Two dimensional modeling of drive induced oscillation in a coupled cavity TWT

Summary form only given.Drive Induced Oscillation (DIO) limits the achievable output power in broadband coupled-cavity TWTs. DIO is attributed to the near synchronism of the slowed-down beam and the 2?? mode of the output circuit under large signal conditions. One-dimensional time-domain ´disk model´ simulations of DIO have been reported. However, these simulations neglect important 2D effects like beam expansion and the radial dependence of bunching. We report on the development of a 2.5D multi-frequency large signal model of coupled-cavity TWTs that is capable of simulating DIO. Our model is based on the 2.5D klystron design code TESLA, to which the Curnow model for coupled cavity circuits has been added. The new code, TESLA-CC, allows for the simultaneous excitation of signals at two independent operating frequencies together with their temporal harmonics. The code preserves TESLA´s capabilities to provide exact solutions of Maxwell´s equations in the presence of an electron beam. TESLA modeling of DIO illustrates the sensitivity of the oscillation to the type of magnetic focusing system used. It is shown that 2D electron beam bunching plays an important role in TWT stability and DIO excitation.