石墨—金刚石复合阴极强流脉冲发射特性研究

Considering that common problems of circular graphite cathodes used by proposed academy of this “NSAF” foundation such as low emission current density, large velocity of cathode plasma and poor emission uniformity and consistency, this project will develop a novel graphite-diamond composite cathode, in which nitrogen-doped N-type ultrananocrystalline diamond thin films are coated on the surface of the circular graphite substrates with transition layer consisting of refractory metal. The novel composite cathode will be prepared by successively depositing refractory metal and nitrogen-doped N-type ultrananocrystalline diamond thin film on circular graphite cathode through electron beam evaporation and microwave plasma chemical vapor deposition techniques. Furthermore, the effect of deposition parameters on the microstructure of the composite cathode will be systematically investigated via several experimental characterizations. Moreover, high-current electron emission performances of the composite cathode with different microstructures will be carefully investigated under the condition of different pulsed fields. Combined with the observations such as plasma lightening at cathode surface, pressure change of vacuum diode, and the microstructure variety of composite cathode before and after emission, the high-current pulsed emission mechanisms of graphite-diamond composite cathode will be intensively explored through theoretical simulations. This project could provide high-performance composite cathode for the vacuum diode of the relativistic backward wave oscillator of high power microwave (HPM) as well as fundamental basis for developing cathodes of other HPM sources, free-electron laser and high-current electron accelerators.

针对本“NSAF”基金课题提出单位现用石墨阴极发射电流密度小、等离子体膨胀速率大、发射均匀性和一致性差等共性问题，本项目拟以带刃环形石墨阴极为基体，采用电子束蒸发镀膜和微波等离子体化学气相沉积技术相结合的方法，在环形石墨阴极表面先后镀覆高熔点金属过渡层和掺氮N型超纳米金刚石薄膜，制备出新型石墨-N型超纳米金刚石薄膜涂层复合阴极，并通过多种实验表征系统地研究复合阴极的微观结构随沉积工艺参数的变化；在此基础上，详细研究具有不同微观结构的复合阴极在不同脉冲强场条件下的强流发射性能；结合脉冲发射过程中阴极表面发光、真空二极管气压变化以及脉冲发射前后复合阴极的微观结构状态，同时通过理论模拟计算，探索复合阴极的强流脉冲发射物理机制。最终为高功率微波相对论返波振荡器真空二极管提供高性能的实用复合阴极，并为其它类型的高功率微波源、自由电子激光、强流电子加速器的阴极研究提供理论基础。

为解决相对论返波管（RBWO）现用石墨阴极发射电流低、稳定性差、材料放气等缺陷，本项目利用掺氮超纳米金刚石（UNCD）优异的电子场发射性能、通过微波等离子体化学气相沉积（MPCVD）技术在石墨阴极发射刃口涂覆掺氮UNCD形成性能优越的新型石墨-金刚石涂层复合阴极。首次采用三乙胺为掺杂氮源制备出电学性能优良的掺氮UNCD，详细研究了液态源组成、通入量及生长温度等因素对薄膜质量的影响，同时系统分析了掺氮UNCD的导电性和直流场发射性能；为克服金刚石薄膜在石墨基底上生长存在形核密度低、膜层完整性差等问题，沉积掺氮UNCD前在石墨基底上分别采用电子束蒸镀和金属有机物化学气相沉积（MOCVD）技术制备钨（W）膜作为过渡层，重点研究了MOCVD过程中以W(CO)6为前驱体沉积温度及退火温度对所制备的金属钨过渡层的影响。利用SEM、多波长Raman、XPS、XRD、霍尔效应等表征技术详细研究了所制备的掺氮UNCD薄膜和金属钨膜过渡层的形貌、结构、成分、电学性能。在掌握了制备掺氮UNCD和W膜生长技术的基础上，将平面基片生长工艺转移到异形石墨阴极上，通过进一步调整优化，成功制备出高质量的掺氮UNCD薄膜涂覆改性石墨阴极，同时RBWO平台上不同脉冲强场条件下的强流发射性能测试表明，新型石墨-金刚石涂层复合阴极电子发射能力、发射稳定性、材料放气特性等均得到大幅度改进，是一种极具潜力的实用型强流电子束源。最后，运用基于第一性原理的VASP软件对掺氮N型UNCD薄膜进行了态密度、能带结构计算，从理论上分析掺氮N型UNCD薄膜的导电及发射机制，有望为研究掺氮N型UNCD阴极的强流脉冲发射机制提供理论依据。