Abstract: A method and apparatus for preventing oscillations in high-current electron guns. Spurious oscillations frequently occur as a result of interaction of the electron stream with the fields of resonant modes of the gun structure. The resonant impedances of the modes are lowered by damping with lossy dielectric or resistive materials which are suited to the high temperature and vacuum environment of electron guns. The lossy materials are located in places shielded from high electric fields applied to the gun. Lossy dielectric materials which are D.C. insulators may be used as insulating supports for gun electrodes.