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A theoretical study is presented on a type of Penning trap configuration referred to as an electron plasma ion trap/source. Ions in the configuration are confined within a three-dimensional electric potential well, which is produced by a combination of the electric field generated by the trap electrodes and the electric field generated by a trapped electron plasma. The ion density is not limited by the Brillouin ion density limit. Instead, the ion charge density must be smaller than the electron charge density. Various mechanisms that may limit the electron charge density are identified. Example calculations are used to find that the most restrictive limit on the electron charge density is likely to be the voltage difference that must be applied to trap the electron plasma parallel to a magnetic field. For confinement of low-charge-state ions, the ion temperature must be smaller than the electron temperature. Relatively long ion confinement times are found to be possible, however, because the equilibration of the ion temperature and the electron temperature is a slow collisional process due to the disparate masses involved. The ions can be easily extracted before the ion temperature reaches a value such that ion loss to the electrode walls becomes a significant source of impurities. Thus, since ion–wall interactions can be minimal, high purity ion plasmas may be generated. A self-consistent finite-differences computation is used to predict a possible plasma equilibrium. © 2003 American Institute of Physics.