The effect of ion trapping in the emitter sheath in ignited thermionic converters is studied. The ion trapping prevents the emitter‐sheath barrier from being higher than approximately 0.1 eV, when the current decreases in the converter. This gives a condition for the constriction of the arc.I‐Vcurves are calculated for an ignited thermionic converter with a hydrodynamic plasma theory that takes into account the effect of Coulomb scattering and volume recombination, but assumes that the electron temperature is constant in the plasma.