Study Of Surface Flashover Mechanisms And Inhibition Methods For Ceramic-vacuum Interfaces In High-current Diodes

Hard-tube high current diodes are needed due to the practical applications of high power microwave?HPM?technology.Ceramic-vacuum interfaces are among the most important components of high current diodes,and the surface flashover seriously restricts their further applications in the field of HPM.In this dissertation,the surface flashover of ceramic-vacuum interface is researched through theoretical analysis,particle in cell?PIC?simulation and experimental methods.A dynamic PIC simulation model of surface flashover development is established,and the influencing factors of flashover are studied.A comprehensive surface flashover plasma diagnostic platform is built,and the development process and characteristic parameters of flashover plasma are obtained.A surface treatment method to inhibit surface flashover is proposed,and the insulation reliability is analysised.These works set a good foundation for the application of ceramic in the repetitive and long pulse high-current diodes.The detailed contents and innovative work includes the followings.1.Dynamic PIC simulation of surface flashover processes along ceramic-vacuum interface.Based on the mechanism of surface flashover,a flashover dynamic simulation model of ceramic-vacuum interface is established via VSim.The flashover process was obtained by analysizing the s econdary electron emission coefficient,secondary electron multiplication and collision ionization.The influences of secondary electron emission coefficient,surface desorption gas,surface charge on surface flashover for ceramic vacuum interface are analyzed.Surface coating treatment wich can reduce the secondary electron emission coefficient and surface corrugation which can block secondary electron multiplication process are proposed to inhibit surface flashover.2.Comprehensive diagnosis of surface flashover plasma for ceramic vacuum interface in long pulse high current diodes.A comprehensive diagnostic platform for surface flashover plasma characteristics of a disk-shaped ceramic vacuum interface?diameter greater than 400 mm?under high voltage with a pulse width of 100nanoseconds was established.The flashover was induced from the cathode triple junction emitter,and the flashover plasma luminescence process is recorded by a high-speed framing camera?HSFC?.The luminescence process can be divided into three stages:discharge channel formation sstage,branch channel discharge stage and the last ending stage.The branch discharge channel appeared in the end of main discharge channel was formed due to the relase of accumulated charge at the ceramic surface defects.The surface flashover luminescence propagation velocity is about 150 cm/?s.Radial velocity component shows that it gradually decreases during the flashover discharge channel formation stage,almost unchanged in branch channel discharge stage,and increases in the final ending stage.The angular velocity component is generally small,and only increases in the branch channel discharge stage.The spectral diagnostic results confirm that both the ceramic material and desorption gases take participate in the flashover process of the ceramic surface.The average flashover plasma temperature is 4.57 eV which leads to stronger gas adsorption than lower voltage conditions,due to it is almost equivalent tosurface adsorption gas desorption activation energy.The large propagation velocity can cause a larger instantaneous loss current and the higher plasma temperature increases the collision desorption cross-sectional area.All these effects can lead to stronger electron stimulated desorption and thermal desorption.3.Based on surface treatment,a ceramic surface flashover inhibition method is proposed and its insulation reliability is analyzed.From the perspective of reducing secondary electron emission coefficient of ceramic,a surface coating treatment is proposed.High temperature?1200°C?and low temperature?800°C?surface Cr₂O₃coating were made on ceramic surface.Surface element composition and crystal phase structure of ceramics with different coating treatments were obtained by energy spectrum analysis?EDS?and X-ray diffraction analysis?XRD?.The results show that the ceramic surface with high temperature coating is much purier than that with low temperature coating.Scanning electron microscopy?SEM?and laser scanning confocal microscopy?LSCM?results show that these two coating treaments can reduce the surface roughness from 0.87?m to 0.32?m and 0.49?m,respectively.With this coating treatment,the secondary electron coefficient decreased to about half of the initial secondary electron emission coefficient,and the maximum secondary electron emission coefficient decreased from 3.5 to 1.5 and 1.7,respectively.From the point of restraining secondary electron multiplication process,two typical surface corrugated structures,uniform surface corrugation and non-uniform surface corrugation which the amplitude of corrugation in the middle region of ceramic is higher than that in the area around anode and cathode,was desigened.High voltage tests of ceramic interfaces with different surface treatment were carried out on a long-pulse repetitive platform.Results show that the surface coating treatment can improve the ceramic flashover voltage under single pulse.Comparing to low temperature coating treatment,the high temperature coating treatment has a larger improvement which can increase flashover electric field to 36.4 kV/cm with a reliability is about 90%.The surface uniform corrugation and non-uniform corrugation can increase the flashover voltage by 6%and 14.4%,respectively.As for repetitive pulses,the high temperature coating treatment and non-uniform corrugated surface treatment can both improve flashover voltage and reliability.Their maximum improvement of flashover electric field is 23.8%and 25.4%,respectively.Surface uniform corrugation treatment can only improve insulation reliability of ceramic interface a little.The low temperature coating treatment just can increase high voltage for high repetitive rate.A basic method for evaluating the insulation reliability of ceramic vacuum interface after different surface treatments is proposed.