Finalist EPFL doctorate Award 2016 – Alexandre Bovet
© 2016 Alexandre Bovet
Special distinction from the selection committee to Alexandre Bovet for his thesis "Suprathermal ion transport in turbulent magnetized plasmas". Thesis n°6527 (2015). Thesis directors: Prof. A. Fasoli, Dr I. Furno.
The vast majority of the visible matter in the universe is in the state of plasma, consisting of partially or totally ionized gas. Fast ions, which have velocities greater than the background plasma, are present in many plasmas on Earth and in space.
In fusion devices, they are generated by the fusion reactions and auxiliary heating. Controlling their transport is essential for the success of future fusion devices that could provide a clean, safe and abundant source of electric power to our society. In space, fast ions include energetic solar particles and cosmic rays and the understanding of their acceleration and transport mechanisms is still incomplete.
In this Thesis, the combination of uniquely resolved measurements and first-principle numerical simulations reveals the general non-diffusive nature of the fast ions transport.
A precise characterization of their transport regime shows that, depending on their energies, fast ions can experience either a superdiffusive or a subdiffusive transport in the same background turbulence. This phenomenon is explained by the interaction between the fast ion orbits and the turbulent structures. A difference is also identified in the intermittency of fast ions time-traces depending on the transport regime they experience.
Finally, a theoretical model extending the Brownian motion to include non-Gaussian statistics and long-range temporal correlation is developed. This model successfully describes the evolution of the particle density and provides information on the microscopic processes underlying the non-diffusive transport of fast ions.
Figure: Experimental setup showing the fast ion beam in the TORoidal Plasma EXperiment (TORPEX).