Spatial characterization of the space charge compensation of negative ion beams

The use of high current negative ion beams is fundamental to neutral beam injectors used in advanced fusion devices. After the acceleration stage, beam transport at reduced divergence is possible only thanks to space charge compensation of the beam. An investigation of this phenomenon is described, aiming to characterize, in particular, the spatial extension of the charge compensated region. A 2D particle-in-cell code was used and modified to include the very last part of the accelerator. Realistic primary beam density profiles can be also implemented, and matching to the ray tracing codes used in accelerator modeling is discussed. The influence of a repeller electrode is also described, both on the field topology in the region and on the slow particles motion. All results are discussed for the reference case of an H- beam.