The research activity done during the three years Joint Doctorate in Fusion Science and Engineering was focused on negative ion sources and dedicated to numerical investigations of beam optics, experimental measurements and development of improvements for experimental devices. In particular, I carried out investigations on plasma source behavior and beam extraction under different operational conditions, paying special attention to beam optics improvement and co-extracted electron suppression efficiency, as well as analysing and developing solutions by means of numerical codes such as SLACCAD and OPERA, supported by dedicated post-processing by MATLAB scripts. I investigated the beam properties for three negative ion beam sources: SPIDER and NIO1 at Consorzio RFX (Padova, Italy) and NITS at National Institute for Quantum and Radiological Science and Technology (QST, Naka, Japan). SPIDER is the full-scale prototype of the negative ion source for ITER negative ion beam injectors (NBIs) and is in the advanced construction and assembly phase, so no further design improvements could have been done. SPIDER is provided with several peculiar design solutions: I have numerically assessed the efficiency of such solutions related to beam optics with 40 beamlets (half SPIDER beamlet group). I participated in the joint experiments of Consorzio RFX and QST in the NITS facility. The aim of this collaboration was to prove experimentally, for the first time, the effectiveness of a specific magnetic field configuration adopted for ITER NBIs requiring the use of the so-called ADCM magnets to correct the residual magnetic deflection of the beamlets induced by the (CESM) magnets devoted to deflect the co-extracted electrons. In particular my contribution was to design the new extraction grid and the new magnets (both CESM and ADCM) to be installed on the source. I also cooperated to the data analysis. Lastly, for the NIO1 source, I participated in the experimental sessions since the beginning, and I gave my contribution to the spectroscopic characterization of the source and to beam optics characterization. These experimental observations suggested some upgrades needed for optics improvement. Hence, I moved in this direction designing a new extraction grid and new magnet configurations. Furthermore, the successful experimental evidences at NITS have made us confident to introduce ADCM also in NIO1. This design work was performed in strict collaboration with Laboratori Nazionali di Legnaro (LNL, Legnaro, Italy). Unfortunately, these upgrades will be not installed on the source before spring 2017 and thus no experimental evidences of beam optics improvements are available yet. Concerning the thesis manuscript, it is constituted by five chapters, followed by an additional summary. Here is a brief summary of the chapter contents. In the first chapter I give an introduction on Fusion and ITER, highlighting the need for the use of NBIs in future fusion devices and the general issues related to MITICA, which is the full size prototype of ITER NBI. In the second chapter there is a general description of the theoretical considerations related to the physics of plasma in the source, beam extraction and optics, and main components constituting the ion sources. The final part of the chapter is dedicated to the presentation of the numerical tools available at Consorzio RFX: SLACCAD, EAMCC, COMSOL and OPERA. Concerning the latter, since it is the most important numerical tool I used during my PhD, I dwell more on the possibilities it can offer and I provide also a sensitivity study in order to increase the calculation precision. While the first two chapters have essentially introductory purposes, the last three present the actual activities I performed on the three ion sources, SPIDER, NITS and NIO1, as briefly described so far. In particular each of the three chapters is focused on the activity done on just one machine. Lastly, two appendixes supplement this thesis concerning further activities, which are loosely related to the rest. These are the development of an Excel GUI, in Visual Basic language, to simplify the interaction with NIO1 database (Appendix A) and investigations on the energy recovery system efficiency that NIO1 will feature in the future, an activity carried out in collaboration with CNR institute of Bari (Appendix B).