Design of X-band coaxial gyrotron cavity driven by intense relativistic electron beam
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摘要: 在爆炸发射阴极驱动下，相对论回旋管常因超高束流电流（＞300A）易发生虚阴极现象，电子束也极易轰击导体内表面，并伴随回旋共振（Cyclotron Resonance）和回旋返波振荡（Gyro-Backward-Wave Oscillation, BWO）模式的非预期激励。本研究采用通过理论分析与三维粒子模拟相结合的方法，系统探究了X波段同轴回旋管腔体在强流相对论电子束（Intense Relativistic Electron Beam, IREB）驱动下的电磁特性。结果表明，通过腔体几何优化与电子书参数匹配可实现强流相对论电子束的稳定传输与TE01单模工作。腔体品质因数Qcav的合理取值对于抑制寄生模式竞争至关重要。当Qcav＜65时，会激励TE21-BWO模式；Qcav＞90时，会激励TE31回旋共振模式。而Qcav=65～90区间内可稳定维持TE01单模振荡，输出功率达35 MW（电压300 kV、电流500 A、横纵速度比1.2），对应效率34.4%。进一步研究表明，该腔体对电子束速度零散（Δβ＜25%）具备显著鲁棒性，为高功率微波源设计提供了重要参考。Abstract: Under the driving of explosive-emission cathodes, relativistic gyrotrons frequently suffer from virtual cathode phenomena induced by ultrahigh beam currents (＞300 A), where electron beams readily impact the inner conductor surfaces, accompanied by unintended excitations of cyclotron resonance and backward-wave oscillation (BWO) modes. This study systematically investigates the electromagnetic characteristics of an X-band coaxial gyrotron cavity driven by an intense relativistic electron beam (IREB), combining theoretical analysis with three-dimensional particle-in-cell (PIC) simulations. The results demonstrate that stable IREB transmission and TE01 single-mode operation can be achieved through cavity geometry optimization and electron beam parameter matching. The cavity quality factor (Qcav) plays a critical role in suppressing parasitic mode competition: TE21-BWO modes are excited when Qcav＜65, while TE31 cyclotron resonance modes emerge when Qcav＞90. Stable TE01 single-mode oscillation with an output power of 35 MW (voltage: 300 kV, current: 500 A, transverse-to-longitudinal velocity ratio: 1.2) and efficiency of 34.4% are maintained within the Qcav range of 65-90. Further studies reveal significant robustness of the cavity against electron beam velocity spread (Δβ＜25%), providing critical insights for high-power microwave source design.
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图 1 电子束传输和腔体结构示意图
Figure 1. Schematic diagram of electron beam propagation and cavity structure
图 3 Ub = 300 kV, B0 = 0.5 T and ɑ = 1.2时不同模式的色散曲线
Figure 3. Dispersion curves under different modes at the Ub = 300 kV, B0 = 0.5 T and ɑ = 1.2
图 5 不同轴向位置处回旋管热腔的归一化振幅和束波互作用效率（Ub=300 kV, Ib=500 A, B0=0.48 T, ɑ=1.2, and Rb=13.1 mm）
Figure 5. Normalized amplitude of hot cavity and efficiency versus distance of the axle of the cavity (Ub=300 kV, Ib=500 A, B0=0.48 T, ɑ=1.2, and Rb=13.1 mm)
图 6 Qcav超过90时三维PIC模拟中方位角电场Ephi的分布和FFT值
Figure 6. The distribution and the FFT of the azimuthal electric field Ephi in 3D PIC simulation under the Qcav more than 90
图 7 TE01和TE31模式的起振电流Istart(实线为Qcav=125，虚线为Qcav=90)
Figure 7. The start oscillation currents Istart of TE01 and TE31 modes
图 8 Qcav小于60时三维PIC模拟中方位角电场Ephi的分布和FFT
Figure 8. The distribution and the FFT of the azimuthal electric field Ephi in 3D PIC simulation under the Qcav less than 60
图 9 TE21模式BWO的腔体长度随起振电流的变化情况
Figure 9. Variations of cavity length thresholds of TE21 mode BWO versus the start current
图 10 Qcav小于60时，20 ns至30 ns的方位角电场Ephi的 FFT 值和输出功率随时间的变化情况
Figure 10. FFT of the azimuthal electric field Ephi at 20 ns to 30 ns and output power variation over time under the Qcav less than 60
图 11 最佳参数下三维PIC模拟方位角电场Ephi的分布和FFT
Figure 11. The distribution and the FFT of the azimuthal electric field Ephi under the optimal parameters of 3D PIC simulation
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