Observation of copious emission at the fundamental frequency by a smith-purcell free-electron laser with sidewalls

A three-dimensional simulation using the Particle-in-cell code “MAGIC” predicted that for a grating with sidewalls, copious emission of coherent Smith-Purcell (SP) radiation at the fundamental frequency of the evanescent surface wave would occur¹. This may be contrasted with the two-dimensional theory proposed by Andrews and Brau (AB) some years ago², for which emission could only occur at harmonics of that frequency. In order to validate this prediction we have performed a demonstration experiment in the microwave domain, using the same set-up as in our previous confirmation of the AB theory³, except that the grating has sidewalls only four cm apart. We observed a forward-moving beam of SP radiation at the predicted frequency after reflection by a plane mirror, since the true emission is in the backward direction, which makes it hard to observe. Agreement with the PIC simulations is satisfactory.