Abstract
Summary form only given. 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 led to a planar "plasma cathode" consisting of metal honeycomb filled with epoxy. The introduction of velvet by Adler provided an important electron source that turned on uniformly at low fields and with low plasma velocity, and velvet has been widely used, for example in laser pumps and high power microwave tubes. The repetitive operation of velvet is sometimes limited by the gas evolved on each pulse and by the material lifetime. Ways to counter these limits are reviewed, including some recent results on materials that give improved performance.
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