SPIDER Beam Homogeneity Characterization Through Visible Cameras

High energy beams of negative hydrogen and deuterium ions are needed to heat and sustain the plasma of future nuclear fusion reactors, in particular, in the experimental reactor International Thermonuclear Experimental Reactor (ITER). Besides the beam energy, low divergence ( $<$ 7 mrad) and homogeneity (greater than 90%) are required, as well as a beam current of 285 $\mathbf{A/}\mathbf{m}<^>{\mathbf{2}}$ in deuterium and 350 $\mathbf{A/}\mathbf{m}<^>{\mathbf{2}}$ in hydrogen. Source for the production of ions of deuterium extracted from a radio-frequency plasma (SPIDER), the full-size prototype of the ITER negative ion source, is equipped with a tomographic system consisting of 15 visible cameras, which observe the light produced by the interactions of the beam particles with the background gas after the accelerator, all around the beam itself, allowing a complete characterization of the beam shape and intensity. In fact, when the beam particles propagate in the background gas, they emit light in the visible range, due to the production of excited neutrals and ionization of the background particles. This light allows studying the beam properties since it is proportional to the beam current density itself. In the SPIDER ion source, magnetic and electric fields are used to optimize the beam current density, by reducing the electron temperature and density close to the extraction region. Also, cesium is evaporated in the plasma as a catalyst of negative ion production. In this work, the impact of these fields and of the cesium presence on beam properties will be discussed by means of visible tomography, using both the 1-D beam profiles and the 2-D tomographic reconstructions.