A study of absolute instability in TWTs

Summary form only given. This paper will present results of our analysis of the onset of absolute instability at the lower band edge of a traveling wave tube (TWT). We begin with the study of an electron beam propagating in a dielectric waveguide because the exact dispersion relation is easily formulated, and it is similar to that of a gyrotron-traveling wave amplifier in which the occurrence of an absolute instability was previously established [1]. When the Briggs-Bers criterion [2] is applied to the dispersion relation for the dielectric waveguide TWT, however, there does not seem to be an absolute instability regardless of the current level for such a dielectric waveguide TWT. That is, all frequencies that satisfy the pole-pinch criterion of Briggs and Bers are real, regardless of the beam current. Adapting the dispersion relation to a coupled-cavity TWT, we tentatively conclude that there is no absolute instability in the sense of Briggs and Bers at the lower band edge of each pass band. Our preliminary conclusion is therefore that oscillation at the lower band edge is more likely due to reflection at the ends of the tube, and not to the dispersion of the waveguide modes that can produce an absolute instability. The potential for absolute instability at the upper band edges will also be addressed.