SPIDER beam homogeneity characterization through visible cameras

High energy beam of negative H and D ions are needed to heat and sustain the plasma of future nuclear fusion reactors, in particular in the experimental reactor ITER. Beside the beam energy, low divergence (<7 mrad) and high homogeneity (greater than 90%) are required, as well as a beam current of 285 A/m^2 in deuterium and 350 A/m^2 in hydrogen. SPIDER, the full-size prototype of the ITER negative ion source, is equipped with a tomographic system composed of 15 visible cameras, which observe the light produced as a result of the interactions of the beam with the background gas at the end of 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 produce light in the visible spectrum, due to the production of excited neutrals and to the ionization of the background particles. This light allows studying the beam properties, since it is proportional to the beam density itself. In SPIDER, magnetic and electric fields are used to optimize the beam current, reducing the electron temperature close to the extraction region. These fields impact on the beam homogeneity and shape, and in this paper visible tomography is used to characterize beam properties with and without cesium, using both the 1D beam profiles and the 2D tomographic reconstructions. These results are compared with other diagnostics, such as Beam Emission Spectroscopy, the STRIKE calorimeter and the electrical measurements, resulting in substantial agreement.