(TH/P8-2) MHD Stability in X-point Geometry: Simulation of ELMs

G.T.A. Huysmans1)
 
1) Association Euratom-CEA, CEA/DSM/DRFC, Centre de Cadarache, 13108 St. Paul lez Durance, France

Abstract.  Edge localised modes (ELMs) associated with the edge transport barrier in H-mode plasmas remain an important issue for ITER. It is generally accepted that the onset of an ELM is caused by MHD instabilities, notably ballooning modes driven by the edge pressure gradient and the external kink(peeling) modes driven by the bootstrap current. In order to study the non-linear evolution of the ballooning and kink modes in a full x-point/separatrix geometry, the code named JOREK is being developed. The current version solves the reduced MHD equations in toroidal geometry using either 3D finite elements or 2D finite elements with Fourier harmonics in the toroidal direction. The finite elements cover both the open and closed field lines and are aligned to the equilibrium flux surfaces. The JOREK code has been used to study the influence of the x-point on the linear stability of external kink(peeling) modes driven by an edge current gradient. The traditional peeling modes are found to be strongly stabilised by the presence of the x-point, both for ideal and resistive peeling modes. A resistive MHD instability is found to remain unstable in the presence of the x-point. This instability is much less sensitive to the specific value of q close to the boundary. Its mode structure is very similar to the conventional peeling mode except close to the x-point where the mode shows a phase inversion as a function of radius. The non-linear evolution of this so-called peeling-tearing mode shows a saturation of the mode amplitude and a local flattening of the density profile just inside the x-point. This could be consistent with the relatively long-lived low-n precursors (`Outer Modes’) to the giant ELMs in JET hot-ion H-modes. The ELM crash is simulated by evolving a medium-n ballooning mode starting from an equilibrium which is linearly unstable to the ideal MHD ballooning mode. The non-linear simulations show the unstable ballooning mode to lead to a small amplitude ballooning perturbation of the flux surfaces. The temperature perturbation follows the magnetic perturbation due to the large parallel transport. The density, on the contrary, is strongly perturbed by the ballooning mode flow pattern. This leads to expulsion of high density ‘blob-like’ structure which are sheared of the main plasma by the poloidal flow.

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