Abstract
A density-modulated electron beam can generate coherent spontaneous radiation at the harmonics of the modulating frequency. This study employs a multicolor laser beating at sub-THz frequencies to modulate the photocurrent from a keV electron gun. The modulated electron beam drives a Smith–Purcell FEL to generate THz coherent radiation. We first built an infrared two-color laser containing a thin etalon in an Nd:YVO4 laser cavity to generate two laser spectral components separated by 124.6 GHz. Quadrupling the laser frequency in nonlinear optical materials generates a UV frequency with adjacent components separated by 124.6 GHz. This UV laser is useful to modulate the photocurrent from a photoinjector at the comb frequencies. As a design example, the photocathode of a 10–25 keV DC electron gun is illuminated with this multicolor laser and the keV electron beam is propagated above a 30 mm-long Smith–Purcell grating with a groove period of 240 um. With 18.2 keV beam energy, our simulation using the MAGIC code confirms coherent radiation at the third harmonic of the 124.6 GHz at a 56-degree radiation angle from the Smith–Purcell grating. Coherent radiation can be observed in simulation with a sub-mA beam current.