Turbulence at the boundary of toroidal plasmas with open and closed magnetic flux surfaces

Abstract

The control and confinement of fusion plasmas are currently limited by a lack of understanding of the physical mechanisms behind the evolution of the turbulent transport experienced by particles and energy. In-situ investigations of plasma turbulence in fusion experiments is strongly hampered by the high temperatures and densities. Basic plasma physics devices represent an alternative solution to perform turbulence studies with the possibility of rigorously validating numerical codes. One of these experiments is TORPEX, in which a comprehensive characterization of plasma turbulence has been conducted in the presence of open helical magnetic field lines in toroidal geometry. These reproduce the main features of the scrape-off layer (SOL), which is the open flux surface region at the edge of magnetically confined fusion plasmas. The SOL has a key role in the balance of the dynamics that determine the overall plasma confinement. The first achievement of this thesis work is a technical upgrade of TORPEX that consists in the installation of a copper toroidal conductor inside the TORPEX vacuum vessel. A poloidal magnetic field is produced by a current flowing inside the conductor, introducing a rotational transform. This allows studying plasma turbulence in magnetic geometries of increasing complexity, starting with the simplest configuration of quasi-concentric flux surfaces. The initial exploration of the main plasma properties, including plasma production mechanisms and particle confinement time, is followed by a detailed spectral characterization of the measured electrostatic quasi-coherent fluctuations. Measurements of the toroidal and poloidal mode numbers reveal field-aligned modes. These present a poloidal localization indicating a clear ballooning feature that is in agreement with the results of a linear fluid code. The first experimental measurements of plasma blobs in the presence of a single-null X-point are performed. Blobs radially propagating outwards across the X-point are conditionally sampled, which allows us to track and analyze in detail the corresponding dynamics. The ExB drifts induced by the background potential gradients and the fluctuating potential dipole are both responsible for the measured blob acceleration in the X-point region. The contribution of the potential dipole is explained on the basis of an analytical model, in which the variation of the magnetic field intensity close to the X-point plays a key role. This results in a blob speed scaling that is in good agreement with the measured values.