Flashover mechanisms of bridged vacuum gaps based on cathode electric field measurement

Abstract

In bridged vacuum gaps, it is believed that the secondary electron emission avalanche (SEEA) and the resultant insulator charging play a crucial role in the progress of discharge leading to the surface flashover. This paper deals with the electric field just before the flashover that includes the accumulated SEEA charge component (E sF ) and applied geometrical field component (E gF ) on a cathode and their relation to the flashover voltage. The test gap consists of parallel disc electrodes bridged by an insulating solid cylinder made of Al 2 O 3 . The test insulator changes its length from 5 to 50 mm. As it has been clarified in our previous works that surface roughness of the insulator affects SEEA charge accumulation, we have examined the roughness ranging from 0.2 to 6 μm. Aluminum electrodes are mainly examined, and copper and stainless electrodes are used for comparison. A ring type electrostatic probe embedded near the triple junction on the cathode is employed to measure the electric field. As a result, the total field strength (E gF + E sF ) for various insulator conditions with aluminum electrodes is 6.7 kVmm -1 as an average. However, the rate (γ) of SEEA charge component that accounts for the total electric field varies according to the insulator length and the roughness. When γ exceeds ca. 15%, the flashover voltage V b depends on the square root of the insulator length. On the other hand, V b is a linear function of the insulator length when the SEEA charging is depressed and thus γ is smaller than 15%. On the condition that γ<15%, the material of electrode makes an impact on the flashover voltage.

Evaluation of a factor for calculating SEEA charge on insulator surface in vacuum

“…As shown in the Figure, the value of k can be expressed as a function of R a , and it increases with the roughness. This means that the SEEA charge density decreases with the increase in R a [2]. By using the same method, we have obtained the values of k for Pyrex® and PMMA.…”

“…Since the bumps on the insulator surface act as barriers for electrons in the avalanche to prevent an SEEA [2], the surface was polished or roughened in order to control the SEEA charge accumulation. The average roughness R a was 0.2, 1, 3 or 6 m. For estimating k value, we used the insulator with H=50 mm in length for each roughness.…”

“…The charging of an insulator in vacuum occurs via a process called Secondary Electron Emission Avalanche (SEEA) [1,2] and is believed to trigger the flashover. The charge accumulated on the surface due to the SEEA reaches an equilibrium state in which the number of emitted electrons equals to that of the injected electrons over the surface.…”

Review of interface tailoring techniques and applications to improve insulation performance

“…[48] and Naruse et al. [49] investigated the influence of surface roughness on flashover voltage for different materials such as Al 2 O 3 , SiO 2 and PTFE, indicating that rougher surfaces present higher flashover voltages. A CO 2 laser generator was adopted to create scratched lines on the surface of alumina ceramics in [50], and it was found that a much higher flashover field strength was gained with a larger scratching density.…”

“…This is possible because rougher surfaces are less sensitive to the impacts of electrons at shallow angles and thus suppress the secondary electron emission avalanche process [47]. For instance, Yamamoto et al [48] and Naruse et al [49] investigated the influence of surface roughness on flashover voltage for different materials such as Al 2 O 3 , SiO 2 and PTFE, indicating that rougher surfaces present higher flashover voltages. A CO 2 laser generator was adopted to create scratched lines on the surface of alumina ceramics in [50], and it was found that a much higher flashover field strength was gained with a larger scratching density.…”