Superradiant emission of Smith-Purcell radiation
Superradiant emission of Smith-Purcell radiation
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DOI：
10.1103/physrevstab.8.110702
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发表时间：
2005-11
影响因子：
--
通讯作者：
H. Andrews;C. Boulware;C. Brau;J. Jarvis
中科院分区：
文献类型：
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作者：
H. Andrews;C. Boulware;C. Brau;J. Jarvis
当电子靠近金属光栅的表面通过时，会发出史密斯-珀塞尔（SP）辐射。当电子聚束的尺寸小于辐射的波长时，辐射变得相干（注量与电子数的平方成正比）。这已经在电子被射频直线加速器预聚束的实验中观察到。当电子以周期性聚束出现时，光谱强度的增强伴随着辐射的角度和光谱分布的大的变化。这就是所谓的超辐射。最近，人们在一台所谓的Smith-Purcell自由电子激光器（SP-FEL）中观察到了超辐射SP辐射，在这种激光器中，电子被激光过程聚束。与其他慢波结构一样，SP-FEL中的电子束与一个相位速度与电子速度相匹配的倏逝波相互作用，并将其放大，该倏逝波的频率低于SP辐射的范围，并且该倏逝波只从光栅的两端辐射。然而，通过与倏逝波的相互作用的电子的聚束增强了普通的Smith-Purcell辐射，并且由于超辐射效应而改变了角度和光谱分布。本文介绍了一种计算三维空间SP辐射的新方法，包括有限光栅长度和任意频率的周期性电子聚束引起的超辐射效应。我们发现，SP辐射的频谱和角度窄峰的聚束频率的谐波。在射频直线加速器中，聚束间距很大，几个间距很近的谐波位于单个电子发射的光谱包络之下。在SP-FEL中，谐波间距很大，SP辐射出现在与聚束频率的谐波相对应的SP角处的窄锥体中。最后，我们计算了一个电子通过一个有限长度的层状光栅辐射的角谱注量，研究其相干增强SP-FEL和RF直线加速器，并与数值模拟和现有的实验数据的结果进行比较。
Smith-Purcell (SP) radiation is emitted when an electron passes close to the surface of a metallic grating. The radiation becomes coherent (fluence proportional to the square of the number of electrons) when the electrons are in bunches whose dimensions are smaller than the wavelength of the radiation. This has been observed in experiments in which the electrons are prebunched by an rf linac. The enhancement of the spectral intensity is accompanied by large changes in the angular and spectral distribution of the radiation when the electrons appear in periodic bunches. This is called superradiance. Recently, superradiant SP radiation has been observed from a so-called Smith-Purcell free-electron laser (SP-FEL) in which the electrons are bunched by the lasing process. As in other slow-wave structures, the electron beam in a SP-FEL interacts with an evanescent wave for which the phase velocity matches the electron velocity and amplifies it. The frequency of this wave lies below the range of SP radiation and the wave is not radiated except from the ends of the grating. However, the bunching of the electrons by the interaction with the evanescent wave enhances the ordinary Smith-Purcell radiation and changes the angular and spectral distribution due to superradiant effects. In this article, we introduce a new method for computing the SP radiation in three dimensions, including the effects of finite grating length and superradiance due to periodic electron bunching at an arbitrary frequency. We show that the SP radiation develops spectrally and angularly narrow peaks at the harmonics of the bunching frequency. In rf linacs, where the bunches are widely spaced, several closely spaced harmonics lie under the spectral envelope of the emission from a single electron. In a SP-FEL the harmonics are widely spaced and the SP radiation appears in narrow cones at the SP angles corresponding to the harmonics of the bunching frequency. Finally, we calculate the angular spectral fluence radiated by an electron passing over a lamellar grating of finite length, examine its coherent enhancement in SP-FELs and rf linacs, and compare the results with numerical simulations and available experimental data.