The plasma in a plane caesium diode with a hot emitter and a cold collector was investigated experimentally with a ribbon-shaped electron beam. The ribbon beam is projected through the diode at an angle of 45 degrees to its axis and allowed to strike a fluorescent screen. Variations in the axial electric field of the diode cause the ribbon beam to be distorted. The image of the distorted beam as seen on the fluorescent screen then constitutes a plot of the axial electric field along the axis of the diode. The field plots so obtained are compared with a theory in which the collisions of the charge carriers are neglected. By means of this comparison it is possible to evaluate the neutralization parameter, the plasma density, and an average drift energy of the charge carriers. The results show that the theory correctly describes the different modes of the potential distribution and especially the transitions between modes of operation as long as the diode is free of oscillations. The stability of the different possible static potential distributions was also investigated. It was found experimentally that the system is unstable if the electron emission is less than the ion emission and the collector potential is positive.