First results of SPIDER beam characterization through the visible tomography

SPIDER, the full-size prototype of the negative ion source for future ITER Heating and Current Drive, is equipped with a tomographic diagnostic to characterize the negative hydrogen (and deuterium) beam. The main goal of the tomographic system is the reconstruction of the beam emission pattern, which is proportional to the beam density, by assuming a uniform background gas, through the measurement of the beam emitted light along a sufficiently large number of lines-of-sight (LoSs), to study the beam homogeneity and divergence. Currently, 11 2D visible cameras observe the beam on a plane at a distance of 400 mm from the last grid of the accelerator, all around the beam; sensors of different sizes with different lenses are used, to maximize the resolution despite the limited number of points of view. The cameras are calibrated both in intensity and spatially and their fans of LoSs have been reconstructed. Once calibrated, the experimental data are compared with the simulated ones, simultaneously for all the cameras in order to have a complete profile of the extracted beam. Simultaneous Algebraic Reconstruction Technique (SART) algorithm as inversion technique is used to reconstruct for the first time the 2D beam emissivity profile, and it is applied to study the spatial uniformity of the beam as a function of the filter field and the extraction voltage.