Recent Developments in Advanced Design Codes for Vacuum Electronic Devices

Computer-aided design of modern vacuum electronic devices such as travelingwave tubes (TWTs), klystrons, and gyrotrons has become standard in both the research and industrial communities. The physical processes at work in these devices are for the most part described by the Maxwell-Lorentz equations. However, the various physical processes that govern device behavior span widely disparate spatial and temporal scales, posing serious challenges to the development of accurate computational design tools with efficient run-times. To address these modeling and simulation challenges, several approaches and techniques have been developed over the past few decades. Ideally, a fully selfconsistent, end-to-end simulation of the device is desired. However, such a simulation is prohibitively computation-intensive for most device types, particularly if parameter scoping and design optimization are involved. Thus, despite all of the advances in processor speed and parallel computing techniques, state-of-the-art computational resources remain insufficient for the magnitude of the task. Instead, the general approach has been to break the problem up into smaller, more manageable sub-problems, to develop algorithms and codes to model basic phenomena for each sub-problem, and to integrate the results by having the various codes communicate with each other. Thus, the entire radiation generation process has been divided into key physical sub-processes: beam generation, beam propagation, beam-wave interaction, beam collection, wave injection (in the case of an amplifiers), wave extraction, and thermal management. Each of these sub-processes (with the exception of thermal management) is described by a set of equations derived using the general set of Maxwell-Lorentz equations where, by necessity, the models are a compromise between sufficient complexity to capture the basic physics of the particular sub-process and simplifications made to reduce computation time. In the past, this approach has been very successful and has led to the development of a number of parametric design codes that have been used to design the current generation of vacuum electronic devices. However, compromises in model and algorithm complexity have had an effect on the accuracy of the codes – it is often necessary to fabricate multiple experimental prototypes before achieving the desired performance in a given device. The inefficiency and cost associated with the need for