Visible cameras as a non-invasive diagnostic to study negative ion beam properties.

Beam tomography is a non-invasive diagnostic that allows us to reconstruct the beam emission profile by measuring the light emitted by the beam particles interacting with the background gas, along an elevated number of lines of sight, which is related to the beam density by assuming a uniform background gas. In the framework of the heating and current drive of future nuclear fusion reactors, negative ion beams of hydrogen and deuterium are required for neutral beam injectors (NBIs) due to their elevated neutralization efficiency at high energy (in the MeV range). Beside the beam energy, beam divergence and homogeneity are two critical aspects in the design of future NBIs. In this paper, the characterization of the negative ion beam of the negative ion source NIO1 (a small-sized radio-frequency driven negative ion source, with 130 mA of total extracted H current and 60 kV of maximum acceleration) using the tomographic system composed of two visible cameras is presented. The Simultaneous Algebraic Reconstruction Technique (SART) is used as an inversion technique to reconstruct the 3 × 3 matrix of the extracted beamlets, and the beam divergence and homogeneity are studied. The results are compared with the measurements of the other diagnostics and correlated with the source physics. The suitability of visible cameras as a diagnostics system for the characterization of the NIO1 negative ion beam is a small-scale experimental demonstration of the possibility to reconstruct more complicated multi-beamlet profiles, resulting in a powerful diagnostic for large NBIs.