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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.

In the bridged vacuum gaps, the secondary electron emission avalanche (SEEA) occurring along the surface of solid insulator and the resultant surface charging play a crucial role in the progress of surface discharge. In order to sophisticate the insulation design of high voltage vacuum devices, analysis and control of SEEA charging are of importance. Boersh et al. have proposed a boundary condition on the surface of an insulator to solve the charge distribution at an equilibrium state of SEEA charging. The boundary condition can be easily adopted for analyzing two dimensional SEEA charge distributions. However, the above condition by Boersh et al. cannot be applied in the case of three dimensional distributions. In this paper, the authors newly developed the boundary condition for three dimensional applications and created a numerical simulation code for analyzing three dimensional SEEA charge distributions. Two types of insulator were examined in this study; one was a solid cylinder and the other a hollow cylinder. In the case of the solid cylinder, electric field strength on the cathode was calculated and compared with the measurement. On the other hand, in the case of the hollow cylinder, the calculated result was compared with the charge distribution experimentally obtained by a movable electrostatic probe system. These calculation and measurement results have shown reasonable agreement and we have validated the newly developed numerical simulation method for three dimensional SEEA charge distributions.

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To characterize factors determining the varietal differences in the fruit size of Japanese plum (Prunus salicina Lindl.), we investigated the relationships among fruit weight, flesh thickness, cell numbers, and radial cell lengths of flesh at the ripening time in 6 cultivars for 2 years. The correlation coefficient between the fruit weight and flesh thickness was higher than 0.9, and the cultivar with large fruits had thicker flesh. There was a varietal difference in cell numbers of flesh, and the cultivar with large fruits had more cells. On the other hand, differences in radial cell lengths of flesh among the cultivars were not clear. In the investigation of correlation coefficients among measurement items, although a correlation was noted between the thickness and radial cell lengths in flesh, the correlation coefficient was low, at about 0.3 to 0.5. Conversely, the correlation coefficient between the thickness and cell numbers in flesh was high, over 0.9. The results revealed that the differences in fruit size among Japanese plum cultivars were markedly influenced by the cell number rather than radial cell length.