Design facts in the axial monotron

Upon reexamining the electron beam radio frequency (RF)-field interaction of the monotron, which is the simplest transit-time microwave tube, it is found that a 20% maximum conversion efficiency can be attained at weakly relativistic beam energies (∼200 keV). It is shown that the conversion efficiency can be cast as a function of three parameters, namely, injection beam energy, resonant frequency, and electric field strength. From this fact, a design procedure of how the optimum operating parameters should be selected is presented. In support of the analytic study, 2.5-D particle-in-cell simulation of a transverse magnetic (TM)₀₂₀, 6.7 GHz axial monotron operating at 10 keV, 70 A beam parameters has given a tube efficiency of 15.4%, while one-dimensional (1-D) analysis assuming a strictly monoenergetic beam has predicted a maximum theoretical efficiency of 18.5% at a beam energy of 10 keV.