| 摘要 |
Cylindrical type sealing electrodes(22a-c) having same length of d1 are mounted downwardly on the separating plate(20) having three holes(18a-c) in a 3rd grid cap(16). Canonical type sealing electrodes(23a-c) are also mounted upwardly on the separating plate, but the center electrode is separated with a distance of d2 from the cap. The distance of d1 is 1/10-1/15 times of d3 which is the distance from the cap to the separating plate, and d2 is 1/3 of d3 so that the mutual-interference of the electric field between the three holes are sealed and the distribution of each electric field generated frdom the hole is constant.
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