Abstract

Frequency-locked Smith-Purcell radiation (FL-SPR), generated by a train of electron bunches traveling above a grating, is characterized by a broad range of frequencies which are locked to the train frequency in a discrete comb and are spatially dispersed in space. We report absolute-scale power measurement of FL-SPR in the millimeter wave range. A 50 ns long train of $170\text{ }\ensuremath{\mu}\mathrm{m}$ electron bunches was produced by a 15 MeV, 17 GHz accelerator with 80 mA of average current. The grating had 20 periods spaced by 2.54 mm. The experimental results were compared, on an absolute scale, with the electric-field integral equation model which takes into consideration the finite length of the grating. Very good agreement was obtained. The present results should be useful in planning SPR applications such as diagnostics of electron bunch length on the femtosecond scale and coherent THz radiation sources.

Highlights

  • Smith-Purcell radiation (SPR) [1] emitted by an electron bunch passing at a relativistic velocity ˆ 1 ÿ ÿ2†1=2 above a periodic structure is characterized by a broad spectrum of frequencies in which the radiated wavelength depends on the observation angle according to the SPR resonance relationship

  • The purpose of this experiment was to test the accuracy of the finite grating length electric-field integral equation (EFIE) model

  • SPR was generated by a train of bunches moving above the grating, and it was shown to be coherent at the millimeter wave range

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Summary

Introduction

Smith-Purcell radiation (SPR) [1] emitted by an electron bunch passing at a relativistic velocity ˆ 1 ÿ ÿ2†1=2 above a periodic structure is characterized by a broad spectrum of frequencies in which the radiated wavelength depends on the observation angle according to the SPR resonance relationship. Though different theoretical models agree on this resonance relationship, substantial differences arise in the calculated radiated energy [2]. The nth harmonic of the radiated wavelength depends on the grating period Dg and the observation angles and by the SPR resonance relationship ˆ Dg 1 ÿ sin sin ; (1). The wave vector of the radiation, k, makes an angle with respect to this y axis.

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