  The three-dimensional NAM-ECRIS model is applied for studying the metal ion
production in the DECRIS-PM Electron Cyclotron Resonance Ion Source.
Experimentally measured extracted ion currents are accurately reproduced with
the model. Parameters of the injection of metal vapors into the source are
optimized. It is found that the axial injection of the highly directional
fluxes allows increasing the extracted ion currents of the highly charged
calcium ions by factor of 1.5. The reason for the gain in the currents is
formation of internal barrier for the ions inside the plasma, which increase
the ion extraction and production efficiency. Benefits of injecting the
singly-charged calcium ions instead of atoms are discussed.
