发散磁场中螺旋波等离子体双电层加速机理研究

The formation and presentation of Current Free Double Layer (CFDL) can accelerate plasma to supersonic speed along a diverging magnetic filed in helicon plasma. Based on this acceleration mechanisms, helicon double layer thruster presents an alternative for future deep-space missions. It is a promising thruster for its figure of merits of compact configuration, high energy coupling efficiency and unrestricted lifetime. However, it is not comprehensive for the acceleration mechanisms. This project mainly focuses on the CFDL acceleration mechanisms and formation in a diverging magnetic field configuration through theoretical and experimental methods, and gives an emphasis on the low pressure, non-uniform plasma discharge status, as well as the acceleration and expansion process simulation under the influence of CFDL, by means of two dimensional Fluid/PIC mixture computation method and laser induced fluorescence(LIF) dynamic diagnosis technology. Effective acceleration mechanism and performance models of the helicon CFDL will be developed, which will be capable of predicting the relationships among those factors, such as the propulsive efficiency influenced by the propellant mass flow rate , magnetic configuration and intensity, radio-frequency power. The expected achievement of this project will be helpful to provide the crucial base for the design and optimization of a helicon double layer thruster.

螺旋波等离子体在发散磁场中运动形成的无电流双电层可实现等离子体超音速加速，以此为加速原理的螺旋波双电层推力器具有结构紧凑、能量耦合效率高以及工作寿命长的特点，是一种很有潜力的新型空间电推力器，但目前对其等离子体加速的认识尚不全面。本项目拟通过理论和实验研究发散磁场中无电流双电层的形成及加速机制，采用二维流体/PIC混合模拟方法，结合激光诱导荧光动态诊断技术，着重解决低气压、非均匀等离子体放电状态及其在双电层作用下加速膨胀过程模拟问题，探索气体工质流率、磁场强度和构型、射频功率等因素与双电层加速性能之间的内在关联规律，为螺旋波双电层推力器的设计与优化提供理论方法。

经过三年的研究，项目组通过螺旋波等离子体-天线放电能量耦合模型及稀薄等离子体在发散磁场流动中的二维流体数值计算模型的建立，探索影响推进性能的推力器几何构型、磁场构型及强度、射频功率、气体工质流量密度等因素的关联规律。建立了更为精确的螺旋波三维瞬态放电模型，可进一步应用于螺旋波放电物理机制研究。结果表明：（1）磁场强度、天线位置是影响螺旋波等离子体能量吸收规律的主要因素，而射频频率、天线半径及其长度、气体通道半径主要影响了其吸收能量的大小，是推力器设计的次要考虑因素；（2）在螺旋波发散磁场加速机制下，等离子体的运动在百微妙时间尺度内已基本达到稳定状态，适当增加电离段等离子体获得的能量，增大发散磁场入口处的电子温度，以提高内能转化为轴向动能的效率，可有效增大双电层加速推进的比冲。.实现了稳态等离子体激光诱导荧光诊断系统集成，发展了一种双向偏振态激光诱导荧光测量方法，提高了等离子体诊断系统的测量准确性与使用便捷性。针对自建的螺旋波等离子体双电层推力器的测量结果表明：双电层形成于较低的气体压力，随着气体流量的减小，离子平均速度出现明显的升高，但是当气体流量为10SCCM时仍然远低于双电层的期望加速值。