Probing into space charge interactions of negative ion beams through imaging diagnostics

Space charge interactions are crucial in multi-beam group ion sources like ITER-DNB, HNB, and SPIDER. Studying the space charge interactions in small-sized ion beam sources like ROBIN will help the operations of full-size ITER-NBI sources. ROBIN has a beamline of ∼ 3200 mm (axial distance from the grounded grid (GG)) with a thermocouple-based beam interceptor (at 2700 mm) and a visible range CCD camera (at 1900 mm) to diagnose the beam. Two models were developed to study the beam transmission in ROBIN transport vessels. The beamlet-sum model tracks 146 Gaussian beamlets (one beamlet per aperture) emerging from the grounded grid throughout the beamline. Another model (Beam group model) uses the analytically reduced expression for beam groups. These models do not incorporate the space charge (SC) interactions, making their predictions close to the geometrical calculations. The geometrical calculations and modeling predictions suggest vertical separation (Δ μ y ) of ∼ 45–50 mm between beam groups at 1900 mm (location of CCD camera). The experimentally determined Δ μ y ∼ 200 mm indicates the presence of interaction between the beam groups. Furthermore, the influence of accelerator parameters on Δ μ y strongly suggests the involvement of SC interaction within the beam groups. A study of the beam group interactions and their dependence on the accelerator is presented.