Frequency domain simulation of drive induced oscillation in a coupled-cavity TWT

Summary form only given.Drive induced oscillation (DIO) is an instability that limits the achievable output power in wide-band coupled-cavity TWTs. It is thought to be due to the synchronous interaction of the slowed electron beam near the output end of the beam-circuit interaction region under large signal conditions. This DIO instability occurs at the high frequency edge of the circuit passband where the beam interaction impedance is high. This phenomenon is therefore highly non-linear, involving the simultaneous presence and interaction of signals of different frequencies. One approach to simulating DIO is to use a large signal time-domain simulation code, as described by Ayers and Zambre. However, this approach can consume considerable computing time. An alternative approach, which we report here, employs a multi-frequency, frequency domain model, in which the stability of the amplifier is tested by introducing a small amplitude test wave with frequency near that of the 2pi-mode; the frequency of the test wave and the drive power of the amplifying signal are swept until a very large value of gain is observed for the test wave, indicating onset of oscillation. We have successfully used this approach to analyze DIO in a new Ka-band communications TWT design, using the CHRISTINE-CC large signal coupled-cavity TWT simulation code. Results from our analysis, including plots of test wave gain vs. frequency for various values of drive power, will be presented. We will also demonstrate the effect of output circuit velocity changes on the threshold power required for DIO onset.