| 摘要 |
PROBLEM TO BE SOLVED: To prevent the lifetime of an electrode from shortening and prevent the inner wall of a bulb from blackening while shortening the transferring time from the glow discharge to the arc discharge and while stabilizing the arc discharge by providing an electron emitting element formed of the granular bulk material for emitting the thermion and a discharge concentrating means for concentrating the discharge to an exposed surface while being arranged close to or contacting with at least one part of the exposed surface. SOLUTION: Arc discharge is generated from an exposed surface 55 of the most surface layer of an electron emitting element 5 housed in a container 6, in detail, from a surface of grains 51 in contact with the inner wall surface of an opening 62. Since the electron emitting element 5 made of the semiconductor porcelain (ceramics) has a large electrical resistance, current hardly flows, and arc spot is generated from grains 51... of the electron emitting element 5, which contacts with or arranged close to the inner wall of the container 6 as a conductor part. This arc spot is moved from the grain 51 to the adjacent grain 51 with the dispersion of the radioactive material, and discharge is thereby continued. As a discharge concentrating means, the container 6 works.
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