Design study of a two-cavity monotron

In its classical embodiment, the monotron has its conversion efficiency limited to 20.0%. But we demonstrate here that on considering a stepped electric-field axial profile the monotron efficiency can attain the theoretical maximum value of 57.0%, almost three times as high as that obtained from the conventional uniform field distribution. This is accomplished by using two cavities electromagnetically coupled by an annular slit crossed by an electron stream. From a one-dimensional analysis, a two-cavity monotron is developed to operate in the TM₀₁₀ mode at 4.0 GHz when self-excited by a 20 keV, 10 A hollow electron beam. The device operation is examined through a 2 1/2-D particle-in-cell (PIC) simulation giving a 47.5% conversion efficiency. Departure from the theoretically predicted efficiency is explained by beam-thickness effects.