Improved Model of Long-Term Gain Increases in Traveling-Wave Tubes

An improved model to predict the gain increases in traveling-wave tubes (TWTs) during long-term operation is presented. The conventional gain growth model describes the pressure variation in a TWT over its life using an exponential decrease from the initial outgassing level to a constant base pressure. This model often shows an inability to capture the gain change behavior of many tubes during the transition between early life burn-in and long-term operation, leading to a significant underprediction of long-term gain increases. The model is improved here first through the introduction of another pressure-related term associated with desorption of gas from the tube´s inner surfaces that exhibits a t^-1/2 time dependence. This new pressure dependence is governed by the behavior of a Langmuirian adsorption isotherm. Second, the exponential pressure decay term is separated into two terms associated with early life and long-term operation with different outgassing time constants. The improved model shows a significantly better matching of long-term TWT gain growth data compared to the conventional model. In addition, the improved model predicts a more physical pressure behavior in the TWT with time.