摘要
高功率微波器件中导引强流电子束传输的常规方法是采用外加磁场来限制电子束的扩张,研究磁场导引系统对高功率微波器件非常重要。而由于高功率微波器件的小型化,实用化的发展需求,研究周期永磁聚焦(PPM)系统是一个非常有前景的方向。本论文的主要工作是对同轴永磁Halbach结构磁路这种周期聚焦系统进行理论分析和数值模拟,对强流环形电子束在该磁路中的传输进行分析,并对以同轴永磁Halbach结构磁路作为摇摆器(Wiggler)的自由电子激光(FEL)放大器进行粒子模拟。
分析Halbach结构磁路性质,求解推导出了同轴永磁Halbach结构磁路磁感应强度各分量的近似表达式,特别对磁场高次空间谐波的忽略进行了讨论,对磁场初始相位进行了修正。分析总结该结构磁路的一些性质,同时还讨论了磁环厚度、磁路间隙和磁场周期对磁场幅值的影响。
理论分析了电子束在磁路中的受力情况,推导出电子束传输的径向力平衡方程,给出束包络平衡条件。通过粒子模拟分析了电子束在Halbach结构磁路中传输的物理过程,主要讨论了磁场强度,电子束电流,电子束厚度,电子束入射角度,入射位置,磁场周期等因素对电子束传输的影响。分析认为:利用同轴永磁Halbach结构磁路导引数kA的电子束,并使之稳定传输是可能的。同时该磁场聚焦形式也为FEL中的束波相互作用提供了一个作用机制。
作为同轴永磁Halbach结构磁路的一个应用,对同轴波导FEL放大器进行粒子模拟研究。选取TE_(01)模式作为FEL放大器的工作模式,在输入信号频率9.65GHz,功率12.4kW,电子束参数束电压525kV,束电流1kA的条件下,自由电子激光放大器在频率9.65GHz处,输出平均功率达到100MW,束波转换效率为19%,放大器的增益为39 dB。
High-power microwave (HPM) devices require the applied magnetic field to guide the propagation of intense electron beams, and indeed to inhibit the diffusion of the electron beams. Therefore, it is necessary to investigate the magnetic-field guide system. HPM technology development drives the need to investigate the periodic permanent-magnet (PPM) focusing system that is able to operate at a compact and practical size. The research work in this thesis mainly focus on the theoretical analysis and numerical simulation of a coaxial permanent magnet Halbach circuit. Meanwhile, investigate the propagation process of annular intense electron beam in this type of PPM focusing system, and free electron laser (FEL) amplifier with such kind of magnetic circuit as the wiggler with Particle-in-cell(PIC) simulation code.
Firstly, the magnetic field of coaxial permanent magnet Halbach circuit is calculated using the finite element code (POISSON SUPERFISH). The approximate expressions for the components of the induction magnetic field are obtained. The problem of how to neglect the higher spatial harmonics of the field components is discussed and the initial phases of the field components are modified. Some properties of the magnet circuits are analyzed and summarized. The effects of structure parameters of the magnetic circuits, including the thickness of the magnetic ring, the gap between the coaxial circuits, and the period of the magnet, on the amplitude of the magnetic field are discussed.
Secondly, the forces acting on intense annular electron beam in such kind of magnetic field are analyzed. The radial force equation with a modified Mathieu function form and the conditions for the electron beam stability are drawn. Then PIC simulation code is used to investigate the physical process of the beam propagation in such kind of magnetic field. The amplitude and period of the magnet field, the electron beam current, the thickness, the initial incident angle of electron beams and the initial emission position related to the stable propagation are discussed. The conclusion is made that several kilo-amperes intense annular electron beam could propagate stably, meanwhile the focusing form of such magnetic geometry provides action mechanism for the interaction between electron beams and microwave in the FEL.
At last, this thesis studys FEL amplifier with coaxial permanent magnet Halbach circuit as the wiggler through particle simulation as the application of the circuit. The TE_(01) mode is chosen as the operate mode of the FEL amplifier. The simulation results show the FEL amplifier, which is generated by a 525kV, 1kA electron beam with a 9.65 GHz, 12.4 kW input signal at TE_(01) mode, operate at 9.65 GHz with averaged power of 100 MW. The efficiency is about 19% and the gain of the amplifier is 39 dB.
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