Simulation of Smith-Purcell radiation using a particle-in-cell code
Simulation of Smith-Purcell radiation using a particle-in-cell code
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DOI：
10.1103/physrevstab.8.060702
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发表时间：
2005-06
影响因子：
--
通讯作者：
J. Donohue;J. Gardelle
中科院分区：
文献类型：
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作者：
J. Donohue;J. Gardelle
使用二维细胞内粒子代码 MAGIC 对微波频率下 Smith-Purcell (SP) 辐射的生成进行了模拟。该模拟假设连续、细（但无限宽）、单能电子束通过衍射光栅，而强轴向磁场将电子限制为基本上一维运动。该代码通过使用有限元方法求解麦克斯韦方程来计算随时间变化的电场和磁场。我们发现电子束的通过在光栅附近激发了瞬逝的电磁波，这反过来又导致最初连续的电子束聚集。聚束的频率和波数被确定，并发现与 Brau 及其同事在最近的工作中提出的频率和波数接近。该频率低于 SP 辐射的阈值。然而，聚束足够强，以至于束流中的高次谐波清晰可见。这些谐波频率对应于允许的 SP 辐射，我们在模拟中看到这种辐射以适当的角度强烈发射，这再次与布劳的预测一致。我们还发现，在光栅的末端，一些倏逝波被衍射远离表面，并且发生低于阈值的辐射。此外，我们观察到第二个倏逝波具有相同的频率，但波数不同。该波的存在也是理论所预测的，尽管它在我们的模拟中的存在是出乎意料的。尽管增益对束流依赖性的精确形式仍然难以确定，但对倏逝波增长的数值估计也与预测相当一致。
A simulation of the generation of Smith-Purcell (SP) radiation at microwave frequencies is performed using the two-dimensional particle-in-cell code MAGIC. The simulation supposes that a continuous, thin (but infinitely wide), monoenergetic electron beam passes over a diffraction grating, while a strong axial magnetic field constrains the electrons to essentially one-dimensional motion. The code computes the time-dependent electric and magnetic fields by solving the Maxwell equations using a finite element approach. We find that the passage of the beam excites an evanescent electromagnetic wave in the proximity of the grating, which in turn leads to bunching of the initially continuous electron beam. The frequency and wave number of the bunching are determined, and found to be close to those proposed by Brau and co-workers in recent work. This frequency is below the threshold for SP radiation. However, the bunching is sufficiently strong that higher harmonics are clearly visible in the beam current. These harmonic frequencies correspond to allowed SP radiation, and we see strong emission of such radiation at the appropriate angles in our simulation, again in agreement with Brau’s predictions. We also find that at the ends of the grating, some of the evanescent wave is diffracted away from the surface, and radiation below the threshold occurs. In addition, we observe a second evanescent wave at the same frequency, but with a different wave number. The existence of this wave is also predicted by the theory, although its presence in our simulation is unexpected. Numerical estimates of the growth of the evanescent wave are also in reasonable agreement with the predictions, although the precise form of the dependence of the gain on beam current remains hard to establish.