Suprathermal ions, i.e. ions with energies exceeding those of the bulk plasma, arise in diverse natural and laboratory plasmas ranging from the solar wind to magnetic confinement fusion (MCF) devices. In tokamaks, they are generated by external heating systems and fusion reactions, and a thorough characterization of their dynamics is crucial for safe operation. In tokamaks, the core plasma is separated from the walls by the last closed flux surface (LCFS), which exhibits a null point (X-point) in the poloidal plane. Understanding plasmas in the X-point region and the transport of fast ions in such a magnetic geometry therefore requires fundamental investigations. Basic plasma experiments, such as the TORoidal Plasma EXperiment (TORPEX), offer a controlled environment for these studies, where low plasma temperatures and densities enable in situ diagnostics and precise plasma parameter control.
This thesis investigates fast ion dynamics and transport in a background plasma featuring an X-point magnetic geometry in TORPEX, and compares experimental results with simulations. The background plasma is characterized in terms of its time-averaged properties, instabilities, and turbulent transport. In particular, varying the magnetic shear is shown to affect the mean plasma density, mode structure, and cross-field turbulent transport, with an abrupt transition observed between two distinct shear values.
Fast ions are injected at two locations in the poloidal plane, within the dominant propagating mode and below the X-point, and the effects of injection position, fast ion energy, and background plasma scenario on their dynamics are examined. It is found that fast ion motion is predominantly governed by the background E x B velocity field, while the fast ion beam spreading is partly attributed to electric field fluctuations of the background plasma. These experimental observations are validated against experimental-based particle tracing simulations. The transport of suprathermal ions is shown to be predominantly non-diffusive and to depend not only on ion energy, as it was previously observed in a simple magnetic configuration (SMT), but also to vary significantly with injection position and background plasma conditions.
Finally, to further extend the diagnostic capabilities of TORPEX, a new laser-induced fluorescence (LIF) diagnostic is presented, enabling previously inaccessible measurements of ion temperature and mean flow velocity in TORPEX. Preliminary results in argon plasmas indicate that ions can be heated by broadband instabilities under specific plasma conditions.
