Pulse shortening in high power microwave sources

Observations show that the ubiquitous pulse shortening in high power microwave devices arises from formation of plasma, electron streaming, high-E-field breakdown and beam disruption. We review experiments in terms of these causes. Linear beam devices exhibit all these mechanisms; in particular beam disruption by ExB drifts in the strong microwave fields and diffusion in turbulent electric fields in background plasma appears common. In many sources the dominant effect is destruction of the resonance between beam and slow wave structure by cathode plasma motion, possibly from water contamination of the surface. Wall plasma effects shorten pulses in most sources. We call for the introduction of improved surface conditioning, cathodes which do not produce plasmas and increased effort on measurements of the high-field and plasma properties of HPM sources. Because of the broad nature of the phenomena in pulse shortening, we appeal for increased participation in this research by the plasma, intense particle beam and traditional microwave tube communities.