Vasilios Vlahos

Ab initio study of the effects of thin CsI coatings on the work function of graphite cathodes

Cesium-iodide (CsI)-coated graphite cathodes are promising electron sources for high power microwave generators, but the mechanism driving the improved emission is not well understood. Therefore, an ab initio modeling investigation on the effects of thin CsI coatings on graphite has been carried out. It is demonstrated that the CsI coatings reduce the work function of the system significantly through a mechanism of induced dipoles. The results suggest that work function modification is a major contribution to the improved emission seen when CsI coatings are applied to C.

Ab initio investigation of the surface properties of dispenser B-type and scandate thermionic emission cathodes

Scandate cathodes (BaxScyOz on W) are important thermionic electron emission materials whose emission mechanism remains unclear. Ab initio modeling is used to investigate the surface properties of both scandate and traditional B-type (Ba–O on W) cathodes. We demonstrate that the Ba–O dipole surface structure believed to be present in active B-type cathodes is not thermodynamically stable, suggesting that a nonequilibrium steady state dominates the active cathode’s surface structure. We identify a stable, low work function BaxScyOz surface structure, which may be responsible for some scandate cathode properties and demonstrate that multicomponent surface coatings can lower cathode work functions.Scandate cathodes (BaxScyOz on W) are important thermionic electron emission materials whose emission mechanism remains unclear. Ab initio modeling is used to investigate the surface properties of both scandate and traditional B-type (Ba–O on W) cathodes. We demonstrate that the Ba–O dipole surface structure believed to be present in active B-type cathodes is not thermodynamically stable, suggesting that a nonequilibrium steady state dominates the active cathode’s surface structure. We identify a stable, low work function BaxScyOz surface structure, which may be responsible for some scandate cathode properties and demonstrate that multicomponent surface coatings can lower cathode work functions.

Surface chemical analysis and ab initio investigations of CsI coated C fiber cathodes for high power microwave sources

CsI coated C fiber cathodes are promising electron emitters utilized in field emission applications. Ab initio calculations, in conjunction with experimental investigations on CsI-spray coated C fiber cathodes, were performed in order to better understand the origin of the low turn-on E-field obtained, as compared to uncoated C fibers. One possible mechanism for lowering the turn-on E-field is surface dipole layers reducing the work function. Ab initio modeling revealed that surface monolayers of Cs, CsI, Cs2O, and CsO are all capable of producing low work function C fiber cathodes (1 eV<Φ<1.5 eV), yielding a reduction in the turn-on E-field by as much as ten times, when compared to the bare fiber. Although a CsI-containing aqueous solution is spray deposited on the C fiber surface, energy dispersive x-ray spectroscopy and scanning auger microscopy measurements show coabsorption of Cs and I into the fiber interior and Cs and O on the fiber surface, with no surface I. It is therefore proposed that a cesium...

Measurements and Analysis of Advanced Field Emission Cold Cathodes

We report measurements and analyses of field emission from both copper and aluminum cathodes. To analyze the data, we have developed a numerical model of electron emission. We note that localized heating of high current density field emission cathodes can provide regime where thermionic as well as field emission effects must be considered. Our analysis of experimental data fitting in both the thermionic and field emission dominant regime provides a robust method for determination of effective work function and field enhancement factor for field emission cathode.

An 80 Watt dual Ka/Q-band mini-TWT

This paper describes RF performance on a Ka/Q-band mini-TWT. An output power of greater than 80 W has been demonstrated in the Ka and Q communication bands.

17.2: Ab initio models of dispenser B-type, scandate, and alloy cathode surfaces

The work function (Φ) and operating stability of dispenser, scandate, and alloy cathodes are investigated with ab initio quantum mechanical modeling. Low Φ, thermodynamically stable, and steady state surface configurations are identified, while the ability of alloy surfaces to improve emission is presented.

Surface chemical analysis of cesium-iodide (CsI) coated carbon (C) fibers and thin films for field emission applications

Carbon (C) fibers with electron emission enhancing cesium iodide (CsI) surface coatings are utilized as field emission cathodes and have been investigated as promising electron sources. Possible applications include not only High Power Microwave (HPM) devices, but also conventional microwave devices, x-ray tubes, and field emission displays [1]. Upon appropriate conditioning of the cathode to remove adsorbed water vapor, stable operation for more than one million pulses, little or no plasma formation, and low turn on E-fields, compared to un-coated C fibers, have been observed.

An ab-initio molecular model of the Scandate cathode

To obtain a fundamental understanding of the surface processes responsible for the superior emission characteristics of scandate cathodes, a first principles investigation was carried out using the Vienna ab-initio simulation package (VASP) [G. Kresse and J. Furthmuller], a quantum mechanical molecular dynamics simulator.

Material analysis and characterization of cesium iodide (CsI) coated C fibers for field emission applications

Field emission cathodes consisting of cesium iodide (CsI) coated graphite fibers have been investigated as promising electron sources for high power microwave (HPM) devices. After conditioning for removing adsorbed water vapor, these cathodes have operated stably for more than one million pulses and release little or no plasma. One of the most useful properties of the Csl coatings appears to be a significant reduction in turn-on electric fields compared to un-coated graphite fiber cathodes. Computational investigations of the modification of the electronic properties of a C surface upon adsorption of both isolated and crystalline thin layers of Csl have been performed, indicating that these adsorbed layers are capable of significantly improving the emission properties of the C fibers by forming surface dipole layers . These layers are predicted to alter the electrostatic potential barrier for electron emission of the surface and lower the overall work function of the system from ~4.5 eV (pure graphite) down to ~1.2 - 1.4 eV. In order to further correlate the microscopic properties of the material system with the computational results obtained and the macroscopic performance characteristics of these cathodes, a series of materials characterizations are being carried out on the cathodes. These studies will facilitate the determination of both bulk and surface elemental composition of the cathodes, provide the value of the surface work function, determine the micro-morphological characteristics of the cathodes surface as well as the micro-structural characteristics of the bulk, and identify the role of any interface regions on the emission properties of the cathode. A more complete understanding of the mechanism(s) responsible for the superior emission characteristics of Csl coated C fibers and the nano-physics of the system will be obtained.

Measurements and analysis of advanced field emission cold cathodes

We report measurements and analysis of metal cold field emission cathodes utilizing an advanced cathode test facility. The facility is designed to measure the field emission current densities from cathodes on the Micro- and Macro-scale.