Excitation of parasitic waves in forward-wave amplifiers with weak guiding fields

To produce high power coherent electromagnetic radiation at frequencies from microwaves up to terahertz, the radiation sources should have interaction circuits of large cross-sections, i.e., the sources should operate in high-order modes. In such devices, the excitation of higher order parasitic modes near cutoff where the group velocity is small and, hence, start currents are low can be a serious problem. The problem is especially severe in the sources of coherent, phase-controlled radiation, i.e., the amplifiers or phase-locked oscillators. This problem was studied earlier¹ for the case of electron focusing by strong guiding magnetic fields. For many applications it is desirable to minimize these focusing fields. Therefore in this paper we analyze the problem of excitation of parasitic modes near cutoff in forward-wave amplifiers with weak focusing fields. First, we study the large-signal operation of such a device with a signal wave only and analyze the effect of a strong signal wave on the axial and transverse motion of electrons. Then, we analyze the self-excitation conditions of parasitic waves near cutoff in the presence of the signal wave. It is shown that the main effect is the suppression of parasitic waves in large signal regimes. At the same time, there is a region of device parameters where the presence of signal waves can enhance excitation of parasitic modes. The role of focusing fields in such effects is studied. A general treatment is illustrated by an example of an X-band traveling-wave tube driven by a thin annular 30 kV, 5 A electron beam and consisting of a 30 period slow-wave structure. It is shown that to provide the beam transport and simultaneously increase the interaction efficiency due to the electron transverse motion in such a tube it is enough to have focusing fields about 100 Gauss or less.