Phase velocity measurements on a broadband TWT

Summary form only given. Phase velocity plays a critical role in TWT operation, but is impossible to measure in conventional tubes. However, using the custom-made XWING (eXperimental WIsconsin Northrop Grumman) broadband TWT we have been able to measure the cold and hot phase velocities in situ. The XWING TWT is a two-stage broadband helix TWT configured with sensors at various points along the axis. These sensors allow us to measure the amplitude and phase of the wave at known positions along the axis of the output stage. By measuring the phase difference between two sensors separated by a known axial distance we have calculated the cold and hot phase velocities in the 2-6 GHz range. Both the simulations and theory predict that the hot phase velocity should be between the cold circuit velocity and the beam velocity, but closer to the cold circuit velocity. The preliminary experimental results are approximately consistent with these predictions. Further investigation is underway to determine the cause of modest inconsistencies.