Abstract
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.
Highlights
There is currently substantial interest in the development of THz sources for applications to biophysics, medical and industrial imaging, nanostructures, and materials science [1]
Available THz sources fall into three categories: gas lasers, solid-state devices, and electron-beam driven devices
The results show that 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
Summary
There is currently substantial interest in the development of THz sources for applications to biophysics, medical and industrial imaging, nanostructures, and materials science [1]. Shibata et al extended the work of van den Berg, Haeberle et al [22], and Schaechter [23] to a lamellar grating of finite length, and discussed its coherent enhancement by individual electron bunches [24] They compared the theory with experimental results they obtained using electron beams bunched by rf linacs. We calculate the angular spectral fluence radiated by electrons passing over a lamellar grating of finite length and examine the coherent enhancement caused by bunching in rf linacs and SP-FELs. we compare the results with numerical simulations carried out using particle-in-cell codes and with available experimental data
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