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Abstract
In this paper, three surface polishing treatments were employed to treat plate titanium electrodes, and microscopic surfaces of the electrodes after polishing were presented. Through comparing the breakdown strength of the 2.5 cm vacuum gap formed by plate titanium electrodes after the three treatments, experimental results showed that the breakdown strength was enhanced by 35% while the micro-surface roughness dropped from 3.5μm to 0.35μm. In view of that, effects of microstructural parameters after polishing on the microscopic field enhancement factor were investigated. The field-uniformity mechanism and the shield effect between micro-protrusions on the rough electrode surface were put forward and demonstrated. Based on the idea that electric field can be shield in a pit, a theoretical model was established to evaluate the maximum field enhancement factor βEmax on the micro-surface. It revealed that 1 ≤ βEmax ≤ 3.96, and βEmax had the maximum decrements of 1.96 and 2.1 both from 3.96 after the mirror polishing and the chemical polishing, respectively. When the surface roughness decreased to the scale from nm to μm, the effort on βEmax reduction through surface polishing was not effective to enhance the vacuum gap breakdown strength any more.
Highlights
Electric breakdown of a vacuum gap means a phenomenon that sparks form in the gap and intensive arc current flows across the vacuum gap when the gap voltage is enhanced to a certain level
Through comparing the breakdown strength of the 2.5 cm vacuum gap formed by plate titanium electrodes after the three treatments, experimental results showed that the breakdown strength was enhanced by 35% while the micro-surface roughness dropped from 3.5μm to 0.35μm
It revealed that 1 ≤ βEmax ≤ 3.96, and βEmax had the maximum decrements of 1.96 and 2.1 both from 3.96 after the mirror polishing and the chemical polishing, respectively
Summary
Electric breakdown of a vacuum gap means a phenomenon that sparks form in the gap and intensive arc current flows across the vacuum gap when the gap voltage is enhanced to a certain level. Vacuum gap breakdown becomes a hard problem which restricts the insulation and stability improvements of high-power vacuum electron devices.[1,2,3] Some researchers believed that electric field on microscopic surface and the field induced electron emission from electrode can be depressed by reducing the field enhancement factor of the microscopic protrusions on electrode surface,[4,5,6] so that the vacuum gap breakdown strength can be enhanced. Other researchers demonstrated that the breakdown strength of a vacuum gap was enhanced when the electrode roughness was reduced, but the limited enhancement was far less than the anticipation.[7,8,9,10] Electrical breakdowns in small vacuum gaps were studied in Refs. Important questions can not be answered yet at present, such as how does polishing treatment change the micro-surface parameters, and how do these parameters influence the microscopic field enhancement factor and the field strength. Theoretical model is established to explain the experimental results and the effects of the three polishing treatments
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