Numerical Simulation Study On Three Dimensional S-Band Double-gap Output Cavity Klystron

Having been occupying an irreplaceable position in high peak and average power applications, klystron has a limitless application prospect. And as one of the most important aspects of the design and manufacture of klystron, computer aided design(CAD) has become a key mean to shorten the lead time and cost.This paper studies on theoretical analysis and numerical simulation of the S-band double-gap coupled-cavity klystron working in 010 TM mode, by analyzing the small signal theory and large signal theory first. The small signal theory includes space charge wave theory based on Maxwell and Lorentz equation, calculation of the plasma frequency reduction factor in the process of clustering; the big signal theory considers the influence of space charge force in clustering, including nonlinear analysis of the one-dimensional electronic disk and two-dimensional electronic ring model.Electro-optical system design is one of the most basic and important link of klystron manufacture. Cathode emission density, surface compression ratio, perveance, normalized radius and current density of the electron beam were studied first, and then numerical simulation of the electronic gun was carried out. Under the direct voltage of 65 k V, a laminar electron beam was obtained, and its current was about 37.5A, perveance was 2.3 μP, as well as the gun pressure was in a permitted range. Theoretical analyses and calculation of focusing magnetic field were also be carried out, and based on brillouin magnetic field, the electro-optical system was simulated under the focusing magnetic field, but the electron pass rate were very low. Through many experiments and analyses of the results, we found that applying a radial magnetic field could rises the pass rate up to nearly 100%, meanwhile that can maintain a well laminarity.The high frequency structure of the S-band klystron has been studied by CST. Frequency, quality factor Q, characteristic impedance R/Q of the resonant cavity were mainly studied, as well as the influence made by structure size such as cavity length, radius, gap length and shape of the drift tube head. The external quality factor, return loss and standing-wave ratio were calculated, and electromagnetic field distributions in TM010 mode of these cavities especially the double-gap output cavity were simulated in cold cavity model.Finally this klystron was modeled on CHIPIC graphical interface platform, considering size of it, the time needed and correctness for simulation, non-uniform grid partition of 0.15~0.5mm was applied. The simulation area is divided into several segments to avoid memory overflow, and these threads calculated in parallel way can also cut down the time. Applied an input signal with frequency of 3.1GHz, power of about 100 W, an pure output signal was obtained with a frequency of 3.1GHz, and its peak power is 1.7MW, average power is 825 k W. The efficiency of this klystron is 34.3%, the gain is about 39.2d B, and band width is 200 MHz.