Excessive ion trapping by electron beams in microwave tubes

The presence of ions trapped by the space-charge depression of electron beams causes instabilities which exhibit themselves as noise phenomena in microwave tubes. It is difficult to experimentally determine the trapped-ion density. A new technique has been successfully developed by the authors which uses a second thin probing electron beam. The residual gas present under normal conditions is replaced by Ar at a pressure of 5.10^-7torr because the various ionization threshold potentials of Ar are suitable for this experiment. The original beam energy is set such that it produces then only Ar⁺. The probing beam energy can generate Ar⁺⁺from Ar⁺, but not Ar⁺⁺from Ar. Finally, the Ar⁺⁺ions are extracted by a lens and the resulting Ar⁺⁺current is measured, which can be used to find the density of Ar⁺in the beam. It has been found in this way that extremely high ion densities exist on the primary-beam axis. The equivalent positive potential depression of these ions is about 100 times higher than the negative potential depression of the primary electron beam, and charge neutrality is only possible by trapping slow secondary electrons created near the axis by the ionizing process. The high ion density is localised on the beam axis and does not interfere directly with the microwave interaction, except by some periodic ion discharging mechanism when strong low frequency instabilities occur which interfere seriously with such applications as radar and television.