A study of the diffraction radiation oscillator for narrowing the diffraction structure

The diffraction radiation oscillator (DRO) differs from the orotron in that the DRO diffraction structure covers only a part of the plane (spherical) mirror rather than the full mirror surface as the orotron grating does. A narrow diffraction structure is easier in fabrication, it allows us to increase the loaded Q of the DRO oscillatory circuit and enhance efficiency of the diffraction radiation feed to the open resonator and it also changes the excited mode content. Considering that the chosen (optimized) width of the DRO periodic structure is more than twice that of the electron beam, we have a good possibility for reducing the grating ohmic loss, which amounts to a half of the DRO total loss, by some more grating narrowing. The problem, however, is that a decrease in grating width together with narrowing the grating-accommodating canal degrades the DRO performance (the combined tuning range is displaced, the starting current rises), and this takes place in spite of the loaded Q enhancement. In this work, our concern will be with the DRO whose diffraction structure about 4-mm. wide is almost equal to the electron beam width, which is about 3.8 mm. The periodic structure is put into a roomy, 10-mm wide canal. The canal is such as to accommodate the optimum-width grating in the previous DRO modification, and it offers more room than the present grating size requires.