Investigation of the surface morphologies and the microstructures of graphite cathodes for explosive emission

Graphite cathodes are ideal for explosive electron emission due to advantages of stable operation and long lifetime of over 106 pulses under repetitive pulsed high voltage. In this study, emission properties of two graphite cathodes, fabricated from petroleum coke (PC) and carbon black (CB) respectively, were examined at operating voltages in the range ∼350-550 kV and pulse duration of 35 ns. The PC graphite cathode has superior emission properties in relation to higher currents and lower current-voltage delay times, which is probably due to greater field enhancement factor of microscopic flakes than that for round carbon black particles. The resonance tunneling of electrons through a potential barrier for the PC flakes with higher graphitization degree is also suggested. Smoothing of the cathode surfaces was found and the graphitization degree of the PC graphite cathode was decreased after 103 pulses as a result of continued actions of strong electric field and ion bombardment from anode plasma during explosive emission process. By contrast, the change of the graphitization degree of the CB graphite cathode was insignificant as the amorphous carbon microstructure of the carbon black was rather difficult to alter.