A beam of 3.6-MeV electrons has been used to study the generation of radiation in the far infrared (FIR) by the Smith-Purcell mechanism. The dependence of wavelength on angle of emission, over angles from 56° to 150° and wavelengths from 350 to 1860 /xm, is in excellent agreement with the SmithPurcell dispersion relation. Comparison of the yield with that from a 5000-K source suggests that the spontaneous Smith-Purcell effect offers an easily tunable alternative to the synchrotron as a coherent FIR source, and that it could also form the basis of an inexpensive, compact free-electron laser.PACS numbers: 41.75.Ht, 41.60.Cr We report the first observations of Smith-Purcell (SP) radiation in the submillimeter-far-infrared region of the spectrum. Unlike both early [1] and more recent work [2], which relied on low-energy electron beams and concentrated on emission in the visible, here a relativistic electron beam was employed and the emphasis is on a spectral region where the population of sources is sparse.Comparison of the measured value of the emitted wavelength (Xsp) and the emission angle iO) for a series of gratings confirms in detail that the process obeys the celebrated SP formula:(1)The parameter XQ is the grating period and p is the relative velocity of a beam electron. In the spectral region where the radiation was observed, the level is comparable to that produced by an infrared beam line on a synchrotron. The emission from compact, linear-acceleratordriven gratings would be much greater. Thus, the process provides a means whereby Fourier-transform spectroscopic techniques can be extended into the technologically important long-wavelength spectral region. The grating is also a much simpler structure than a magnetic undulator and with adequate feedback it offers another basis for the free-electron laser (FEL) [3].The inverse SP process has also been considered as a possible means of accelerating electrons to high energy I4]. Although in the present work acceleration was not the object, coupling of near speed of light electrons and gratings has been demonstrated.The electrons were accelerated in a modified 10-MV Van de Graaff accelerator formerly used for a program of nuclear structure research at Oxford. Conversion to accelerate electrons had been started in preparation for a FEL project which was then not funded. Work necessary to enable efficient electron transmission [5l was completed and an electron gun was kindly lent by the Department of Physics, University of Glasgow.First, acceleration of the electrons was obtained at an energy of 3.6 MeV. The grid of the gun was pulsed, giving 6-//S bursts of electrons at 1 Hz. Considerable difficulties were experienced with both the yield and short lifetime of the cathodes; these were not fully overcome. Another pair of unsolved problems were a slight positional jitter in the beam and a beam size slightly larger than expected. The beam size, shape, and transverse position could be controlled using the up-stream deflecting magnets and quadrupole (Fig. l).The electr...
The radiation produced by the interaction of an electron beam with a metallic grating (the Smith - Purcell radiation) can be described in terms of a theory based on the acceleration of the surface charges induced on the grating surface by the passing electrons. The calculated spectral distribution of the emitted power is compared with recent experimental results; the agreement between the two is found to be satisfactory.
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