A compact electron gun for generation of FIR radiation

We report here on an experiment in which picosecond electron pulses, produced by a compact RF electron gun, were used to excite rectangular waveguide modes, generating 7 ps pulses of radiation with a bandwidth of approximately 200 GHz. The interaction of the electron pulses with the waveguide modes can be modeled quite simply by performing a harmonic expansion of the pulse train produced by the electron gun and employing Poynting´s theorem to compute the power coupled into the modes of the waveguide by each harmonic. The resulting model for the distribution of spectral power yields good agreement with the observed spectrum. We also report of the production of sub-picosecond electron bunches at an energy of ∼1.2 MeV to generate picosecond pulses of radiation via the mechanism of coherent transition radiation. We show that, though the high frequency cutoff for the radiation generated when the beam impacts a target at normal incidence is reduced by transverse beam size effects, it is nevertheless possible to generate much higher frequencies by a judicious choice of the angles of incidence and observation. These radiation pulses have a broadband spectrum containing frequencies from below 100 GHz to above 1 THz and have a number of potential applications in spectroscopy. Experimental results and comparison with theory are presented.