XCEPT: extended Computation of Edge Plasma Turbulence

The edge of an inhomogenous magnetised plasma is a complex physical system, which exhibits structure formation and self-organisation processes that are linked to self-sustained turbulent, fluid-like drift motion of the quasi-two- dimensional plasma. The genesis of zonal flow structures out of a homogenous isotropic vortex field and the resulting shear-flow suppression of the initially driving turbulence is a unique example of a self-organising regulation process in a dynamical system. Morover, the process of global flow formation in the plasma edge region is thought to be the main cause of the low-to-high (L-H) confinement transition in magnetically confined toroidal plasmas for fusion research. The realisation of the resulting H-mode in the international large tokamak experiment ITER is regarded as a prerequisite for the success of this major next step in fusion energy research. But in contrast to the wide experimental experience with the H-mode, there is still a lack of any first-principle predictive theory or simulation. In the present project, we envisage appropriate extensions of the models and codes for computation of edge turbulence and flows, so as to make them suitable to access additional dynamics necessary for obtaining a transition. The proposal is directed at extending a particular plasma edge turbulence model and code ("TYR") and applying it to fusion plasma problems, such as the L-H transistion. In particular, we intend to include global profile evolution, a coupling of the closed-flux-surface steep gradient region to the scrape-off layer, and adaptation to realistic magnetic field geometry including treatment of an X-point separatrix geometry associated with a divertor. The computational results gained by the new model and code developments will be used for improvement of transport models for Integrated Tokamak Modelling and for comparison with experimental fluctuation measurements.

Hot plasmas for fusion energy research are rather chaotic, difficult to access, and expensive to produce. All the more important is the role of computer simulations as a complement to experiments with fusion plasmas. In the FWF project XCEPT ("Extended computation of edge plasma turbulence", 2006-2009) in this respect models for numerical simulation of instabilities and turbulence at the edge of fusion plasmas have been further developed, applied to recent problems of interest, and compared with experimental measurements. The numerical investigations focussed on genesis and evolution of unstable structures with various scales and lifetimes under different experimental conditions. For example, the propagation and decay of coherent vortices ("blobs") at the outer plasma edge have been computed with nonlinear simulation models, whereby turbulent structures measured with probes at the fusion experiment ASDEX Upgrade (IPP Garching, Germany) could be identified and characterised by statistical methods. A direct influence of rational magnetic surfaces (where magnetic field lines wind around the torus by an integer ratio) on turbulent flows, which has been suggested by other experimental results, could however not be confirmed within the employed models (with fixed background magnetic field and small fluctuations). Therefore, rather other self organization phenomena (like magnetic reconnection) have to be assumed as causes for the observation. Furthermore, the so far most complex and realistic computations on the cataclysmic periodic eruptions, which occur in experiments with high plasma confinement, have been realized with a high resolution electromagnetic gyrofluid model. It has thus first been shown that multiple scales are participating in the bursts of energy and particles, where even smallest vortices by their large number have a decisive role for the overall strength of these ELM ("edge localized mode") eruptions. The models and numerical computations developed during this project have thus contributed to a more profound understanding of nonlinear dynamics and structure formation at the edge of fusion plasmas, which is of relevance for the design and operation of the next generation of fusion experiments like ITER.