Abstract <p>A numerical study of a proton beam source with an ion-optical system of the acceleration-deceleration type is carried out. The main attention is paid to the behavior of the secondary plasma entering the source and affecting the formation of the proton beam. To minimize the secondary plasma flow in the system, an additional electrode with a small negative potential was used for the first time, reflecting electrons and collecting ions of the secondary plasma. The influence of the secondary plasma and the source parameters on the angular and spatial characteristics of the formed proton beam with an energy of 2–5 keV and the efficiency of ion collection by the additional electrode were studied. The POISSON-2 numerical code was used to determine the shape of the secondary plasma boundary in the slit cell. The angular divergence of the beam proton velocities does not exceed 0.04–0.06 rad. It is shown that the third electrode located before the cell exit absorbs secondary plasma ions, reducing the secondary ion current to the accelerating electrode with a potential of 13–15 kV by ∼50–90%.</p>