Dispenser Cathodes Research Articles

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.

Physical operating principles of scandate cathodes for microwave devices

The electronic structure of barium oxide crystallites determining the emission properties of both dispenser and scandate cathodes has been studied using electron spectroscopy for chemical analysis, electron energy loss spectroscopy, and optical spectroscopy. It has been established that the other elements (calcium, aluminum, scandium, and tungsten) contained in cathode materials are diluted in barium oxide and significantly affect its electronic structure and, consequently, emission properties. The obtained results give an idea about the physical and physicochemical mechanisms of the effect of scandium on the reduction of the work function of scandate cathodes relative to that of the cathodes of other types.

Applications of deep ultraviolet laser photo-and thermal-emission electron microscope in thermal dispenser cathode research

The research of micro-region emission state for thermal dispenser cathode surface,especially in-situ observation and analysis,is an important subject in the field of thermal cathode.A newly developed instrument aiming at meeting the special operation requirements of thermal dispenser cathode is used to carry out this research.This instrument combines the functions of deep ultraviolet laser photo-emission electron microscope and thermal-emission electron microscope,so it is called DUV-PEEM/TEEM.In this paper,its basic principle is introduced emphatically.In addition,the actual applications of the microscope system to the electron emission investigation of thermal dispenser cathode are displayed. This system is equipped with the heating unit,which is used for activating the thermal dispenser cathode sample,and the temperature of sample can reach 1400℃.The system has three imaging modes,namely,photoemission electron imaging, cathode thermal emission electron imaging,and united imaging by integrating cathode thermal emission electron and photoemission electron.By applying new microscope system to traditional thermal dispenser cathode,we acquire the photoemission electron images of impregnated barium aluminate cathode surface at room temperature.In the heating process,we observe the thermal electron emission phenomenon originating from thermal dispenser cathode and record the variation process with temperature change.A high emission cathode which we developed before,is also studied with DUV-PEEM/TEEM.Fortunately,we find that some bright stripes appear on the surface of high emission cathode when the cathode temperature reaches 800℃.The widths of these bright stripes are about 100 nm.We calculate the thermal emission electron imaging resolution of this system by using these thermal electron emission stripes and the obtained resolution reaches 28 nm.Conveniently,the emission performance and uniformity of this high emission cathode are compared with those of traditional impregnated barium aluminate cathode directly at same temperature. Using united imaging mode of the system,in-situ observation and analysis of thermal electron emission spots on high emission cathode surface are carried out successfully.The results indicate as follows.For thermal dispenser cathode,the deep ultraviolet laser photoemission electron imaging can be used to show the surface fundamental micro-morphology of cathode;cathode thermal emission electron imaging is suitable for revealing the intrinsic emission uniformity of the thermal dispenser cathode;with the united imaging by integrating cathode thermal emission electron and photoemission electron,the positions of effective emission points on cathode surface can be fixed accurately.Based on these applications and findings,we believe that DUV-PEEM/TEEM also has ability to investigate the processes of cathode poisoning and recovery.

Back-gated graphene anode for more efficient thermionic energy converters

Thermionic energy converters (TECs) are a direct heat-to-electricity conversion technology with great potential for high efficiency and scalability. However, space charge barrier in the inter-electrode gap and high anode work function are major obstacles toward realizing high efficiency. Here, we demonstrate for the first time a prototype TEC using a back-gated graphene anode, a barium dispenser cathode, and a controllable inter-electrode gap as small as 17µm, which simultaneously addresses these two obstacles. This leads to an electronic conversion efficiency of 9.8% at cathode temperature of 1000°C, the highest reported by far. We first demonstrate that electrostatic gating of graphene by a 20nm HfO2 dielectric layer changes the graphene anode work function by 0.63eV, as observed from the current-voltage characteristics of the TEC. Next, we show that the efficiency increases by a factor of 30.6 by reducing the gap from 1mm down to 17µm, after a mono-layer of Ba is deposited on graphene by the dispenser cathode. Finally, we show that electrostatic gating of graphene further reduces the graphene work function from 1.85 to 1.69eV, leading to an additional 67% enhancement in TEC efficiency. Note that the overall efficiency using the back-gated graphene anode is 6.7 times higher compared with that of a TEC with a tungsten anode and the same inter-electrode gap.

Commercialization of an S-band standing-wave electron accelerator for industrial applications

An electron accelerator system has been developed for use in industrial, as well as possible medical, applications. Based on our experiences achieved during prototype system development and various electron beam acceleration tests, we have built a stable and compact system for sales purposes. We have integrated a self-developed accelerating cavity, an E-gun pulse driver, a radio-frequency (RF) power system, a vacuum system, a cooling system, etc. into a frame with a size of 1800 × 1000 × 1500 mm3. The accelerating structure is a side-coupled standing-wave type operating in the π/2 mode (tuned to~3 GHz). The RF power is provided by using a magnetron driven by a solid-state modulator. The electron gun is a triode type with a dispenser cathode (diameter of 11 mm). The system is capable of delivering a maximum 900-W average electron beam power with tight focusing at the target. Until now, we have performed various electron beam tests and X-ray beam tests after having built the system, have completed the beam assessment for commercializations, and have been preparing full-fledged sales activity. This article reports on our system development processes and on some of our early test results for commercializations.

Technology Development of Continuous Synthesis of Active Substance for Dispenser Cathode

Barium aluminates have been obtained by the method of continuous precipitation at different values of pH. The method of thermal analysis showed that the main mass losses which are connected with dehydration processes occur in the temperature interval from 50 °C to 100 °C and from 150 °C to 200 °C. In addition, mass loss occurs in the temperature interval from 780 °C to 1000 °C which can indicate barium aluminate formation. XRD analysis was applied to determine phase composition of obtained substance and showed formation of Ba1.17Al10.67O17.17 and BaAl2O4 at pH 6.5; 7.0 and 7.5 respectively. Significant influence of pH on precipitation composition has not been detected.

Multisource inverse-geometry CT. Part II. X-ray source design and prototype.

This paper summarizes the development of a high-power distributed x-ray source, or "multisource," designed for inverse-geometry computed tomography (CT) applications [see B. De Man et al., "Multisource inverse-geometry CT. Part I. System concept and development," Med. Phys. 43, 4607-4616 (2016)]. The paper presents the evolution of the source architecture, component design (anode, emitter, beam optics, control electronics, high voltage insulator), and experimental validation. Dispenser cathode emitters were chosen as electron sources. A modular design was adopted, with eight electron emitters (two rows of four emitters) per module, wherein tungsten targets were brazed onto copper anode blocks-one anode block per module. A specialized ceramic connector provided high voltage standoff capability and cooling oil flow to the anode. A matrix topology and low-noise electronic controls provided switching of the emitters. Four modules (32 x-ray sources in two rows of 16) have been successfully integrated into a single vacuum vessel and operated on an inverse-geometry computed tomography system. Dispenser cathodes provided high beam current (>1000 mA) in pulse mode, and the electrostatic lenses focused the current beam to a small optical focal spot size (0.5 × 1.4 mm). Controlled emitter grid voltage allowed the beam current to be varied for each source, providing the ability to modulate beam current across the fan of the x-ray beam, denoted as a virtual bowtie filter. The custom designed controls achieved x-ray source switching in <1 μs. The cathode-grounded source was operated successfully up to 120 kV. A high-power, distributed x-ray source for inverse-geometry CT applications was successfully designed, fabricated, and operated. Future embodiments may increase the number of spots and utilize fast read out detectors to increase the x-ray flux magnitude further, while still staying within the stationary target inherent thermal limitations.

Control over the emission properties of impregnated dispenser cathodes via surface pore density

Metal injection molding (MIM) was used to fabricate impregnated dispenser cathodes. The emission properties of tungsten cathodes were adjusted by controlling the number of exposed surface pores via mechanical polishing after completion of the impregnation process with 4BaO:CaO:Al2O3 (4:1:1) at 1600 ∼ 1900°C. The average distance between neighboring pores decreased as the number of pores increased. Three cathode samples having various surface pore densities were compared. Emission improved as the surface pore density increased. An average distance between exposed pores of 3.32 μm resulted in an emission current of 2.14 A (7.57 A/cm2) at 1030°Cbr, corresponding to a work function of 1.92 eV, which is comparable to that of an M-type cathode. Thus, control of the surface pore density of impregnated cathodes provides a simple, cost-effective means to produce high current density thermionic electron guns.

Study on the compositions of emission active materials and the emission properties of dispenser cathodes made from Scandia-doped powders

Scandia-doped dispenser cathode provided significantly better performance compared to standard dispenser cathodes in the application of microwave devices. Emission active materials are the key to dispenser cathodes. Scandium doping can enhance the performance of dispenser cathodes. In this paper, active materials are fabricated by co-precipitation method. Comparison is made between the active materials undergoing different heat treatments. XRD and SEM methods are employed to analyze the compositions and microstructure of active material samples. The cathode matrix powders are prepared by the sol–gel method using different recipes of scandium content. The contents of scandium oxide are, respectively, 3.5wt%, 4wt% and 5wt%. Emission tests are carried out in dedicated test vehicles. The results show that there are different aluminates including Ba4Al2O7, CaAl2O4, Ba7Al2O10, Ba3CaAl2O7, and Ba3Al2O6 come into being after heating using different techniques. The cathode impregnated with self-developed emission materials which numbered LN4 can achieve an inflection point emission current density of 122A/cm2 at 1165°C. On the study of different cathode matrix powders, when impregnated with the conventional material (S), the cathode made from 3.5wt% Scandia-doped powders has the best emission performance. The 4wt% cathode sample performs better when the cathodes are impregnated with the self-developed emission materials.

Noise parameters of defects of the emitting surface of thermionic cathodes

A noise model in which fluctuation noise of output current is determined by weakly emitting fragments of thermionic cathode working in the saturation regime (defects on the emitting surface) is proposed. The measured noise characteristics of the shot and flicker components of noise can be used to determine distribution of defects with respect to emission current densities and the noise parameters of defects. The relation of noise parameters and the degree of degradation of cathode is analyzed using the pulsed emission current at relatively long working times of dispenser cathodes with the W–Re sponge and Os coating. Preliminary results show that that proposed noise parameters can be used for the development of working methods for estimation of cathode quality.

The study of neutron burst shape of a neutron tube driven by dispenser cathode

A slim-shaped portable DD-neutron generator is developed at Budker institute of Nuclear Physics. The generator is a combination of Cockcroft–Walton voltage multiplier and a sealed gas-filled neutron tube driven by dispenser cathode. Neutron burst shape in pulsed mode of neutron tube operation is measured with stroboscopic time spectrometry, implemented on scintillation detector, and modeled with Comsol Script 1.3 and Comsol Multiphysics 3.5. Modeling appears to be in good agreement with experimental results. Measured pulse rise and fall times are 110ns and 100ns respectively.

Investigation of Nanosized-Scandia-Doped Dispenser Cathodes With Machined Surfaces

Since the cathodes with machined surfaces are required for the practical applications in some high-power vacuum electron devices, the characteristics of nanosized-scandia-doped dispenser (SDD) cathodes with machined surfaces have been investigated, especially in dc applications. In order to reopen the surface pores, which may be smeared by machining procedure, the cathode surface was washed by Ethylene Diamine Tetraacetic Acid (EDTA) solution and prolonged time was taken for the cathode activation. The emission test results show that the SDD cathodes with machined surfaces exhibit better emission performances after treatment. The space-charge-limited dc emission current density of over 20 A/cm2 at 900 °Cb has been measured with temperature compensation for electron cooling after full activation. An on-going life test with a dc load of up to 40 A/cm2 at 970 °Cb has been completed $\sim 5000$ h of operation to date. Since the emission capability can be fully restored after exposing to air and reactivating for a short time, a preheating procedure is favorable for SDD cathode with machined surface in the practical applications. Surface analysis indicates that a recrystallization of tungsten grains occurs during the prolonged heating process, leading to further reopening of the surface pores. As a result, cathode surface is covered uniformly by active elements of Ba and Sc (as well as O) as on an unmachined surface, thereby maintaining the high emission property of the SDD cathode.

Size effects in finish machining of porous powdered metal for engineered surface quality

Porous tungsten is conventionally machined with the aid of a plastic infiltrant to achieve acceptable surface finish. For dispenser cathode application, both high surface porosity and low surface roughness are necessary. Cryogenic machining has already been demonstrated to be capable of eliminating the need for plastic infiltration by greatly reducing smearing of pores. In order to address the problem of undesirable brittle fracture during cryogenic machining, the importance of uncut chip geometry is investigated. The value of critical chip thickness, beyond which brittle fracture occurs, is found to be closely linked to the microstructure of the workpiece material. While machining with very low uncut chip thickness leads to ploughing and spalling of the workpiece surface, ductile mode machining of porous tungsten with cryogenic cooling is found to yield excellent surface quality. When the maximum uncut chip thickness is approximately equal to the average ligament diameter of 80% density porous tungsten (d≈8–9μm), ductile mode machining is possible under both dry and cryogenic conditions. Changes in shock compaction behavior of the workpiece material, leading to altered physical properties, is hypothesized to be the underlying mechanism of ductile mode machining of porous tungsten.

Study and Development of Active Sintered Controlled Porosity Dispenser Cathode

The sintered wire cathode offers a significant improvement in density and uniformity of emission. In the present technique of active sintering, an activator is introduced, which relaxes the sintering temperature of tungsten (W)-wire bunch significantly. The technique also offers well-opened columnar pores with a uniform distribution all across the cross section. The pellets are made out of the sintered bunch of W-wires, each of diameter $50~\mu \text{m}$ . The pellets are impregnated using triple carbonates and are integrated with potted heaters. The cathode assemblies are tested in an analytical system containing an anode and an Auger electron spectroscopy facility. The pulse emission measurements revealed that the emission density of these cathodes is higher than twice that of a conventional B-type cathode. This high emission is attributed to a relatively high Ba/W and O/W ratios on the surface as compared with those of a B-type cathode. The Miram curves, generated out of pulse emission data, exhibit sharp temperature-limited fully space charge-limited transition, indicating lower patchiness of emission as compared with that of a B-type. The theoretical studies are carried out to estimate the optimum wire gauge that can produce good coverage and recovery to the ion bombardment.

Design and Experimental Evaluation of Dual-Anode Electron Gun and PPM Focusing of Helix TWT

A dual-anode convergent electron gun with a perveance of 0.14 $\mu \text{p}$ has been designed and developed for a high-efficiency helix traveling-wave tube (TWT), which would ultimately have application in a satellite communication system. The electron gun with an isolated beam forming electrode and an M-type dispenser cathode of 3.1-mm button diameter has been designed using EGUN, OMNITRAK-3-D, and OPERA-3-D codes. It consists of two isolated anodes, namely, control anode and ion-barrier anode, in addition to a ground accelerating anode to meet the requirement of long life and high reliability. Initially, in cathode selection, life considerations are ignored, and the gun is designed with the available cathode. Periodic permanent magnet (PPM) focusing has been designed using a Sm2Co17 high-intensity magnet in order to reduce the size and the weight of the overall TWT. Confined flow beam focusing has been adopted. A compact, small-size electron gun assembly has been designed using a ceramic insulator and a suitable support structure with due considerations for high-voltage and thermal requirements. A high-voltage analysis has been carried out using ESTAT code, and a thermal analysis has been carried out using ANSYS code. Designed electron gun and PPM focusing has been experimentally validated by developing and testing a beam-stick tube with the application of rated potentials at various electrodes in a gun and a collector. The design and experimental results for the electron gun and PPM focusing have been found in close agreement.

Decoupling the Thermal and Plasma Effects on the Operation of a Xenon Hollow Cathode With Oxygen Poisoning Gas

Hollow cathodes are used as the electron source for generating the plasma discharge in electric thrusters. These cathodes contain porous tungsten emitters impregnated with BaO material to achieve a lower surface work function and are operated with xenon propellant. Reactive contaminants such as oxygen in the xenon gas can modify the surface chemistry and morphology of BaO dispenser cathodes and degrade the electron emission properties. Hollow cathodes that operate with reactive impurities in the propellant will experience higher operating temperatures, which increase the evaporation of the emission materials and reduce cathode life. A significant amount of work has been previously done to understand the effects of oxygen poisoning on vacuum cathodes; however, the xenon plasma adds complexity and its role during cathode poisoning is not completely understood. A deeper understanding of the mechanisms initiating cathode failure will improve the thruster operation, increase the lifetime, and ultimately reduce the cost. In this paper, we present the results of experiments in which the cathode was operated with 100 ppm of oxygen gas in the xenon plasma at two different discharge currents. Poisoning was indicated by an increase in the emitter temperature that was measured using a two-color pyrometer and thermocouples. The emitter temperature rose monotonically during poisoning and decayed to its original value when the oxygen flow was turned OFF, indicating that short exposures to oxygen do not result in permanent cathode damage. The experiments show that removal of oxygen adatoms from the cathode surface following poisoning is dominated by thermal desorption rather than xenon ion sputtering. Operation at higher discharge currents improves the resistance to poisoning as a result of the higher operating temperatures at these conditions.

Increased Surface Integrity in Porous Tungsten from Cryogenic Machining with Cermet Cutting Tool

In order to eliminate the process of backfilling porous tungsten with a plastic infiltrant during machining to prevent unwanted smearing of surface pores, cryogenic machining is investigated as a viable alternative. Porous tungsten is mainly used in the manufacture of dispenser cathodes where demands for surface quality and dimensional tolerances are extremely high. For these reasons, the ability of cryogenic machining to provide increased surface integrity and tool life compared to conventional dry machining is explored. Moreover, some preliminary results of machining with various cutting edge radii and effects on surface stress state are presented. Overall, cryogenic machining does provide significant surface quality and tool wear improvements over conventional dry machining practices.

Structural Engineering of Impregnated Dispenser Cathodes

Targeted research efforts focusing on the properties and structure of impregnated dispenser cathodes (IDCs) have been monitored. The data are summarized in terms of scale structural hierarchy in inorganic materials to develop principles for controlling their formation in the design of high-emission and long-life IDCs. The performance of IDCs of different types is modeled using the materials science triad ‘chemical composition ↔ structure ↔ properties’ and the concept of structural hierarchical levels in IDCs. Basic structural levels in IDCs are determined: electronic, nanostructured, mesoscopic, microscopic, and macroscopic. Their structural elements are analyzed: electrons, emitter layer, film coating, matrix and emission material, and cathode structure. It is found out that the electronic level is the key one in the hierarchy of IDC structural levels; its effectiveness depends on the nanocrystalline, mesoscopic, and microscopic levels. The principles of structural engineering are developed for the design of high-emission and long-life IDCs and for the control of their formation at nanostructured and microscopic levels by variation in the chemical composition and structure.

Composition and work function relationship in Os–Ru–W ternary alloys

Os–Ru thin films with varying concentrations of W were sputter deposited in order to investigate their structure–property relationships. The films were analyzed with x-ray diffraction to investigate their crystal structures, and a Kelvin probe to investigate their work functions. An Os–Ru–W film with ∼30 at. % W yielded a work function maximum of approximately 5.38 eV. These results align well with other studies that found work function minima from thermionic emission data on M-type cathodes with varying amounts of W in the coatings. Furthermore, the results are consistent with other work explaining energy-level alignment and charge transfer of molecules on metal oxides. This may shed light on the mechanism behind the “anomalous effect” first reported by Zalm et al., whereby a high work function coating results in a low work function for emitting cathode surfaces. An important implication of this work is the potential for the Kelvin probe to evaluate the effectiveness of dispenser cathode coatings.

Про ефективні термоелектронні катоди

A review of scientific works devoted to the most efficient modern thermionic cathodes – oxide cathodes and dispenser cathodes – is made. The main attention is focused on the analysis of emission mechanisms from those electron sources. In addition, the relation between the emission of dispenser cathodes and their physical and technical parameters is considered. The influence of fabrication methods on the cathode performance is discussed.