(TH/P82) MHD Stability in Xpoint Geometry: Simulation of ELMs
G.T.A. Huysmans^{1)}
^{1)} Association EuratomCEA, CEA/DSM/DRFC, Centre de Cadarache, 13108 St. Paul lez Durance, France
Abstract. Edge localised modes (ELMs) associated with the edge transport barrier in Hmode 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 nonlinear evolution of the ballooning and kink modes in a full xpoint/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 xpoint 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 xpoint, both for ideal and resistive peeling modes. A resistive MHD instability is found to remain unstable in the presence of the xpoint. 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 xpoint where the mode shows a phase inversion as a function of radius. The nonlinear evolution of this socalled peelingtearing mode shows a saturation of the mode amplitude and a local flattening of the density profile just inside the xpoint. This could be consistent with the relatively longlived lown precursors (`Outer Modes’) to the giant ELMs in JET hotion Hmodes.
The ELM crash is simulated by evolving a mediumn ballooning mode starting from an equilibrium which is linearly unstable to the ideal MHD ballooning mode. The nonlinear 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 ‘bloblike’ structure which are sheared of the main plasma by the poloidal flow.
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