Characterization of SPIDER beam emission profile through beam imaging

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 beam. The main goal of the system is the reconstruction of the 2-dimensional beam emission profile, in the plane x-y perpendicular to the beam propagation direction (z), which is proportional to the beam density by assuming a uniform background gas. The light emitted is mainly produced by the deexcitation of fast neutrals produced by the interaction of the beam particles with the background gas, with the emission of a photons with ?=656.28 nm (the Balmer line H?). Eight visible cameras observe the beam at a distance of 50 cm from the last grid, all around the beam: 1 linear camera and 7 cameras equipped with 2D sensors of different sizes and with different lenses, to have a spatial resolution in the order of the single beamlet dimension (centimetre), despite the limited number of point of views. To reconstruct the profile of the extracted beam, the cameras are calibrated in intensity, aligned and focused on the plane perpendicular to the beam propagation direction. Their fans of lines of sight are reconstructed and the experimental data are compared with the simulated ones. This analysis allows to characterize the beam homogeneity, deflection and divergence, which is estimated by assuming Gaussian profiles for the beamlets at different positions along the beam propagation direction, in different experimental conditions. The evolution of these quantities as a function of the main source parameters is presented.