Backward- and traveling-wave tubes with dielectric-lined rippled-wall waveguides

This paper presents the concept of a dielectric backward-wave tube or a dielectric traveling-wave tube, in which the relativistic electron beam guided by a strong magnetic field propagates through a dielectric-lined cylindrical waveguide with a sinusoidally rippled wall, producing very high-power coherent microwave radiation. A linear fluid model is used to study the effects of a dielectric liner in the device. The dispersion relation is derived and then solved numerically. It is found that, in the dielectric-lined sinusoidally rippled-wall cylindrical waveguide, the wave modes can be excited either in the backward-wave case or in the traveling-wave case, and the presence of the dielectric slows the normal modes of the structure, making it easier to achieve traveling-wave tube operation. Numerical results show that a rapid increase in the growth rate occurs when the dielectric constant reaches an optimum value, where a high-frequency mode may possess the largest peak growth rate provided other parameters are reasonably chosen. Therefore, one would expect the device to operate in the high-frequency regime with a much larger growth rate. However, in the limit of no dielectric liner, the results reduce to the regular ones, which show that the growth rate of the fundamental mode is the largest. In addition, if all of the beam parameters remain the same, the dielectric liner enhances the space-charge-limited current of the electron beam, which may also be beneficial for the generation of high-power microwave radiation. © 1996 The American Physical Society.