Two different mode interactions in an electron tube with a Fabry—Perot resonator—The Ledatron

An electron tube with a Fabry-Perot resonator for the generation of millimeter and submillimeter waves, the Ledatron, has been investigated both theoretically and experimentally. Two different mode interactions, Fabry-Perot mode and surface wave mode, were predicted and found to exist. These two modes can be separated by proper selection of the mirror spacing of the Fabry-Perot resonator in the tube. These two mode oscillations have different characteristics. In the case of the Fabry-Perot mode, the oscillating frequency is tuned mainly by variation of the mirror spacing, that is, mechanical tuning is predominant. On the other hand, in the case of the surface wave mode, electronic tuning predominates. For gratings of the same physical size, the surface wave mode oscillator needs a larger electron accelerating voltage than the Fabry-Perot mode oscillator in order to obtain the same wavelength. The experimental results are in good agreement with our theory of operation.