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Title of the thesis: Plasma flows and turbulence in gyrokinetic simulations

Thesis defender: Francis Albert Devasagayam
Opponent: Professor Minna Palmroth, Department of Physics, University of Helsinki
Custos: Professor Mathias Groth, Aalto University School of Science

I did my doctoral thesis in the field of nuclear fusion. The aim of nuclear fusion is to produce electricity without the release of green house gases. In my research, I used two gyrokinetic codes ELMFIRE and GENE to study plasma turbulence and meso-scale flows in the FT-2 tokamak.

To predict scrape-off layer physics in the FT-2 tokamak, several code changes in ELMFIRE recycle particles exiting the simulation domain in the toroidal direction closer to the poloidal limiters. Radial particle source profiles from ASTRA simulations and recycling plasma ions and electrons near the limiters agree with Langmuir probe electron density profiles within 30%. However, electron temperature profiles are predicted systematically higher by a factor of 1.5 to 2 with the respective Langmuir probe measurement, still within uncertainties.

Using GENE, the Geodesic acoustic mode (GAM) intermittency in the core of FT-2 is studied by performing a global non-linear simulation. The simulation results reveal intermittency in GAMs near the low-order rational surface (q=3), the first observation of GAM intermittency in a gyrokinetic simulation. The underlying cause is attributed to the non-linear coupling between GAMs and turbulence at this radial location. Density fluctuations also exhibit intermittency at the same location, confirming the coupling.

The impact of Krook-type source operators on GAM dynamics in GENE simulations is investigated. Global non-linear simulations show that the GAM amplitude increases with increasing Krook-type source operator values, driven by an increased GAM drive. Zonal flow amplitudes decrease with increasing operator values in the center region. Notably, GAM damping in the center region increases by 15 to 100% with increasing operator values. Despite GAM intermittency, their quiescent and active periods differ with operator values.

Key words: plasma physics, turbulence, nuclear fusion, energy, renewable energy 

Contact information: francis.albertdevasagayam@aalto.fi 

Thesis available for public display 10 days prior to the defence at .