There is nowadays a large interest to employ helicon plasmas as sources of negative ions for Neutral Beam Injectors (NBIs). Helicon plasma sources offer a number of advantages compared to current Inductive Couple Plasma (ICP) negative ions source, such as high energy efficiency, high plasma density production, low electron temperature and a considerable amount of H- and D- produced through volumetric processes, without caesium. Therefore, helicon plasma sources are promising candidates to be employed as a negative ion source for future NBIs, such as in DEMO, a next generation tokamak closer to commercial devices. However, many physics and technical aspects of helicon plasmas have to be deeply investigated, before their possible integration in future NBIs. The goal of this thesis is to advance the understanding of the physics of helicon-based negative ion sources in the helicon plasma source RAID (Resonant Antenna Ion Device) at the Swiss Plasma Center of Ecole Polytéchnique Fédérale de Lausanne, Switzerland. The investigations are carried out by employing a variety of plasma diagnostics, and the results are interpreted and compared to numerical models.