The elimination of "Ring mode" oscillations in a high-power traveling-wave tube

A traveling-wave amplifier using a cross wound helix or the approximate equivalent, the ring bar circuit, when designed near the maximum limits of power and gain is susceptible to oscillations in a mode known as the "ring mode." The cutoff region of this mode has a range of phase velocity that generally encompasses that of the fundamental helix mode used for amplification. If other factors such as impedance and gain are sufficiently competitive with the fundamental mode, strong oscillations can ensue preventing the tube from functioning as a useful amplifier. Such oscillations have, in fact, been observed on a tube udng a ring bar circuit and at a frequency in the region of ka = 1. The approach used toward eliminating these oscillations was a hot and cold test study to identify the nature of the oscillation, consideration, and trial of methods intended to shift the frequency or phase velocity of this mode beyond the range of usual fundamental mode operation, decoupling of the beam, and use of preferential methods of attenuation. The suppression of "ring mode" oscillation was achieved without excessive deterioration of those characteristics desired in a high powered traveling-wave amplifier through the combined use of several of the above approaches to the problem. Although a description of the "ring mode" has appeared in the literature, oscillation in this mode and its elimination are believed to be novel and of fundamental importance.