Discussion of factors of backward wave oscillation in millimeter wave helix TWTs

A challenge of high-power millimeter wave TWT is to realize high-power operation without incurring oscillation from backward-wave (BW) interaction. In this paper, the influences of beam parameters such as voltage, current and radius and helix structure parameters of pitch profile on BW oscillation are investigated. In order to get 500W output power, above 12% electron efficiency is considered and 4080W electron-beam power is given. According to the small-signal theory of BW oscillation, starting length and frequency of BW oscillation are calculated; the optimized operating voltages of TWTs for different pitches are calculated by 3-D large-signal interaction codes MTSS. By analysis of the results, factors affecting the BW oscillation length and frequency are discussed in different cases and the conclusion is that larger pitch, higher voltage, lower current and smaller beam radius can be chosen to avoid BW oscillation.