The frequency of the oscillations observed in ion‐neutralized electron streams such as that of an ion‐trapping gun or of a tetrode has been shown to agree well with the Langmuir‐Tonks theory for plasma ion oscillations. The existence of a dipole mode of oscillation in addition to the longitudinal mode has been observed for a plasma column. A mechanism is proposed for the excitation of these self‐sustained oscillations by which the secondary electron current from electrodes bounding the plasma is modulated by the vibration of the ions in such a way as to deliver energy to the ac field associated with the ion oscillations. Experimental observations verify this picture as far as the source of energy is concerned.