Ideal interchange instability near the separatrix in the SOL and its relation to ELMs

Abstract

The linear stability of the `scrape-off-layer' (SOL) with respect to interchange-type modes is studied using the reduced MHD model and applying a ballooning approximation to the perturbations. `Line-tying' boundary conditions are used at the target plates. Employing a metric determined by the magnetic field geometry, the influence of the X-point on stability is assessed. The effect of relevant parameters like X-point height, magnetic shear and plasma shaping is qualitatively determined. It is demonstrated that the calculations for experimental configurations and those based on an analytical equilibrium model yield good qualitative agreement. It is shown that the SOL plasma just outside the separatrix can become unstable more easily (i.e. for lower pressure gradients) than the plasma just inside the separatrix where the magnetic well is stabilizing. This finding can be important for explaining the occurrence of giant ELMs. The interchange instability in the SOL may act as a precursor and is mainly localized near the X-point, but may also have a strong signature just above the outer midplane. The scaling of the ELM repetition time with heating power and current is derived.