Observation of Frequency-Locked Coherent Terahertz Smith-Purcell Radiation

We report the observation of enhanced coherent Smith-Purcell radiation (SPR) at terahertz (THz) frequencies from a train of picosecond bunches of 15 MeV electrons passing above a grating. SPR is more intense than other sources, such as transition radiation, by a factor of $N_g$, the number of grating periods. For electron bunches that are short compared with the radiation wavelength, coherent emission occurs, enhanced by a factor of $N_e$, the number of electrons in the bunch. The electron beam consists of a train of $N_b$ bunches, giving an energy density spectrum restricted to harmonics of the 17 GHz bunch train frequency, with an increased energy density at these frequencies by a factor of $N_b$. We report the first observation of SPR displaying all three of these enhancements, $N_g{N}_e{N}_b$. This powerful SPR THz radiation can be detected with a high signal to noise ratio by a heterodyne receiver.