| http://www.w3.org/ns/prov#value | - Therefore, when an electron emitter is operated, in particular, under a medium degree of vacuum, that is, in a residual gas atmosphere of not less than 10???5 Pa, there is a problem that residual gas ions ionized by emitted electrons bombard an emitter, destroy a crystal array of graphite composing the emitter, and thus result in a reduced electric conductivity and deteriorated emission characteri
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