Observation of coherent smith-purcell emission at 32 GHz from a multi-channelgrating with sidewalls

Summary form only given. The two-dimensional theory of Smith-Purcell free-electron laser predicts that coherent radiation can occur only at harmonics of the frequency of the evanescent wave that is resonant with the beam. Particle-in-cell simulations and a demonstration experiment at 5 GHz have shown that in a three-dimensional context, where the lamellar grating has sidewalls, coherent Smith-Purcell radiation is copiously emitted at the fundamental frequency with 10 % efficiency. Theory shows that a rescaling of all grating parameters at constant energy leads to a rescaled wavelength for the emitted radiation, accompanied by a reduction in power proportional to the square of the scaling factor. Both simulations and antenna array theory suggest that N-fold scale reductions can be partially compensated by laterally juxtaposing N copies of the reduced grating. Both the power and the solid angle into which it is emitted are reduced by 1/N. To confirm these predictions, a new experiment, with a seven-channel grating operating at 32 GHz has been performed. We present in this talk the main results in comparison with MAGIC 3D simulations. Provided that further reduction in scale can be achieved, a coherent Smith-Purcell radiation source in the Terahertz frequency range can be foreseen.