Tungsten Dispenser Cathodes Research Articles

Understanding the surface structure evolution and electron emission behaviors during the activation of Ir-coated dispenser cathodes

Metal coated dispenser cathode is of great interest due to its low operation temperature, high current density and long lifetime, however, the emission mechanism is still far from being fully understood. In this work, the tungsten dispenser cathodes were coated with iridium film of 300 nm in thickness. The phase transition and surface structure evolution during cathode activation was systematically investigated. The as-deposited iridium coating shows a columnar grain morphology with ultrafine size. Upon heating for 1 h at 1150 °C, a multiple-layer of Ir/Ir3W/IrW was detected and it fully converted to IrW layer after 4 h heat treatment. Such a structure evolution can be well explained based on thermodynamics. The Ir → Ir3W → IrW phase transition induces obvious film thickening and grain coarsening. The full activation could increase the emission current density from 17.35 A/cm2 to 30.92 A/cm2 at 1050 °C. Density function theory (DFT) simulation results demonstrate the enhanced emission of IrW-coated cathode may be due to its balanced adsorption behaviors with Ba and O atoms. Our findings will provide deeper insights into the design and development of metal coated cathodes with better performance.

Frontiers in Thermionic Cathode Research

The high electron emission resulting from low-work-function scandate-added dispenser cathodes has generated a great deal of interest and research since the late 1970s. Despite the reported high current density at low temperatures, scandate cathodes have not yet seen wide-scale industrial adoption due to poor emission uniformity, inadequate reproducibility, and short lifetimes. This lack of industrial adoption stemming from the above issues is likely due to insufficient fundamental understanding of the role that scandium plays in the emission process. Recent work by five research teams under the U.S. Defense Advanced Research Projects Agency Innovative Vacuum Electronics Science and Technology Program aims at advancing this fundamental understanding and the cathode manufacturing processing. Emission microscopy of model surfaces of adsorbed Ba–Sc–O on single crystal tungsten and detailed characterization techniques, such as Wulff shape analysis, are being used to understand the morphology, composition and phase of each of the species that comprise the cathode emitting surfaces. The structure of tungsten grains at the cathode surface of activated and unactivated scandate cathodes and the resulting emission performance are observed. 3-D printing is being investigated as an advanced manufacturing method to meet the demanding requirements of modern vacuum electron devices. Finally, density functional theory is being employed to study the work function of perovskite oxides as a novel class of alternative materials to tungsten dispenser cathodes.

Properties of quarter-wavelength coaxial cavity for triode-type thermionic RF gun

A quarter-wavelength coaxial cavity with a longitudinal radio-frequency power supply was fabricated and tested. The cavity was designed as a pre-buncher for a thermionic triode-type radio-frequency gun of a mid-infrared free electron laser facility. The triode structure was formed to ensure the reduction of the back-bombarding effect, which usually appears in thermionic radio-frequency guns. The coaxial cavity was tested using a tungsten dispenser cathode. From the results of the cold test, a cavity voltage of about 25 kV can be attained, which corresponds to designed characteristics. In contrast, the hot test showed a sudden drop in voltage, resulting in an unstable operation. The small dimensions of the cavity caused some low-field effects, which led to multipactoring. In this paper, we report the tested characteristics of the pre-bunching cavity.

Interplay of composition, structure, and electron density of states in W-Os cathode materials and relationship with thermionic emission

The presence and composition of W-Os alloys have been found to significantly affect the thermionic emission properties of Os-coated tungsten dispenser cathodes. However, the comprehensive understanding of structure–property relationships needed to design improved tungsten cathodes with larger thermionic emission is still lacking. In this study, composition–structure–property relationships governing thermionic emission from W-Os alloys were investigated using quantum mechanical calculations. Low-energy W-Os atomic configurations at various compositions were determined from first-principles calculations based on density functional theory in combination with cluster expansion calculations. Electronic properties were investigated in terms of the electron density of states. The relative position of the Fermi level with respect to peaks and pseudogaps in the density of states for different W-Os structures can be used to explain, at least in part, observed variations in thermionic emission from Os-coated tungsten dispenser cathodes.

Life Test Studies on Dispenser Cathode With Dual-Layer Porous Tungsten

This paper introduces a newly developed dual-layer porous tungsten dispenser cathode, which can meet the growing requirements on both life and current density. In comparison with the conventional dispenser cathodes, the new-type dispenser cathodes have higher emission current density, lower evaporation rate and longer lifetime. According to the results of the life tests and the related analysis, their longest lifetime at a 1.5 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> current density under 974 °C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub> is estimated to be over 1.9 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> h.

Development of alloy-film coated dispenser cathode for terahertz vacuum electron devices application

High power terahertz vacuum electron devices demand high current density and uniform emission dispenser cathode. It was found that the coating of noble metals e.g., Os, Ir, and Re on the surface of tungsten dispenser cathodes enhances the emission capabilities and uniformity. Hence metal coated cathode might be the best candidate for terahertz devices applications. In this study, ternary-alloy-film cathode (2Os:2Re:1W) and Os coated cathode have been developed and the results are presented. The cathodes made out of this alloy coating showed 1.5 times higher emission and 0.02eV emission uniformity as compared to those of simply Os coated cathodes which can be used in terahertz devices application.

Scandate Dispenser Cathode Fabrication for A High-Aspect-Ratio High-Current-Density Sheet Beam Electron Gun

A high-current-density scandate tungsten dispenser cathode was used for the demonstration of a 25 : 1-aspect-ratio 750-A/cm2 -current-density sheet beam for the Defence Advanced Research Project Agency High-Frequency Integrated Vacuum Electronics (HiFIVE) program intended for the realization of a wideband ( ~ 30%) 220-GHz traveling wave tube. The elliptical cathode with homogeneous microstructure was made from 1-2-μm-size tungsten powder added with nanosized Scandia using the sol-gel method; it has a current density of up to 160 A/cm2 at 1050 °C. A sheet beam gun analyzer was built to test the terahertz sheet beam gun and determine the size and current density of a sheet electron beam produced by the impregnated scandate tungsten dispenser cathode. A sheet electron beam with an aspect ratio of 12.5 : 1 with a current density exceeding 375 A/cm2 has been obtained using a BVERI impregnated scandate dispenser cathode without magnetic compression; further magnetic field compression would give the final current density of 750 A/cm2.

Effects of Substrate Bias on Microstructure of Osmium-Ruthenium Coatings for Porous Tungsten Dispenser Cathodes

Osmium-ruthenium (Os-Ru) films used as coatings for porous tungsten (W) dispenser cathodes were investigated in this study. By applying different levels of substrate biasing power during sputtering, the texture of deposited Os-Ru films varied between {0002}, {10-10}, and {10-11} in this hexagonal close-packed alloy. Furthermore, film texture changed from one preferred orientation to other preferred orientation(s) during annealing (at 1050degCB for 10 min), during which the microstructure of certain Os-Ru films changed from a columnar to an equiaxed grain morphology. Due to a low degree of texture transition during annealing and due to the high compositional and structural stability of the Os-Ru grains, a 5 W bias power appears to be best for film deposition. In addition, the 5 W biased Os-Ru films transformed completely into a basal plane texture, which has the highest planar density, and grain size increased significantly (from ~20 to ~100 nm) during annealing, all while maintaining a columnar grain structure. These characteristics are believed to be helpful in inhibiting interdiffusion between the W substrate and the Os-Ru film. Preventing or slowing the interdiffusion of W atoms can improve the lifetime and even the performance of dispenser cathodes.

Emission poisoning studies on impregnated tungsten dispenser cathode under CO 2 and O 2 environment

An impregnated cathode of 18% porosity has been fabricated using barium-calcium-aluminates of molar ratio 5:3:2. Effects of carbon dioxide and oxygen on emission of this cathode have been investigated experimentally. After poisoning, recovery of emission and activation time have also been studied. The results are discussed and compared with the work of other investigators.

Os-coated cathode for very high emission-density applications

Emission densities of 35 to 100 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , demanded by free-electron lasers (FEL's), have set far more stringent requirements on thermionic cathodes than what is considered state of the art with current technology. The purpose of the work described here is to evaluate Os-coated cathodes for these ultra-high-current-density applications. Organometallic CVD techniques were used to coat "B" type tungsten dispenser cathodes. The coatings were found to show good adhesion and rapid activation. The activation process involves not only the standard formation of a BaO layer on the surface but also the formation of an optimum surface alloy composition of near 50-percent Os and 50-percent W. These cathodes showed excellent emission capabilities, and at 1085°C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</inf> , a zero-field emission density near 50 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was obtained. Subsequent life studies at 1085° C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</inf> indicated that at an emission level of 40-50 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> over 800 h of life is obtainable with this type of cathode. During life, the pulsed emission density and surface composition were monitored. Surface Ba/BaO coverage as well as surface Os content were observed to decrease with life, but an S impurity was found to increase with life. However, the surface alloy composition change was identified as being the major cause for the emission degradation.

Surface composition and barium evaporation rate of “pedigreed” impregnated tungsten dispenser cathodes during accelerated life testing

A study has been made of the surface composition and barium evaporation rate of “pedigreed” impregnated tungsten dispenser cathodes. The effect of air exposure on coated cathodes was examined and was found to have no significant effect on barium evaporation rate although in some cases longer reactivation times were required. No changes in surface topography were apparent following air exposure and reactivation. Life testing was done at 100°C above the typical operating temperature for the cathode, where the typical operating temperature was taken to be 950°C for coated cathodes and 1050°C for uncoated cathodes. The cathodes were examined at different stages of life testing, up to 1200 h. Significant decreases in barium evaporation rates were found after as few as 500 h of life testing. After 1000 h the evaporation rate had decreased more than an order of magnitude. Changes in surface composition were also found. The effects of tungsten particle size, used in manufacture of the billet, on barium evaporation rate were also studied but no correlation was found.

Surface structure of thermionic-emission cathodes.

We have used surface extended x-ray absorption fine structure (SEXAFS) to study the local geometry around barium atoms on thermionic cathodes. On the surfaces of tungsten and tungsten-osmium alloy dispenser cathodes Ba is bonded to oxygen with a well-defined short-range order. The Ba-O distance is similar (2.62\ifmmode\pm\else\textpm\fi{}0.04 A\r{}) for the two cathodes, with oxygen atoms occupying hollow sites of the substrate. However, the alloy cathode has Ba bonded to two oxygen near neighbors (compared to one for the tungsten cathode), which will enhance the surface dipole, thus explaining the observed lower work function.

Ion scattering spectroscopy studies of barium and oxygen on tungsten and tungsten-based dispenser cathodes

In order to use Ion Scattering Spectroscopy (ISS) for studies of tungsten dispenser cathodes, the relevant ISS sensitivities must be measured. Calibrations have been made using a polycrystalline tungsten ribbon with controlled coverages of oxygen, barium and combinations thereof. Auger Electron Spectroscopy (AES) was used to monitor these controlled surfaces and the escape depths of the tungsten Auger electrons in barium and oxygen have been measured. The absolute ISS sensitivities of all three elements were found to be strongly dependent on the barium coverage of the tungsten surface. This effect has been attributed to the lowering of the work function of the tungsten surface caused by the barium adsorption. However, the relative ISS sensitivities of the three elements are not affected in this way when both barium and oxygen (or oxygen alone) are present on the tungsten surface. ISS spectra of such surfaces have been analyzed quantitatively and found to be in reasonable agreement with AES measurements. The analysis has also been applied to ISS spectra of uncoated tungsten matrix dispenser cathodes in an active state and following exposure to oxygen. Compared to AES, these spectra indicate less oxygen on the active cathode surfaces as a result of the oxygen (associated with barium) not contributing to the oxygen ISS signal. Comparisons of the spectra from the active and oxygen poisoned cathodes suggest that oxygen adsorbed during the oxygen exposure sits on the topmost barium layer whereas the oxygen on the active cathode surface does not.

Diffusion and desorption of barium and oxygen on tungsten dispenser cathode surfaces

Auger measurements have been made on the concentration of barium and oxygen diffusing out of a pore and slot onto the surface of a simulated tungsten dispenser cathode. Profiles of concentration vs distance from the source were obtained at different temperatures. It is found that at cathode operating temperatures very little gradient of oxygen concentration exists on the surface, and under steady state conditions Ba is the main diffusing species. Ba diffusion distances derived are consistent with previous measurements. However it is found that this distance increases with concentration in spite of the fact that the Ba surface lifetime decreases with concentration. Time dependent measurements made on a clean surface show that the rate of Ba and O build up is limited by the supply rate of oxygen to the surface. This supply rate is not diffusion limited but seems to be limited by the mechanism generating free oxygen. A comparison of emission microscope measurements on a tungsten matrix dispenser cathode with the simulated cathode data indicates that similar oxygen generation processes may be controlling the activation of these cathodes.

Relative work function, surface composition and topography of “pedigreed” impregnated tungsten dispenser cathodes

A study has been made of the variation in work function, surface composition, and topography of 5:3:2 impregnated tungsten dispenser cathodes made under carefully controlled conditions (pedigreed cathodes). Despite these conditions several cathodes had unexpected deposits on their emitting surfaces, and one showed a variation in work function and composition across the surface during activation.

XPS studies of the chemical state of Ba on the surface of impregnated tungsten dispenser cathodes

Several impregnated tungsten dispenser cathodes of different manufacture and type have been studied using X-ray photoelectron spectroscopy (XPS). The purpose of this study was to determine what changes occur in the chemical states of the elements present on cathode surfaces both before and after activation. These results are compared with those reported in the literature by other workers, as well as with standards prepared in situ for the case of barium and barium oxide. The effects of air exposure and reactivation on the 500 nm osmium-ruthenium coating were also examined for M-type cathodes.

XPS studies of the chemical state of Ba on the surface of impregnated tungsten dispenser cathodes☆

Several impregnated tungsten dispenser cathodes of different manufacture and type have been studied using X-ray photoelectron spectroscopy (XPS). The purpose of this study was to determine what changes occur in the chemical states of the elements present on cathode surfaces both before and after activation. These results are compared with those reported in the literature by other workers, as well as with standards prepared in situ for the case of barium and barium oxide. The effects of air exposure and reactivation on the 500 nm osmium-ruthenium coating were also examined for M-type cathodes.

The electronic activation by barium and oxygen of different tungsten crystal orientations

The reduction in work function caused by the deposition of barium and oxygen onto a tungsten surface has been studied for five samples (grown by chemical vapor deposition, CVD) with different preferential orientations. The electronic activation was measured under steady state conditions at a typical tungsten based dispenser cathode operating temperature (1400 K) using a B type tungsten dispenser cathode as a source of barium and oxygen. Thermionic emission measurements and Auger electron spectroscopy were used to characterize the degree of activation and estimate the coverage for each surface. The 〈001〉 and 〉411〈 oriented surfaces activated to the lowest work functions.

Determination of barium and calcium evaporation rates from impregnated tungsten dispenser cathodes

The evaporation rates of barium and calcium from impregnated tungsten dispenser cathodes have been determined by both a vapor-collect method and line-of-sight mass spectrometry. Cathodes having molar ratios of BaO, CaO, and Al 2O 3 of 4:1:1, 5:3:2, and 1:1:1 have been studied. All measurements were conducted in ultrahigh vacuum. For the vapor-collect method, a W(110) collector was found to be suitable for monolayer growth. The procedure involved plotting the ratio of the adsorbate Auger peak-to-peak height to that from the collector as a function of collection time. Break-points in these plots characterized the collection of one monolayer of adsorbate. A geometrical correction then allowed the evaporation rates to be determined. Evaporation rates were determined at cathode temperatures of 1050, 1100, and 1150°C. For the line-of-sight mass spectrometry an Extranuclear quadrupole was used. The quadrupole was capable of measuring species up to 300 amu. Measurements with the quadrupole were made on a 5:3:2 and a 4:1:1 cathode. Results obtained using these two methods are compared.

Surface studies of the tungsten dispenser cathode

Synchrotron radiation photoemission spectroscopy measurements have been performed on type “M” and “S” tungsten dispenser cathodes and on model surfaces consisting of barium and barium oxide on clean tungsten. All measurements were performed at room temperature using new cathodes. It was found that in the type S cathode BaO,Ba, and chemisorbed oxygen are present, while only metallic Ba and adsorbed oxygen are found on the type M cathode. The superior performance of the type M cathode implies that Ba, and not BaO, is the critical component of the cathode surface.