Efficiency improvements in a 12 GHz-50 W space TWT

Two main directions have been followed in the research and development of space traveling-wave tubes (TWTs) in order to improve overall efficiency. First there was the reduction of microwave losses along the helix line using a copper coating on the tungsten wire. Losses have been reduced from 14% of the output power to less than 9%. This means a direct enhancement of 3% to 4% in efficiency and makes it possible to achieve close to 60% overall efficiency on a 50-W, 12-GHz tube and over 60% on a 130-W, 12-GHz TWT. Improvement of the collector was the second main direction of development. By analyzing the energy spectrum of the electrons as they enter the collector, it is possible to estimate the loss due to the interception of high-energy electrons by low-voltage electrodes. It appeared that a theoretical efficiency of more than 67% could be obtained with an optimized four-stage collector. More accurate computation of electron trajectories made it possible to design a collector and measure an overall efficiency (including heater power) of 63% on a 50-W, 12-GHz TWT.>