Numerical and experimental investigation of beam transport in a DC broad-beam ion source with concave grid focusing

Abstract

This study presents a three-dimensional kinetic simulation model based on the particle-in-cell with Monte Carlo collisions (PIC/MCC) method to systematically investigate the physical behaviors of planar and concave grids during the extraction and transport processes in a DC broad-beam ion source. A closely packed 10-mesh grid configuration was employed, and an experimental testing platform was constructed to perform diagnostics via a mechanical scanning probe. The results demonstrate that the concave configuration reshapes the equipotential surfaces in the extraction region, forming an electrostatic focusing lens with a centripetal convergence effect that imparts a radially inward initial velocity component to the ions. Although the space charge force within the high-density beam core induces non-linear distortions and increases the root-mean-square (RMS) emittance, the macroscopic beam width is effectively constrained, significantly suppressing beam divergence. The experimentally measured radial current profile exhibits an approximately Gaussian distribution, which is in good agreement with the theoretical PIC distribution after applying a convolution correction to account for the spatial integration effect of the probe.