Gain increases through end of life in traveling wave tubes

The fundamental mechanism for the observed gain increases in traveling wave tubes (TWTs) over burn-in and through end of life has been identified as a damage of the carbon attenuator material inside the tubes due to ion bombardment. The ions are generated by electron beam ionization of the finite base pressure generated in the tube during operation by heating and electron-beam desorption of residual gases on the internal surfaces. A theory that predicts this behavior was previously published and validated over burn-in for different families of tubes from S-Band through Ka-Band. Recently released life test data over periods of one to 15 years has shown the presence of very long-term gain increases of several dB in magnitude as predicted by this theory. In this paper, this data is presented and analyzed in terms of the gain growth theory. Predictions of very long-term gain increases through end of life of various TWTs are made based on these results.