Infoscience

Thesis

Basic Investigation of Turbulent Structures and Blobs of Relevance for Magnetic Fusion Plasmas

Similarly to neutral fluids, plasmas often exhibit turbulent behavior. Turbulence in plasmas is usually more complex than in neutral fluids due to long range interactions via electric and magnetic fields, and kinetic effects. It gives rise to many interesting phenomena such as self-generated magnetic fields (dynamos), zonal-flows, transport barriers, or particle pinches. Plasma turbulence plays a crucial role for the success of nuclear fusion as a potentially clean, safe, and long-term source for electric power production. Turbulent processes in the edge and scrape-off layer (SOL) of magnetic fusion plasmas determine, to a large extent, the overall confinement properties. They also influence the life time of plasma facing components, impurity production and influx, main chamber recycling, tritium retention, and helium ash removal. Edge turbulence is often dominated by blobs or filaments, magnetic-field-aligned plasma structures observed in the edge of virtually all magnetized plasmas. This thesis investigates basic aspects of edge turbulence and blobs in simple magnetized toroidal TORPEX plasmas. TORPEX includes important ingredients of SOL physics, such as pressure gradients, "∇B" and curvature of the magnetic field, together with open field lines. A relatively simple magnetic geometry, full diagnostics access and the possibility of controlled parameter scans allow isolating and studying instabilities and turbulence effects that occur in more complicated forms in fusion and astrophysical plasmas. Using a number of optimized probe diagnostic methods, the mechanisms for the generation of blobs from ideal interchange waves and for their subsequent propagation are elucidated. A blob velocity scaling law is introduced that takes into account several damping effects of blob cross-field velocity. This scaling law is in good agreement both with blob simulations and experiments on TORPEX. Studies on blob parallel dynamics shed light on blob induced parallel currents and the transport of parallel momentum. Based on this understanding of blob motion, several tools to influence blobs and turbulence as a whole are developed. A methodology for plasma turbulence code validation is established. Using a large set of observables, the agreement between experiments and both 2D and global 3D two-fluid simulations is quantified.

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