Uniformity Optimisation of the Negative Ion Beam Source for the ITER Neutral Beam Injector

Negative ion sources are fundamental components of high energy Neutral Beam Injectors (NBI), one of the main heating systems for fusion reactors. The ITER prototype negative ion source is SPIDER, hosted in Padua as part of the Neutral Beam Test Facility (NBTF), which also hosts MITICA, the prototype NBI. SPIDER is composed of 8 cylindrical drivers, capable of igniting the plasma through the inductive coupling with 4 radio frequency (RF) generators, each delivering up to 200kW. Given the large scale of the experiment, one of the main problems is matching the ITER’s requirement for uniformity (+/-10%). This manuscript addresses the issue of beam and source non-uniformity in negative-NBI, especially focusing on MITICA and SPIDER, with the purpose of meeting the ITER target parameters and hence being able to produce a uniform beam suitable for ITER HNB. To achieve this goal, this thesis focuses on the development and optimization of both numerical and experimental tools for studying particle transport, exploring the behavior of plasma and particle interactions, particularly focusing on the nonuniformities that arise during negative ion production and beam formation. A custom test particle Monte Carlo code has been developed to simulate particle trajectories, energy distributions, and charge states within SPIDER’s plasma source. This tool enables detailed analysis of the impact of various source parameters, such as magnetic field configuration and plasma grid bias, on particle transport and negative ion extraction. The code was validated by comparing simulated results with experimental data obtained from the SPIDER diagnostic systems, including the Retarding Field Energy Analyzer (RFEA). The thesis also investigates how precursor ions contribute to surface-based negative ion production and how non-uniformities in the plasma affect the resulting ion beams. The integration of advanced diagnostics systems like VICTOR and VERA in SPIDER provides a comprehensive approach to monitoring plasma profiles and beam uniformity. These systems enable real-time feedback, improving control over source parameters and helping to mitigate beamlet inconsistencies caused by spatial variations within the ion source. The results of this work contribute to the optimization of negative ion sources, laying the foundation for future improvements in NBI systems like MITICA. The findings provide key insights for overcoming challenges related to beam uniformity and offer valuable guidance for the design and operation of high-performance ion sources in nuclear fusion reactors. Combining diagnostic, experimental and numerical methods allows a thorough comprehension of the problem, ranging from the theoretical understanding of physical phenomena, to the application of computational techniques, to the practical aspects related to diagnostics design and data analysis.