NDLI: Life-limiting mechanisms in Ba-oxide, Ba-dispenser and Ba-Scandate cathodes

Content Provider	IEEE Xplore Digital Library
Author	Gaertner, G. Barratt, D.
Copyright Year	2004
Description	Author affiliation: Philips Res. Labs., Germany (Gaertner, G.; Barratt, D.)
Abstract	Sufficient thermionic cathode life of several years in a vacuum electron tube environment at increased emission current density is one of the key parameters in advanced tube development. When improvements are investigated, accelerated life tests are applied in order to quantify their impact. Hence the investigation of cathode life limiting effects is the basis of accelerated life predictions. A quite general dependency of cathode life for different types is given by the dependence on operating temperature, due to the fact, that a lot of processes involved scale with temperature, such as chemical supply reactions, transport, diffusion, desorption and evaporation processes. Typically these processes are accelerated when the temperature is increased, and hence lifetime decreases. Other life limiting mechanisms are changes induced in the cathode when drawing current, which can be intrinsic changes, which are most pronounced in oxide cathodes and usually are irreversible, and/or surface changes by ion bombardment, where the ions are created by the extracted electrons via ionization of the residual gas. The removal of surface monolayers containing Ba by ion bombardment usually is reversible especially for Ba dispenser or I (impregnated) cathodes (by resupply of Ba), if the ion current is reduced for example, by lowering the residual gas pressure by continuous pumping. The third class of life limiting mechanisms is given by the presence of poisoness chemicals usually in form of gases in vacuum tubes, which can either affect chemical supply (dispensing) reactions or directly poison emission by adsorption of high work function surface monolayers on top of the emitting surface or f.i. oxidize base metal surfaces. This influence usually is the strongest in the activation and early life stage, where the amount of poisonous gases, e.g. containing oxygen, is highest, but is then continuously reduced by pumping. Usually a clean handling of the vacuum components and a good vacuum processing (baking and degassing, efficient pumping) is needed to avoid detrimental influences on cathode life.
Starting Page	59
Ending Page	61
File Size	187548
Page Count	3
File Format	PDF
ISBN	0780384377
DOI	10.1109/IVESC.2004.1413958
Language	English
Publisher	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Publisher Date	2004-09-06
Publisher Place	China
Access Restriction	Subscribed
Rights Holder	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Subject Keyword	Surface cleaning Temperature dependence Gases Thermionic emission Life estimation Electron tubes Acceleration Current density Chemicals Cathodes
Content Type	Text
Resource Type	Article

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