Influence of surface microstructure on explosive electron emission properties of graphite cathode doped by silicon carbide whiskers

Abstract

Explosive emission cathode (EEC) is a pivotal component in high power microwave source (HPMS), of which the ultimate properties are significantly dependent on the quality of electron beams generated from the cathode. Short lifetime and poor emission uniformity are the persistent drawbacks of conventional field EEC. Improvement of cathode material by changing its compositions and modifying surface micromorphology, is a feasible way to solve this problem. Graphite is one of the frequently used materials for EECs due to its long life-time and sturdy performance under high voltage and repetition frequency. Meanwhile silicon carbide (SiC) whiskers are distinguished by high aspect ratio (ratio of height to diameter) and low work function which is in favor of the fast onset of electron emission. In this work, the novel composites, composed of SiC whiskers, pitch and major graphite powders, are prepared by the conventional mingling and sintering. The cathodes are installed on TPG1000 system with a parameterized pulse of 970 kV, 9.2 kA, and 50 ns. By analyzing the changes of the rise edge of measured diode current and output microwave pulse duration, the effects of material composition and surface micromorphology on electron emission properties for the cathode are disclosed in detail. The results, based on the comparison of emission properties between graphite cathodes with and without SiC whiskers doped, reveal that SiC whiskers play an important role in accelerating the field emission of cathode, which is demonstrated by the eclipse of displacement current peak on the rise edge of measured current waveform after doping. Meanwhile, the duration of output microwave pulse is enhanced by about 11% after doping, which could be explained by the lower expansion speed of Si plasma. Moreover, the surface micro-protrusions of graphite cathode doped by SiC whiskers are constantly “polished” by heating effect and cathode plasma as the number of emission pulses increases to 11000. This is in quite good agreement with the appearance of the displacement current peak on the rise edge of measured current curves after 6000 and 11000 pulses treatment. These changes imply that the initial speed of field emission from cathode is slowed down gradually. The output microwave pulse starts early, which is benefited from the homogeneous surface micromorphology of the cathode due to “polishing” effect. The quantity of releasing absorbed gases, including water and vacuum pump oil vapor, decreases with increasing emission pulses. Then the pulse shortening phenomenon is restrained and the falling edge of output microwave pulse is extended. The duration of output microwave pulse is increased by about 5%, for graphite cathode doped by SiC whiskers after experiencing 11000 pulses. In conclusion, the reaction mechanism of SiC whiskers in the process of explosive electron emission (EEE) is considered as being due to accelerating the onset of felid emission and reducing the expansion speed of cathode plasma. Therefore, combination with SiC whiskers is an effective way to improve the electron emission properties of graphite EECs, especially in the output microwave pulse width and energy conversion efficiency of HPMS.