Meniscus and beam halo formation in a tandem-type negative ion source with surface production

A meniscus of plasma-beam boundary in H ^- ion sources largely affects the extracted H ^- ion beam optics. Although it is hypothesized that the shape of the meniscus is one of the main reasons for the beam halo observed in experiments, a physical mechanism of the beam halo formation is not yet fully understood. In this letter, it is first shown by the 2D particle in cell simulation that the H ^- ions extracted from the periphery of the meniscus cause a beam halo since the surface produced H ^- ions penetrate into the bulk plasma, and, thus, the resultant meniscus has a relatively large curvature.