Super-exponential Amplification of Wavepacket Propagation in Traveling Wave Tubes

Abstract: We analyze wavepacket propagation in traveling wave tubes (TWTs) analytically and numerically. TWT design in essence comprises a pencil-like electron beam in vacuum interacting with an electromagnetic wave guided by a slow-wave structure (SWS). In our study, the electron beam is represented by a one-dimensional electron flow and the SWS is represented by an equivalent transmission line model. The analytical considerations are based on the Lagrangian field theory for TWTs. Mathematical analysis of wavepacket propagation in one-dimensional space is based on the relevant Euler-Lagrange equations which are second-order differential equations in both time and space. Wavepacket propagation analysis is not simple and we develop a numerically efficient algorithm to perform the analysis efficiently. In particular, when the initial pulse has a Gaussian shape at the input port, it acquires non-Gaussian features as it propagates through the TWT. These features include: (i) super-exponential (faster than exponential) amplification, (ii) shift of the pulse frequency spectrum toward higher frequencies, and (iii) change in the shape of the pulse that becomes particularly pronounced when the pulse frequency band contains a transitional point from stability to instability.