Laser ablation models and experimental activities in support of the implementation of a LIBS diagnostics for the BiGyM linear plasma device for fusion-relevant applications

Laser-Induced Breakdown Spectroscopy (LIBS) is a powerful and versatile laser-based diagnostic technique widely used to analyze the chemical composition of solids, liquids, and gases. By applying optical emission spectroscopy to a laser-induced plasma, LIBS enables rapid and minimally invasive elemental and isotopic characterization of a thin superficial layer, typically without the need for sample preparation. Owing to these intrinsic features, LIBS is currently considered a promising diagnostic tool for in-situ monitoring of Plasma Facing Components (PFCs) in magnetic confinement fusion devices, such as tokamaks, as well as in Linear Plasma Devices (LPDs). Compared with other post-mortem diagnostics, LIBS enables remote measurements in harsh environments and can provide depth-resolved information, key capabilities for investigating processes such as impurity deposition and fuel retention, which are critical issues for future nuclear fusion devices. The main objective of this thesis is to develop some preparatory activities, including conceptual design, numerical modeling, and experiments, for the installation of an in-situ LIBS diagnostics dedicated to fusion-relevant plasma-material interaction studies on the LPD BiGyM, the upgraded version of GyM (CNR-ISTP, Milan). The first outcome of the thesis is the definition of the diagnostic components and a preliminary layout. Based on a literature review of existing LIBS systems and experiments, together with some simple spectral sensitivity analyses, a picosecond Nd:YAG laser and a compact, high-resolution IsoPlane 320 spectrometer design were identified as suitable instruments to perform in-situ fuel retention studies on BiGyM. In addition, a preliminary technical drawing of the optical path connecting the laser system to the linear device has been designed. A substantial contribution of this work is represented by the development of a finite element numerical model of the laser ablation process, conceived as a support tool for LIBS diagnostics. The modeling activity, implemented using COMSOL Multiphysics, initially focuses on the nanosecond pulse regime, which is currently the most widely adopted approach in fusion-related LIBS applications. The model describes laser energy deposition, heat diffusion, phase transitions, and material removal, enabling the prediction of ablation crater features (depth and diameter) as well as the temperature evolution within the computational domain. The modeling framework was successively extended to the picosecond regime, in order to progressively approach the diagnostic parameters that will be used on BiGyM. This extension enables the investigation of a different computational approach, describing the temperature evolution through separate heating dynamics for the electron and lattice subsystems. Both approaches were successfully validated by comparing predicted crater characteristics with ablation crater data obtained from dedicated laser irradiation experiments. The final part of this thesis includes an experimental activity on in-situ retention LIBS measurements performed on the PSI-2 linear device (FZJ, Jülich). This experiment provided practical insight into hydrogen isotope detection in fusion-relevant materials and enabled the extraction of retention data for tantalum, a potential but still less explored alternative to tungsten as a PFC. Overall, the results obtained in this thesis, combining preliminary diagnostic design, numerical modeling, and experimental results about laser ablation and retention measurements, provide a solid basis for the development and implementation of an in-situ LIBS diagnostic system on the BiGyM linear plasma device.