Study Of Dielectric Window Breakdown And Feed Technqies For High Power Microwave Systems

The power capacity of feed has been an important limitation for the development of high-power microwave (HPM) technology, and it is very meaningful to enhance the power capacity of feed for the practicality of HPM. The power capacity of feed is greatly dependent on the structure form and breakdown characteristics of dielectric window materials. In this dissertation, with several commonly used window materials, the breakdown characteristics of dielectric window and the surface characteristics of dielectric window and theirs effects on the dielectric breakdown process are studied through comprehensive theoretical analysis and experimental verification. Moreover, the HPM mode convertor, rotary joint, and the design technology of the feed are discussed. The work in this paper includes:X-ray photoelectron spectroscopy (XPS), optical microscopy, and infrared spectrum were employed to analyze surface composition. It was found that there is significant difference in chemical composition and atomic structure between material surface and body. At surface, besides the certain composition of dielectric material, such elements as O, N, and K were also found, which are believed to be a major reason for the dramatic decline in surface breakdown threshold.The phenomena of dielectric window breakdown under microwave field are investigated theoretically and experimentally. The general physical process of dielectric breakdown under microwave field is described, and the generation of primary electron, the multipactor process of secondary electron and their effect factors are discussed. The effect of maching and treatment technics of dielectric window on window surface state and dielectric characteristics are also studied. It was found that a long time (24 hours) roasting affects the dielectric loss and body resistance largely. The electrification on the dielectric surface during breakdown process was measured, and the effect of electron trap density distribution on dielectric surface charge accumulation was analyzed. By observing specimen surface before and after breakdown, it was found that at the initial stage, impurities participates breakdown and becomes the weakest point in the breakdown process, the breakdown threshold of dielectric material increases with the improvement of dielectric material surface finish, and surface scratches parallel to the electric field direction reduces the breakdown threshold obviously and deteriorates the breakdown of dielectric surface.A solar blind ultraviolet detector and an X-ray detector were developed for observing surface electroluminescent phenomenon under microwave field. The experimental method of investigating dielectric window breakdown with photoelectric diagnostic tools was established, and the breakdown experiments of 100 kW-class waveguide end dielectric were carried out. Breakdown criterion of dielectric window was obtained, the phenomenon of breakdown point shift was diagnosed with optics and explained theoretically, and it was believed that plasma is the reason for the premature cutoff of transmission wave through the window. The electricity phenomenon at the "triple-point" of waveguide end dielectric window can be explained by adopting the concept of work function,The experimental device investigating the dielectric breakdown caused by high power (GW-class) and narrow pulse (ns-class) microwave was set up, and the experiments were conducted. The breakdown thretholds for several dielectric window materials were achieved, the images of tree-like breakdown channel within the dielectric were obtained, and the reason for this was analyzed. Ultraviolet and X-ray emitted by HPM sources and dielectric window discharging were detected, and the mechanism of primary electron generation during dielectric window breakdown under HPM was brought forward based on band theory. The electricity phenomenon of dielectric window under HPM mainly results from the HPM-sourced X-ray bombardment on dielectric window as Well as local micro-discharge happened at dielectric window surface. It has been proved that HPM-sourced X-ray and ultraviolet are the major reason for desorption and ionization of the gas absorbed at the dielectric window surface. Based on the surface micromorphology analysis after breakdown of dielectric window, the damage of HPM dielectric window can be categorized into two different types. One is caused by micro-discharge sparkling'scorching'the dielectric window surface. The other is caused by the breakdown of dielectric window itself.The feed with equal E and H patterns that was used for an experiment prototype was designed and manufactured, and a high power test of the dielectric window life was carried out. The results showed that the feed window life is more than 6×104 pulses under incident microwave with power larger than 1 GW and pulsewidth of 20 ns. A dual-reflector antenna fed by the three-reflector beam waveguide, a TM01 circular waveguide elbow, and a knockdown rotary joint were proposed and developed, which meet the power capacity requirement of HPM transmission line successfully.