Study of Coherent Smith–Purcell Radiation in the Terahertz Region Using Ultra-Short Electron Bunches

Smith–Purcell radiation (SPR) can be generated nondestructively, providing valuable applications in light sources and beam monitors. Coherent SPR is expected to enable single-shot measurements of very short bunch lengths on the fs scale. Since the reconstruction of the longitudinal bunch shape from the coherent SPR is based on the reliable SPR spectrum, a more detailed understanding of the properties of the radiation is important in this context. Employing a 100 fs ultrashort electron bunch at the t-ACTS test accelerator, the spectrum, angular distribution, and polarization of the produced coherent SPR were measured in the terahertz frequency region and compared with a model calculation. In addition to the widely known surface current model evaluation, the effect of the geometrical shading effect on induced currents on metal surfaces was evaluated using 3D numerical calculations. The obtained SPR characteristics are also presented. In the evaluation of the grating with a shallow blaze angle, it was found that the shading effect has a non-negligible effect on the generated SPR intensity; the measured angular distribution and polarization results were in good agreement with this result.