Explosive Electron Emission Properties Of Dielectric-doped Graphite Cathodes

With the development of high power microwave(HPM) technology, it has broad applications in direct energy weapon, high-sensitivity long-distance radar communication, plasma heating etc. Cathode is one of the most significant components in HPM sources, and the characteristic of HPM is highly dependent upon the quality of the electron beam generated by the cathode. Nowadays, graphite is ideal material as explosive emission cathodes for HPM sources. Graphite cathode takes advantage on low outgassing rate, stable operation and long-lifetime(>106 shots), it still has a few drawbacks, such as, low current density, emission nonuniformity and so on. According to the field-induced explosive theory and dielectric band bending theory of emission, microscopic morphology on the cathode surface and its composition have great effects on explosive electron emission. Therefore, this paper focuses on improving the explosive electron emissionproperties of graphite cathode by doping dielectrics. Besides, under the low voltage-current response characteristics of different doped dielectrics in graphite cathode, we explored surface properties of the cathode and the influences of electron emission properties induced by micromorphology. Moreover, through testing the microwave generation characteristics of different dielectrics doped in graphite cathode under high voltage, we preliminary investigated the microwave output pulse width influenced by the electron emission process of the cathode surface.To research the vaieties and morphology of the doped dielectrics, we have prepared Si C, Ti C, BN and La B6 doped graphite cathodes and CNTs, carbon fiber and Si C whisker doped graphite cathodes by the method of powder mixed directly. The graphite cathode doped with Si C has also been prepared by the method of chemical vapor deposition. Then, we have studied the microstructure and physical properties of doped graphite materials. The results show that the carbon and Si or Ti reaction is transformed into the corresponding carbide though the treatment of graphitization under 2200? and 1h, and the doped dielectrics is beneficial to improve the graphitization degree of the material. The distribution of doped dielectrics in graphite matrix becomes more uniform. Besides, the enhancement of graphitization is advantageous to reduce the resistivity of doped graphite materials as well as improving its thermal conductivity and oxidation resistance.We test the diode voltage and current curves of doped graphite cathodes on the pulse testing platform with the voltage of 160~190k V and the pulse width of ~20ns. Comparing the delay-time of electron emission and the diode perveance curves, we found out the factors, dielectric properties and micromorphologies, which affect electron emission properties. The results show that graphite cathode doped with Ti C, La B6 and carbon nanotubes, silicon carbide whiskers performs better voltage response characteristics in the stage of field emission. The two graphite cathodes show better characteristics of current response in the stage of explosive emission, which doped with Si C and Ti C by the chemical vapor deposition method and electroplating method, repectively. By comparison, surface micromorphology is the greater factor on the electron emission performance of the cathode. it's useful to reduce the emission threshold for the increase of flake-graphite crystals and existence of nano structures. Besides, the smaller surface microprotrusions, and the more nano structures, the larger amount of emission micropoints.In the pulsed accelerator test platform with voltage of 950~1000k V and pulse width of 40 ns, the voltage-current response of the doped graphite cathode corresponds to the low voltage pulse. Graphite cathodes doped with Si C show better characteristics of microwave, and their microwave amplitude are higher than that of POCO-CZR pure graphite cathode. Especially, the microwave width imporves for CZR after chemical vapor deposition with Si C at 1600?. The more obvious changes have taken place in the surface micromorphology of Si C whisker and carbon fiber doped graphite cathodes after 105 shots, which result in the improvement of microscopic uniformity. Although the slightly lower in amplitude of diode current, the increase of microwave pulse width and amplitude show that the emission uniformity of electron beam is imporved.