Kilovolt extraction of dense electron beams from synthesized-plasma cathodes

Electron beams with a continuous current density of 800 A/cm²were extracted from a synthesized-plasma hollow cathode. The beams were accelerated up to potentials of 1.2 kV, through the potential of maximum ionization probability at about 16 volts. There was no accompanying oscillation or gas breakdown. This high voltage operation was made possible by collimating the cesium vapor entering the hollow cathode. The collimation, which was accomplished by use of a narrow slit at the "top" of the cathode, caused the vapor to flow along the cathode wall surface and away from the exit aperture. This minimized escape of neutrals into the high voltage region. The cathode was tested for 200 hours without any signs of degradation. The cathode lifetime is predicted to be several thousand hours at the highest cathode temperature (2425°K) employed in these studies. The extracted 0.030-in diameter beam of 0.250-in length had an electron temperature of less than twice cathode temperature. Longer beams, such as would be used in actual microwave tubes, would require magnetic confinement.