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The vacuum requirements to obtain stable field emission cathodes are discussed. To keep the average work function of a tungsten emitter constant, the total pressure of all chemically active gases should not exceed 10−14 Torr. To minimize ion bombardment of the emitter surface the helium pressure should be reduced by ion pumping even with tubes made of an alumino-silicate glass. The relationship between the current density and the slope of a Fowler-Nordheim plot has been calculated. Experimental evidence is given that variations in the work function can be compensated by changes in the field strength in such a way that stable field emitters are obtained. A qualitative explanation of such a mechanism is suggested.
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A. van Oostrom (1962) studied this question.
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