Beam physics via tomographic diagnostics

The goal of this thesis work is the study of the beam physics of the negative ion beam for ITER HNB. A new diagnostics is installed on SPIDER, the full-size prototype of ITER negative ion source: the visible tomography. It is composed by a set of visible cameras which measure the light emitted by the beam particles when they interact with the background gas. An algorithm to reconstruct through tomographic inversion the two-dimensional pattern of the beam emission is developed, and the reconstructed profiles are used to study the homogeneity of the beam current, also with the support of a model to directly correlate the beamlet emission with the beamlet current density. Thanks to the masking of most of the apertures composing SPIDER multi-beamlets negative ion beam, the single-beamlet divergence is estimated through the Gaussian fit of the 1D beam profiles. \\ The results obtained by this new technique are used to investigate the beam features as a function of the main source and accelerator parameters, integrating the information provided by all the other diagnostics available. A strong correlation between the beam properties and the plasma features is found, thus a deep investigation of the source plasma is carried out. The beam homogeneity depends on the uniformity of both electrons and negative ions at the extraction region, in order to obtain identical beamlet optics at all the apertures and to avoid localised heating of the extraction grid due to the co-extracted electrons. The estimation of the single-beamlet current density is exploited to better interpret the spectroscopic measurements both close to the grid system (together with the electrostatic probes data), and inside the drivers. This experience is fundamental for the future operation at full performances, when the characterization of the single beamlet will be more challenging. The various operational regimes explored, both with and without caesium evaporation, are investigated to improve the understanding of the physics behind the generation and extraction of a large negative ion beams, when the principal source and accelerator control parameters are varied. This is the first comprehensive analysis of the experimental data measured during the experimental campaign of SPIDER experiment, and the performances achieved in term of beam divergence and homogeneity, as well as beamlet current density and co-extracted electrons currents are presented.