The three-dimensional particle orbit simulation is carried out to analyze the plasma generation in a bucket-type ion source. The characteristics of ion source plasma is analyzed when the filament current is changed. When the filament current is increased, the electron is trapped by the magnetic field made by a filament current, and makes less contribution to the plasma discharge. In this study, the filament configurations are studied in viewpoints of efficiency of discharge, where electron emission is examined by tracing electron orbits from and near the filaments. The uniformity of plasma distribution is also studied due to the filament configuration and current.©1998 American Institute of Physics.