-ballooning modes with a relatively large radial extent. The double null configuration needed to replace Type I ELMs by smaller Type II ELMs greatly increases the stability against these modes while the stability against n = ∞ ballooning modes is not affected. When this is combined with high collisionality (which is the other requirement for Type II ELMs), we find that the plasma can not reach the Type I ELM triggering peeling-ballooning mode stability boundary before it is destabilised by high-n ballooning modes resulting in more benign ELMs. The ELM mitigation by magnetic perturbation causes the edge stability to be limited by pure peeling modes with a narrow radial extent. This explains the smaller ELM size and also why the ELMs are not fully suppressed. The transition from Type I ELMs to Type III ELMs by increasing the edge radiation fully stabilises the edge plasma against ideal MHD modes. Therefore, the Type III ELMs are due to be triggered by some other mechanism than an ideal MHD instability.
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