Numerical study on high frequency characteristics of slow plasma wave in cylindrical waveguide loaded with annular plasma beam
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摘要: 等离子体相对论微波发生器（PRMG）可以产生宽带高功率微波输出，同时又具有良好的频率可调谐性，因此在雷达、通信、电子对抗和物体探测等诸多领域均具有良好的应用前景。PRMG通常采用加载环形等离子体束的圆柱波导作为其波束互作用区，工作模式为慢等离子体波TM01模（下称P-TM01模）。P-TM01模的色散特性及其变化规律对PRMG输出性能有着重要影响。利用全电磁粒子模拟程序对加载环形等离子体束的圆柱波导中P-TM01模的色散特性和场分布进行了粒子模拟和分析，获得等离子体束密度np、径向厚度Δrp和径向位置rp以及外加引导磁场强度Bz和波导半径rw等参数对P-TM01模的色散特性和场分布的影响规律。主要研究结果包括：（1）一定范围内，np 和Δrp的变化对色散特性影响较大，rp，Bz和rw的变化对色散特性影响较小。值得关注的是，由于波导中环形等离子体束的存在，随着波导半径rw的增加，相同纵向波数kz对应的P-TM01模的频率没有降低而是略有提高。因此，在实际应用时，可以适当加大波导径向尺寸以提高器件功率容量；适当降低磁场，则有利于提高器件的紧凑性。（2）P-TM01模的纵向电场的方向不随径向位置变化，径向电场的方向在等离子体束内外两侧相反，外侧的场分布与同轴波导中TEM模相似。（3）主要物理参数变化时，场分布基本特点不会改变。但随着纵向模式数N和kz相应增加，电场能量向等离子体束收拢，不利于波束相互作用和电磁场的耦合输出。因此为了PRMG的高效运行，束波互作用的共振点最好落在kz相对较小的区域。上述研究结果对PRMG的设计和优化具有一定的理论参考价值。
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关键词:
- 等离子体相对论微波发生器 /
- 慢等离子体波 /
- 色散特性 /
- 场分布 /
- 粒子模拟
Abstract: As a kind of high power microwave generator, the plasma relativistic microwave generators (PRMGs) have the virtues of wideband high power microwave output and fine frequency tunability. Thus PRMG is very useful for a wide variety of applications. The beam-wave interaction region in the PRMG is generally a cylindrical metal waveguide with preformed annular plasma. The dispersion characteristics of the operating slow plasma wave TM01 mode (called as P-TM01 mode below) in the interaction region are critical to the output properties. Therefore, the dispersion characteristics and field distributions of the P-TM01 mode in a cylindrical waveguide loaded with annular plasma beam is studied numerically using the all electromagnetic PIC (Particle-in-Cell) code. Variation trends of the dispersion characteristics and the field distributions of the P-TM01 mode with the density np, radial thickness Δrp and radial position rp of the plasma beam, the intensity of the guiding magnetic field Bz and the radius of the waveguide rw are obtained. Simulation results show that: (1) Both np and Δrp affect the dispersion characteristics markedly and the frequency of the P-TM01 mode increases with the increasing of either np or Δrp at the same axial wave number kz. (2) Variations of rp, rw or Bz have very slight influence on the dispersion in the interested range. It is indicated that one can choose relatively larger dimensions of the waveguide for larger power capacity and lower guiding magnetic field for compactness if necessary. (3) The basic features of the field distributions of the P-TM01 mode will not change with the variations of the above mentioned physical parameters. But with the increasing of axial mode number and kz, the electromagnetic energy will be trapped inside the plasma beam gradually and no effective beam-wave interaction will happen in the end. Therefore, it is suggested to choose the operating point with relatively small kz for the efficient operation of PRMG. -
图 1 加载环形等离子体束的圆柱波导rz截面图
Figure 1. Schematic drawing of the axial cross sectionin a plasma waveguide
图 2 PIC模拟和解析公式计算得到的不同等离子体密度对应的P-TM01模色散曲线图
Figure 2. Dispersion curves of the P-TM01 mode in a plasma waveguide with different plasma density calculated by PIC simulations and theoretical formulation
图 5 不同强度的外加引导磁场对应的P-TM01模色散曲线图
Figure 5. Dispersion curves of the P-TM01 mode in a plasma waveguide with different intensity of the guiding magnetic field
图 3 不同等离子体束厚度对应的P-TM01模色散曲线图
Figure 3. Dispersion curves of the P-TM01 mode in a plasma waveguide with different plasma radial thickness
图 4 不同等离子体束径向位置对应的P-TM01模色散曲线图
Figure 4. Dispersion curves of the P-TM01 mode in a plasma waveguide with different plasma radial position
图 6 不同波导半径对应的P-TM01模色散曲线图
Figure 6. Dispersion curves of the P-TM01 mode in a plasma waveguide with different waveguide radius
图 7 点A、B和C处P-TM01模的轴向和径向电场分布等高图
Figure 7. Contour plots of the electric field of the P-TM01 mode at points A, B, and C
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