Influence of electron impact ionization on the efficiency of thermoemission ion source

The electron impact ionization by accelerated thermionic electrons is included into the numerical model of thermoemission ion source. Mean free paths of atoms in the ionizer are comparable with the ionizer size at high working temperatures (3500 K). The dependence of the source efficiency on the ionizer temperature and the electron impact ionization cross-section is studied. In the case of substances having a small ionization coefficient (β ≪ 1) the contribution of the electron impact ionization may be dominant at high temperatures and the predictions based on previous surface ionization model can be changed significantly.