Modeling of Drive Induced Oscillation in a coupled cavity TWT

Summary form only given. Broadband amplifiers employing an electron beam interaction with slow waves supported by a periodic structure are often vulnerable to spurious oscillations at stop-band frequencies. The Drive Induced Oscillation (DIO) is a spurious oscillation that develops in the large signal regime and is attributed to the near synchronism of the slowed-down beam and the 2π mode of the periodic structure. Broadband coupled-cavity TWTs (CC-TWT) are limited in their achievable output power primarily due to DIO. Further increasing the output power of CC-TWTs requires developing adequate simulation tools that are capable of accurate prediction of DIO onset. Major computational challenge in DIO simulations is the non linear nature of this phenomenon. The computer codes, CHRISTINE-CC and TESLA-CC, are capable of accurate modeling of the non-linear self-consistent beam wave interaction and have been expanded to the multi frequency regime for DIO simulations. We compare levels of agreement between one-dimensional (1D), CHISTINE-CC, and two dimensional (2D), TESLA-CC, approaches and measurements of broadband Ka-band CC-TWTs. The effects of the focusing magnetic field and the structure period profile on DIO onset have been studied and compared for the 1D and 2D approaches. Measurements show that the frequency of the DIO in a broadband Ka-band CC-TWT is equal to the frequency of 2π mode of the periodic structure and the maximum achievable power before the DIO starts is practically constant across the operating bandwidth.