Pulse shortening in high-peak-power Reltron tubes

Pulse shortening has been observed in nearly all high-power (>100 MW) microwave tubes that have attempted to extend the useful duration of the power pulse beyond shout 100 ns. Although Titan's high-peak-power Reltron tubes are notable in that they have produced 300 J/pulse in L-band, we have observed and eliminated several pulse-shortening phenomena to reach this level of performance. In this paper, we will present the results of several experiments related to pulse shortening that have examined the use of different materials and construction techniques; the importance of a good vacuum system, especially when operating under repetitive pulse conditions; and the effects of various conditioning processes. We believe that a major pulse-shortening problem has been the result of a mechanism involving rf electric-field-induced gas evolution and subsequent ionization. With our present construction techniques, using explosive emission cathodes and plastic insulators, we believe that it is possible to achieve 1 kJ/pulse at perhaps a few tens of Hertz pulse repetition rate. To exceed these levels will most likely require the use of conventional microwave tube construction techniques. Including thermionic cathodes, ceramic insulators, and brazed joining with high-temperature bakeout.