Electrostatic self-focusing of electron streams

Abstract

The electrostatic self-focusing mechanism for electron streams is shown to be a stronger focusing process than previously realized in that most of the potential drop that obtains between the stream axis and the surrounding tube wall occurs in a rapid potential variation in and near the stream itself. Streams are self-focusing when the particles have transverse energy spreads less than a well defined value that depends on the stream particle energy and the choice of ambient gas. The stream diameter is inversely proportional to the current for low values of stream perveance, but is independent of the current for high values of this parameter. The average random energy of the ambient electrons is high in the stream vicinity as compared with values near the tube wall.