Effects of Different Cathode Materials on Submicrosecond Double-Gap Vircator Operation

The operation of a double-gap S-band vircator been investigated at submicrosecond duration of a electron beam generated in a planar diode. The experiments performed at accelerating voltages of <= 550 kV and diode of up to 17 kA using a radio-frequency cavity with a wide window between its two sections. Three types of cathodes been studied, namely, metal-dielectric, carbon fiber, and cathodes. The main features of the operation of the vircator each cathode are analyzed. The microwave pulse duration with the metal-dielectric and carbon fiber cathodes reached similar to 250 ns at the peak power level of similar to 100 MW; with the velvet cathode, a duration of similar to 400 ns was achieved. It has been found that, in addition to the common limitations of the microwave pulse duration related to the dynamics of the diode impedance governed by the cathode plasma expansion, there is another factor, namely, the anode-cathode gap, which determines the delay at the beginning of the microwave generation. The latter effect is explained by the role of electrons oscillating between the virtual and real cathodes in the generation process. The issue of radiated microwave frequency behavior is discussed as well.