SearcharxivSearch

arXiv subjects

Nicola Offeddu

Publications and source records attributed to Nicola Offeddu.

4 recordsLinked to original sources

Tracking Blobs in the Turbulent Edge Plasma of a Tokamak Fusion Device

The analysis of turbulence in plasmas is fundamental in fusion research. Despite extensive progress in theoretical modeling in the past 15 years, we still lack a complete and consistent understanding of turbulence in magnetic confinement devices, such as tokamaks. Experimental studies are challenging due to the diverse processes that drive the high-speed dynamics of turbulent phenomena. This work presents a novel application of motion tracking to identify and track turbulent filaments in fusion plasmas, called blobs, in a high-frequency video obtained from Gas Puff Imaging diagnostics. We compare four baseline methods (RAFT, Mask R-CNN, GMA, and Flow Walk) trained on synthetic data and then test on synthetic and real-world data obtained from plasmas in the Tokamak `a Configuration Variable (TCV). The blob regime identified from an analysis of blob trajectories agrees with state-of-the-art conditional averaging methods for each of the baseline methods employed, giving confidence in the accuracy of these techniques. High entry barriers traditionally limit tokamak plasma research to a small community of researchers in the field. By making a dataset and benchmark publicly available, we hope to open the field to a broad community in science and engineering.

physics.plasm-ph

X-point and divertor filament dynamics from Gas Puff Imaging on TCV

A new Gas Puff Imaging (GPI) diagnostic has been installed on the TCV tokamak, providing two-dimensional insights into Scrape-Off-Layer (SOL) turbulence dynamics above, at and below the magnetic X-point. A detailed study in L-mode, attached, lower single-null discharges shows that statistical properties have little poloidal variations, while vast differences are present in the 2D behaviour of intermittent filaments. Strongly elongated filaments, just above the X-point and in the divertor far-SOL, show a good consistency in shape and dynamics with field-line tracing from filaments at the outboard midplane, highlighting their connection. In the near-SOL of the outer divertor leg, shortlived, high frequency and more circular (diameter $\sim$15 sound Larmour radii) filaments are observed. These divertor-localised filaments appear born radially at the position of maximum density and display a radially outward motion with velocity $\approx$400\,m/s that is comparable to radial velocities of upstream-connected filaments. Conversely, in these discharges ($B\times\nabla B$ pointing away from the divertor), these divertor filaments' poloidal velocities differ strongly from those of upstream-connected filaments. The importance of divertor-localised filaments upon radial transport and profile broadening is explored using filament statistics and in-situ kinetic profile measurements along the divertor leg. This provides evidence that these filaments contribute significantly to electron density profile broadening in the divertor.

physics.plasm-ph

Magnetic Reconnection in Turbulent Diluted Plasmas

We study magnetic reconnection events in a turbulent plasma within the two-fluid theory. By identifying the diffusive regions, we measure the reconnection rates as function of the conductivity and current sheet thickness. We have found that the reconnection rate scales as the squared of the inverse of the current sheet's thickness and is independent of the aspect ratio of the diffusive region, in contrast to other analytical, e.g. the Sweet-Parker and Petscheck, and numerical models. Furthermore, while the reconnection rates are also proportional to the square inverse of the conductivity, the aspect ratios of the diffusive regions, which exhibit values in the range of $0.1-0.9$, are not correlated to the latter. Our findings suggest a new expression for the magnetic reconnection rate, which, after experimental verification, can provide a further understanding of the magnetic reconnection process.

physics.plasm-ph

Hall Effect in Diluted Plasmas

We study the Hall effect in diluted plasmas within the two-fluids theory. Composed by two distinct species with opposite charge, such as electrons and ions in fully ionised hydrogen, the plasma is driven by an electric field through a channel in the presence of a transversal magnetic field. As a consequence, a separation of charge is induced producing an electric potential difference. We have found a general relation for the Hall voltage as function of the mass and viscosity ratios, which converges to the usual expression in the limit of solid matter, i.e. when ions are much more massive than electrons. All the simulations have been performed using a three-dimensional Lattice-Boltzmann model, which has been also validated for some relevant applications. Finally, we discuss the importance of our findings in the light of recent developments in plasma physics, in particular in magnetic reconnection.

physics.plasm-ph