Tackling plasma instabilities for better fusion performance
Paul Mulholland defended his PhD thesis at the Department of Applied Physics and Science Education on October 15.
Paul Mulholland has investigated how high plasma pressure affects instabilities and turbulence in nuclear fusion devices with three-dimensional geometry. His research provides new insights into plasma behavior in stellarators and introduces tools to help optimize reactor designs, bringing us closer to efficient, clean fusion energy.
Why high pressure matters for fusion
Nuclear fusion promises a sustainable energy source that produces no greenhouse gases, relies on abundant fuel, and offers exceptional efficiency. To achieve fusion on Earth, plasma must be heated to over 100 million degrees Kelvin and confined using strong magnetic fields, a method known as magnetic confinement fusion (MCF).
However, high-pressure conditions, essential for fusion, also create steep gradients that drive plasma instabilities. These instabilities can stir the plasma into turbulence, causing heat and particles to escape from the core and reducing reactor performance. Understanding and mitigating these effects is crucial for future fusion power plants.
Stellarators: a geometry with potential
Mulholland focused on stellarators, fusion devices with a twisted toroidal shape that offer many degrees of freedom for geometric optimization. Unlike tokamaks, stellarators can be strategically designed to minimize instabilities and turbulence. His work examined how high-pressure scenarios influence plasma behavior in these complex geometries.
Using advanced gyrokinetic simulations on the Wendelstein 7-X stellarator, Mulholland discovered a new type of instability: the sub-threshold kinetic ballooning mode (stKBM). This instability can appear at much lower pressure levels than previously expected and may significantly impact turbulent transport.
New theory and tools for optimization
To explain and address this phenomenon, Mulholland developed new analytical theory and a simplified computational model called Key. This tool reproduces results from high-fidelity simulations at a fraction of the cost and time, making it a promising candidate for guiding reactor design and turbulence optimization.
His studies also explored how magnetic shear and curvature influence instability thresholds and turbulence levels, revealing strategies to stabilize harmful modes while maintaining favorable plasma conditions.
Toward efficient fusion reactors
By uncovering a previously unknown instability and creating tools to predict and mitigate its effects, Mulholland鈥檚 research opens new pathways for optimizing stellarator designs. These advances bring us closer to realizing fusion energy as a practical, clean power source for the future.
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Supervisors
Roger Jaspers and Josefine Proll