Superimposed-harmonic Smith-Purcell free-electron lasers driven by periodic electron-bunches.

To meet the requirements of high power and broad-tunable terahertz sources, we proposed and investigated a modified Smith-Purcell free-electron laser (SP-FEL) driven by periodic electron-bunches (PEBs). We first designed an electron-gun with a photocathode, which generates a train of PEBs with tunable macro-bunching frequency. These PEBs then interact with surface waves on a grating and are further micro-bunched. The macro-bunching frequency together with its harmonics is superimposed on the micro-bunching one such that the multi-color superradiant Smith-Purcell radiation is achieved at the superimposed harmonics. By adjusting the macro-bunching frequency, the superradiant frequency can be tuned from 0.5 to 1 THz—a region hard to reach by conventional SP-FELs and vacuum electron devices. Compared to DC-beams, the PEBs have higher peak current density and generate higher peak power. Thus, the proposed scheme can be promisingly developed as high power and broad-tunable terahertz sources. [ABSTRACT FROM AUTHOR]