Insulation Structure Design And Vacuum Sealing Study For The High Current Diode

Vacuum diodes are among the most important parts of high electron beam accelerators. With the development and application of High Power Microwave (HPM) technology, there are needs to miniaturize the diodes, increase the hold-off voltage, and maintain high vacuum sealed state for long time. In the thesis, use of alumina ceramic as the water-vacuum insulator is discussed. The design of insulation structure is investigated theoretically, numerically, and experimentally. Meanwhile, the material outgassing characteristic and ceramic-metal jointing are also investigated.Firstly, the mechanism of vacuum surface flashover and affecting factors, and the design rules for radial insulation are generalized. Then cone and disk type insulators are compared numerically: the disk type is better for its simple machining and uniform electric field distribution. Further, three disk insulation structure designs are proposed and evaluated. Area weighted statistical method is applied to calculate the surface breakdown field. Results show that the design with a grading ring near the cathode is effective in reducing the triple junction field and satisfies the requirement of voltage hold-off. Based on disk type insulators, the insulation structure is further optimized: a"cone-column"anode outline and a cathode shielding ring were adopted. By adjusting the outline of the anode and the shielding ring, the electric fields were more evenly distributed and the field around the cathode triple junction was controlled. High voltage experiments were carried out for above design on water-isolated spiral PFL accelerator. Results show that diode can work stably under 420kV, 200ns conditions.In order to make vacuum sealing good, outgassing properties of materials is also investigated. A pumping model is set up to simulate the pump down time and material outgassing rate. Experimentally, the outgassing characteristics of materials between normal temperature and heated state are compared. Results show that heating can efficiently accelerate the outgassing rate. From above data, the time of maintaining vacuum under 10-2Pa is estimated.Finally, considering the special structure details used in ceramic-metal jointing, suggestion is given to avoid field enhancement around those details: a bushing structure with application small size ceramic is presented. Mechanical strength is also critical for these details, as electric water hammers can destroy the ceramic, so the inherence frequency of ceramic vibration is calculated by the method of mode analysis, which can aid the analysis of mechanical strength.