The application of the glow discharge to material processing

Abstract

The mechanism and characteristics of the glow discharge are outlined. Energetic electrons and ions, which are generated in the cathode fall region, may be focused or guided to form a beam by geometrical arrangements of the discharge electrodes and by magnetic fields. Two practical arrangements are described: a spherical hollow cathode device, which produces a hot zone at its centre; and a hollow anode device, which produces wellcollimated electron and ion beams. Glow discharge beam devices work at “rough vacuum” pressures, in the range 10−2 to 1 mm Hg, and are particularly suited to the processing of glass and oxide ceramics since, owing to the presence of plasma, no electrical charging of target insulating materials takes place. Limitations are set on the operating conditions of glow discharges by the heat input to the electrodes, by erosion due to sputtering, and by the glow-to-arc transition. Proper engineering, however, should allow cathode current densities of the order of 1 A/cm2 when operated at an anode voltage of the order of 10 kV, and focusing should realise electron beam power densities of up to the order of 1 MW/cm2, with efficiency of about 50%. Thus, the glow discharge may have a wide range of applications to material processing, from etching by sputtering at low power to processing by the most intense heating. New methods of forming and fabricating oxide bodies based on condensation of vapour and on powder deposition are discussed.