ELMs as Ideal Interchange Instabilities near the Separatrix
In this paper the linear stability of the Scrape-off-Layer (SOL) with respect to interchange-type modes is studied using the reduced magnetohydrodynamic (MHD) model and applying a ballooning approximation to the perturbations. 'Line-typing' boundary conditions are used at the target plates. The influence of the X-point on stability is also assessed. The effect of relevant parameters like X-point height, magnetic shear, and plasma shaping is qualitively 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 than the plasma just inside the separatrix where the magnetic well is stabilizing. This finding is used to propose a model for the occurrence of giant edge localized modes (ELMs). The model provides a natural explanation for the occurrence of three different time scales in a giant ELM, and an estimate for the ELM repetition time is given. The scaling of the repetition time with power and current is shown to agree with experimental results.
Bibliographic Reference: Article: JET-P(96)17 EN (1996) 33pp.
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon, OX14 3EA (GB)
Record Number: 199610847 / Last updated on: 1996-08-16
Original language: en
Available languages: en