Dielectric cathodes: A review and some recent improvements

Dielectric cathodes in which plasmas formed by vacuum flashover are the sources of electrons in a vacuum tube or accelerator have been used in a variety of forms. Work at AWRE in the 1960s is reviewed. The introduction of velvet by Adler provided an important electron source that turns on uniformly at low fields and has low plasma velocity, widely used, for example in laser pumps and high power microwave (HPM) tubes. The repetitive operation of velvet is sometimes limited by the gas liberated on each pulse and by the material lifetime. A sparse array of rayon fibres was devised by Miller and used in Reltron HPM tubes, to reduce gas production and increase repetition rate, at further expense of life time. Recently a sparse array of glass fibre bundles has been used at L-3 Pulse Sciences to further reduce gas production with increased lifetime. Other inorganic dielectric cathodes may have advantages. The ceramic honeycomb of Friedman is another example of a dielectric cathode with desirable properties.