Analysis of high-perveance electron guns

Abstract

High-power microwave tubes such as traveling wave tubes and klystrons still make use of electron guns as the main source of current. The design of these guns can become complicated, and the use of computer-aided design (CAD) techniques will ease and enhance their design. A CAD system based on a supermicrocomputer with an integrated array processor was used to analyze the electric fields in the electron gun of a traveling wave tube. The analysis of the fields was motivated primarily to assess the sensitivity of the space-charge and current distributions to the focus and anode electrode structures and the applied electric field. The space charge has a great influence on the formation of the electron beam and as such on overall device performance. An iterative procedure based on the boundary-element method (BEM) was used to determine the space-charge distributions and the electrostatic fields. The method and its underlying assumptions will be discussed in detail. Electron trajectories are determined by solving the equation of motion using Hamming’s predictor–corrector method, allowing for the effects of space charge. Due to the computational demands of these methods, the inclusion of an array processor in the workstation was essential to accelerate the calculations. The algorithms had to be vectorized to make optimal use of the array processor. The analysis of the electron gun is mainly concerned with shaping the focus and anode electrodes to optimize the electron-beam current entering the interaction area of the tube where amplification takes place. Focusing of the electron beam in the interaction area is taken over by a parallel magnetic field. The optimization of the beam in the electron gun includes finding the correct perveance of the beam, minimizing the interception current to the anode, and finding the correct current distribution of the electron beam where it enters the interaction area. The above CAD analysis techniques allow an accurate design of these parameters, and modifications are easily made.