Analysis of negative ion velocity distribution at the extraction holes in a negative ion source

Negative H- ion beams produced by a tandem-type H- ion source have good beam optics with a typical beam divergence angle of less than 5 mrad. However, the physical reason has not yet been made clear. To clarify the mechanism, we investigate a velocity distribution of negative ions at the extraction holes by a three-dimensional (3D) Monte Carlo transport code. In this code, surface-produced H- ion trajectories are calculated by numerically solving the 3D equation of motion for H- ions. The energy of the H- ions is relaxed through Coulomb collision with the background plasma (H+ ions), which has a low temperature of similar to1 eV in the tandem-type H- ion source. Therefore, energy relaxation by Coulomb collision plays an important role in good beam optics. (C) 2002 American Institute of Physics.