The duoplasmatron source was specially designed as a component of the hydrogen and deuterium negative polarized ion source, of Lamb–Shift type, for equipping the FN–Tandem Accelerator in Bucharest. The source can operate long‐term at an arc power of 2.5 kW with an intensity of the arc current of 10–11 A. The duoplasmatron source anode is of a special construction in order to allow a strong cooling and a quick adaptation of some cups for the plasma expansion. Three variants of the extraction ion‐optics configuration have been analyzed by computer and then experimentally carried out and tested. All the extraction ion‐optics configurations employ plasma expansion cups and the process of deacceleration of the beam down to energies of 500–1000 eV is taking place inside of a magnetic lens. The proton and deuterium beam intensity obtained beyond the magnetic lens may exceed 10 mA, function of the discharge conditions in the arc and the ion‐optics configuration employed. The ion composition of the beam (H1, H2, H3) function of the discharge parameters in the arc, has also been analyzed and the results showed that the percentage of the atomic ions is higher than 70%. ©1996 American Institute of Physics.