Use of velvet cathodes for the generation of intense relativistic electron beams in pulse vacuum diodes

The technology used for the intense relativistic electron beam production has advanced significantly and is wide spread in use for variety of pulsed power applications generating several mega-watts of beam power. The production of such intense electron beams begins from electron source. In majority of applications the main requirement for electron beam quality is its uniformity and low divergence and these properties are dominated by type of source used. The most commonly used cold cathode explosive emission source includes wide variety of materials like aluminum, graphite and stainless steel. In the non-explosive filed emission category common velvet cloth has been found to be an excellent field emitter. It has low electric field threshold for plasma initiation, emits very uniformly, has low gap closure velocity and is moreover inexpensive and easy to use. The additional qualities in velvet cathodes as compared to other cathode materials is found due to large number of field enhancement points on the fiber and non conducting nature of the material. Though the fibers are initially non conducting surface flash over along the fibers play an important role in plasma production. Velvet cathodes provide electron beams with brightness two orders of magnitude higher than conventional thermoionic cathodes. This paper mainly focuses upon the mechanism and modeling of plasma initiation in the fast pulse vacuum diodes having velvet cathode along with experimental results and the setup. Experiments in this reference have been carried out in axially symmetric planar diode configuration.