The cathode fall region of the glow discharge contains fast electrons and fast ions which are accelerated in the electric field between the cathode and a plasma at near anode potential. When operated at low pressure and high voltage (i.e. on the left-hand branch of the Paschen breakdown curve), the fast electrons tend to form a mono-energetic group, although the ions, owing to relatively large cross-sections for interaction with the gas, have their energies spread over a wide range and also give rise to fast neutrals by charge exchange. Both types of particle can be efficiently brought out of the discharge through orifices placed in the electrodes. Thin or broad beams or sheets of particles, converging or diverging, can be designed over a wide range of current and voltage. At lower voltages, electron beams may be applied to a target placed within the discharge. As the voltage is increased, extracted electron beams become penetrating and can be manipulated magnetically outside the discharge. The gas pressure required for operation depends on the gas, the anode to cathode distance, and voltage and current. By appropriate design, operation at a pressure of about 100 μm or more can usually be arranged thus demanding only the simplest vacuum techniques. Both electron and ion beams may be applied to insulating materials as well as conducting materials. This is possible because electrical charging difficulties are avoided owing to the associated presence of ionised gas. A number of different types of glow discharge gun of novel design have been constructed and their characteristics investigated. This paper discusses the design principles employed and illustrates applications in the fields of crystal growing, vapour deposition, welding, thermal milling and etching and milling by sputtering. © 1969 Chapman and Hall.
