| 摘要 |
PROBLEM TO BE SOLVED: To extend life of a filament by preventing elements constituting a source gas from infiltrating into an organization of the filament to degrade it. SOLUTION: A sheath 22 is provided with some space left with the filament 12 around a part 13, that is a part of the filament 12, reaching a temperature in which at least an element constituting a source gas 4 (for instance, boron) exists in a liquid phase. COPYRIGHT: (C)2004,JPO
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