Isato Watanabe

Formation Mechanism of a Monoatomic Order Surface Layer on a Sc-Type Impregnated Cathode

The electron emission of a Sc-type impregnated cathode is known to be enhanced by a monoatomic-order surface layer composed of Ba, Sc, and O. This layer reduces the work function much lower than those of conventional impregnated cathodes.1,2) It also makes the cathode resistant to gas contamination.3) In the present work, a basic impregnated cathode is coated with a thin tungsten film containing a certain amount of Sc2W3O12 (about 5–10 atomic percent) to produce a (W-Sc2W3O12) coated cathode. The supplying mechanism of free Sc atoms, and hence the monoatomic-order layer formation mechanism,4) is clarified by Auger electron analysis and X-ray diffraction analyses. Free Sc atoms are produced through the following surface chemical reaction between Sc2W3O12 and Ba atoms supplied from the substrate4) as Sc2W3O12+3Ba=3BaWO4+2Sc. This monoatomic layer formation improves the electron emission current density measured at 850°Cb (brightness temperature) about threefold compared to that of conventional Os coated impregnated cathodes.

Electron emission properties and surface atom behavior of an impregnated cathode coated with tungsten thin film containing Sc2O3

A new cathode has been developed which shows similar electron emission characteristics as a previously reported Sc2O3 mixed matrix impregnated cathode (Sc2O3 MM Cathode). Contrary to the Sc2O3 MM cathode, the new cathode is resistive to prolonged heating at high temperatures and to ion bombardment. This has been made possible by applying to a standard impregnated cathode a tungsten thin-film containing about 5 weight percent Sc2O3. The electron-emission property is found to be strongly linked to the surface atom composition as well as to the distribution of surface atoms.

Impregnated cathode coated with tungsten thin film containing Sc2O3

An impregnated cathode of a novel structure is proposed, fabricated, and evaluated. A thin tungsten film 100–400 nm in thickness containing various amounts of Sc2O3 is coated on a standard impregnated cathode composed of a porous tungsten body in which electron emissive materials are impregnated. The electron emission property measured with a diode configuration is found to be dependent on Sc2O3 content and surface atom distribution. Surface atom distribution is depicted by means of Auger electron spectroscopy. For high electron emission enhancement it is necessary for Sc2O3 content to be 2.5–6.5 wt. % and for a layer of the order of a monolayer in thickness composed of Ba, Sc, and O to develop on the cathode surface.

Electron emission enhancement of a (W−Sc2O3)coated impregnated cathode by oxidation of the coated thin film

A monoatomic order surface layer composed of Ba, Sc and O reduces the work function and, thus, enhances electron emission of a (W-Sc2O3)-coated impregnated cathode surface. The dependency of the electron emission property of the new impregnated cathode on the coated film property is evaluated by intentionally oxidizing the coated film during the film deposition process. The degree of oxidation is monitored by measuring the resistivity ratio (room temperature/liquid nitrogen temperature) of the coated film. It is found that the electron emission is enhanced when the resistivity ratio is in the range of 0.8 to 0.9. This leads to a novel electron emission enhancement model: Electron emission enhancement is caused by free Sc atoms produced by the reaction of Ba atoms with Sc2W3O12 which is formed as the result of the oxidation process and heat treatment followed.

Application of an impregnated cathode coated with W-Sc2O3 to a high current density electron gun

Abstract The electron emission properties of a previously proposed [1,2] W-Sc2O3-coated impregnated cathode are evaluated in a high current density electron gun. It is found that the cathode can be operated at a temperature 100–150°C lower than that of Os-coated impregnated cathodes usually operated at 1000°C for a beam average current density of ∼5 A/cm2. The superiority of this cathode increases when used in high resolution, high brightness electron guns where the electron extracting field is high at the cathode surface.

Work Function Measurements of (W-Sc2W3O12)-Coated Impregnated Cathode by Retarding Potential Method Utilizing Titaniated W(100) Field Emitter

The work function of a (W-Sc2W3O12)-coated impregnated cathode, known as a high current density cathode operable at low temperature, is measured for the first time by the retarding potential method utilizing a titaniated W(100) field emitter as an electron source of high current stability and high brightness. A gradual change in the work function of the cathode is depicted during the activation procedure. Activation is completed after 2 h heating at 1150°Cb, when the work function becomes 1.15 (-0.02, +0.112) eV.

Absolute work function measurements with the retarding potential method utilizing a field emission electron source

Abstract The absolute work function of a scandate cathode is measured by a newly developed retarding potential method utilizing a titaniated W(100) field electron emission source reference. The electron trajectories in the lens system are computed to determine the conditions of the parallel beam impinging on the sample. The experimental results show a good agreement with the computations. The extremely low work function (∼ 1.2 eV) of the scandate cathode is obtained for the first time with this method. A systematic work function drift is observed during repeated work function measurements, suggesting the possibility of surface atom movement caused by the very low-energy electron beam irradiation of 1 to 2 eV.

Work function and microstructure of a monoatomic order surface layer grown on a (WSc2W3O12)-coated impregnated cathode

Abstract Activation processes of a (WSc2W3O12)-coated impregnated cathode are clarified by analysing the change in coated film structure before and after activation, the change in atom concentration on the cathode surface as well as the work function of the cathode during activation processes, and the activation time required to attain electron emission enhancement. It is found that there are three steps necessary to complete the activation of the cathode; (1) separation of Sc2W3O12 from W in the coated film; (2) production of free Sc atoms as a result of the reaction between Sc2W3O12 and Ba atoms from the substrate cathode; and (3) formation of an ordered monoatomic surface layer composed of Ba, Sc and O. It is also found that the overall activation energy is about 4.6 eV, and the work function after activation is about 1.15 eV.

Possibility of atom movement by very low energy electron beam irradiation

The change in the work function of a (W-Sc2W3O12) coated impregnated cathode is observed during the course of work function measurements by means of the retarding potential method utilizing a titaniated W(100) field emitter electron source. The change in work function is negligibly small at the work function minimum but it increases as the final steady state work function value increases. This paper discusses how low energy electron beam irradiation on the order of 2–3 eV, which reduces the effective coverage of the surface monolayer on the cathode, increases the work function.