Translated Abstract
Dielectric window is an important component of high power microwave (HPM) generation device, through which HPM can radiate from vacuum to atomphere. However, breakdown easily occurs on vacuum/dielectric interface when HPM passes through dielectric window. Surface breakdown limits the generation and transmission of HPM, and becomes the bottle neck of HPM technology development. Because of the complexity of breakdown on dielectric window in vacuum, the mechanism is yet unknown. Thus, there is great economic and academic significance to improve the progress of HPM technology in our country by researching breakdown phenomenon, understanding the mechanism and seeking the effective suppression technology of dielectric window breakdown under HPM in vacuum.Organic polymers materials can be processed into complicated and large size dielectric window easily, which can meet the requirements of large power microwave equipment. Therefore, several typical organic dielectric window materials have been investigated in this research, such as, the polytetrafluoroethylene (PTFE), low-density polyethylene (LDPE), high-density polyethylene (HDPE) and polymethyl methacrylate (PMMA). Basic dielectric properties, surface trap parameters, secondary electron emission yield and desorbed gas properties of these mateials have been measured. The measurement results indicate that, PTFE has the higher resistivity and electron trap density, and have the lower dielectric loss, secondary electron emission yield and desorbed gas content. PMMA has the higher dielectric loss and desorbed gas content. Except the higest secondary electron emission yield, other dielectric properties of PE are between PTFE and PMMA. Comprehensive analysis shows that, PTFE exhibits the excellent dielectric properties, which is an ideal organic dielectric window materials, and PMMA has the worst properties as dielectric window material, while PE is between PTFE and PMMA.The surface breakdown properties of typical organic dielectric window materials have been investigated under GW HPM. It is showed that, dielectric properties of materials have influenced surface breakdown threshold, under the same HPM, PTFE has the highest threshold for its lower secondary electron emission yield and desorbed gas content. PMMA has the lowest threshold because of its higher desorbed gas content. Breakdown thresholds of HDPE and LDPE lie between PTFE and PMMA. The characteristics of destroyed samples have been analyzed, research shows that, the surface damage channels of sample are parallel to microwave electric field, and there are dot-like damage, filament-like damage and tree-like damage on its surface. With the increase of breakdown time, the tree-like damage developed along the surface and also to the interior of material. A model is proposed to describe this phenomena, it is suggested that the electric field results in the tree progress along the material surface, and the tree developing into the material inside under magnetic field and surface electrostatic field. The mechanism of tree-like damage has been analyzed, it is suggested that surface local defect induces tree-like damage, while multipactor heat deposition and discharge in micropore promote tree-like damage development.As PTFE is an excellent dielectric material, surface breakdown suppression technologies have also been studied, i.e., surface coating, surface polishing and surface notching treatment. It is indicated that, surface coating is invalid to change the breakdown threshold for the process limitation in our country. Surface polishing with the 3000 grit sandpaper can increase the breakdown threshold, and reduce the damage of material. The periodic rectangular grooves perpendicular to microwave electric field can improve the threshold greatly in a certain power range, and the effect is influenced by the depth and width of grooves. The basic principle of periodic rectangular grooves suppression is that, when the multipactor electrons from groove bottom impact on the side, the electron energy is reduced, then the multipactor electrons are suppressed on groove side, and the number of electrons is decreased, therefore surface breakdown is suppressed.Based on particle-in-cell (PIC) and Monte Carlo method, a simulation model of dielectric surface breakdown under HPM in vacuum is established, and the electron movement and the change of electron multipactor have been simulated. The simulation results reveal that the shape of electron movement trajectory under GHz is parabola-like, The emergence angle of electrons and microwave field parameters affect the electron movement obviously. Both the multipactoring electron number and electrostatic field oscillate periodically with the microwave field， and their oscillating frequency is the twice of microwave. The periodic oscillation is considered because of electron impact energy periodically oscillating with the phase of microwave field. The physical process of rectangular grooves suppression breakdown has also been simulated, and the movement of multipactoring electron in grooves is obtained. The results show that the electron impact energy on groove side is decreased significantly, especially the impact energy of emissive electron from side is only several eV, which is not enough to generate new secondary electron. So the multipactor is suppressed on groove side, these results are consistent with experimental results and theoretical analysis.
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