Influence of plasma formation and expansion on beam extraction in large negative ion sources

Several countries worlwide are collaborating for building ITER, an international experiment with the goal of showcasing the feasibility of generating energy through controlled thermonuclear fusion. To achieve the necessary conditions for producing energy by fusion reactions, it is essential to confine an extremely hot plasma for a sufficiently long period. To this purpose additional heating systems are envisaged in ITER, including Neutral Beam Injection (NBI): this technique allows to deliver additional heating power via a high-energy neutral beam, created by neutralizing a precursor negative ion beam. In the ITER case, the NBI requirements in terms of beam current, energy, focussing, and uniformity are very demanding, and they have never been achieved so far. In addition, the ITER NBI will employ radiofrequency (RF) driven ion sources to produce and extract negative ions, differently from the negative NBI systems of other fusion experiments which instead are based on the more established arc-filament discharge method. The R&D activities on the ITER NBI are mainly being carried out in the Neutral Beam Test Facility (NBTF), hosted by Consorzio RFX in Padua (Italy). The goal of the NBTF is to build and successfully operate the full scale prototype of the ITER NBI, named MITICA, which is currently under construction. The NBTF also includes the SPIDER experiment, namely the full size prototype of the ITER NBI ion source equipped with a 100 kV accelerator. Recent operation of the SPIDER experiment highlighted the need for a more thorough characterisation of the plasma properties in the ion source, as the latter were found to directly influence the extracted negative ion beam features. In this framework, the aim of this thesis work is to investigate the plasma behaviour in large negative ion sources, possibly defining their correlation with the extracted beam current density uniformity and the beam divergence. To this purpose, both experimental tools such as Langmuir probes and numerical tools have been exploited. In particular, a Particle-In-Cell simulation code was improved and used to study the main plasma mechanisms in different regions of the ion source.