On the feedback in the Smith-Purcell experiment

Refers to the well understood mm-wave region with a standard accelerating voltage 3-5 kV. The positive feedback is held not on the zeroth but minus first space harmonic of the grating surface wave, i.e. the modulated electron beam is self-excited in BWT mode at /spl lambda//spl ap/ 11 mm. The high time harmonics of the modulated electron beam are radiated under certain conditions nearly orthogonal to it as fast (volume) waves. Just an experimental verification of the near-millimeter radiation of the fast (volume) waves not from externally modulated but self-excited surface eigenwave mode electron beam is proof that the coherent radiation of light on fast (volume) waves is due to the nonrecognized Smith-Purcell beam automodulation by infrared surface eigenwaves of the planar reflecting grating.