Thermionic Electron Research Articles

Effect of the Gap Size between the Cathode and the Wehnelt Cylinder Bore on the Emittance of the Thermionic Electron Gun

This paper includes simulating an electron gun design to investigate the effect of the distance between the cathode and the Wehnelt cylinder bore on the quality of the ion beam emittance and to obtain the best distance of that geometrical factor. A study of a group of designs was conducted to find this distance. The SIMION8.0 program has been used to calculate the trajectories of the electrons with initial kinetic energy of (0.1 eV) and electron current of (1×10-12 A). The investigation also includes comparisons between the equipotential line trajectories for each design as well as the field distribution and the electron trajectories. The best emittance distance was concluded to be (3 mm).

Enhancing the thermionic electron emission performance of hafnium with nanocluster doping

Scandate cathode as a thermionic electron emission material has attracted broad interest owing to its high electron emission capability. Despite the large number of studies on the electron emission of the scandate tungsten cathode, there is a lack of understanding of the performance of scandium oxide incorporated in other refractory metal matrix. In this work, by applying first-principles calculations, we study the role that Sc2O3 nanocluster plays in enhancing the electron emission of the Hf cathode (labeled as Sc2O3@Hf). We find that Sc2O3@Hf is both thermally and mechanically stable, exhibits complex surface structures, and possesses inverse relationship between surface energy and work function. Moreover, the work function of the Hf (0001) surface can be reduced to 3.50 eV due to the change in the vacuum level when Sc2O3 nanocluster is doped, while the work function of the Hf (101¯2) surface can be reduced to 3.39, 3.39, and 3.03 eV when doped with Sc2O3, Y2O3, and CeO2 clusters, respectively.

Demonstration of Low Work Function Perovskite SrVO3 Using Thermionic Electron Emission

AbstractEngineering a material's work function is of central importance for many technologies and in particular electron emitters used in high‐power vacuum electronics and thermionic energy converters. A low work function surface is typically achieved through unstable surface functional species, especially in high power thermionic electron emitter applications. Discovering and engineering new materials with intrinsic, stable low work functions obtainable without volatile surface species would mark a definitive advancement in the design of electron emitters. This work reports evidence for the existence of a low work function surface on a bulk, monolithic, electrically conductive perovskite oxide: SrVO3. After considering the patch field effect on the heterogeneous emitting surface of the bulk polycrystalline samples, this study suggests the presence of low work function (≈2 eV) emissive grains on SrVO3 surface. Emission current densities of 10–100 mA cm–2 at ≈1000 °C, comparable to commercial LaB6 thermionic cathodes, indicative of an overall effective thermionic work function of 2.3–2.7 eV are obtained. This study demonstrates that perovskites like SrVO3 may have intrinsically low work functions comparable to commercialized W‐based dispenser cathodes and suggests that, with further engineering, perovskites may represent a new class of low work function electron emitters.

Simulation study for the characterization of anode shape effect on the electron beam emittance of thermionic electron gun

A systematic method had been used to develop the anode electrode shape of the thermionic electron gun by studying its effect on the emittance value, where the emittance parameter has been considered an important criterion for determining the fineness of the beam propagation. In this study, it has been found that the extended field outside the electron gun has a negative effect on the electron beam emittance parameter. A new shape of the anode has been developed and introduced in this research work to eliminate the unwanted field at the electron beam side. The cylindrical shape of the anode, which encapsulates the Wehnelt cylinder, has the effect of eliminating the adverse stray field, hence the enhancement of the beam emittance values. The new design is developed to be used in the illumination system of the electron microscopes where the highest laminar flow of the electron beam is needed.

Ultrafast Thermionic Electron Injection Effects on Exciton Formation Dynamics at a van der Waals Semiconductor/Metal Interface.

Inorganic van der Waals bonded semiconductors such as transition metal dichalcogenides are the subject of intense research due to their electronic and optical properties which are promising for next-generation optoelectronic devices. In this context, understanding the carrier dynamics, as well as charge and energy transfer at the interface between metallic contacts and semiconductors, is crucial and yet quite unexplored. Here, we present an experimental study to measure the effect of mutual interaction between thermionically injected and directly excited carriers on the exciton formation dynamics in bulk WS2. By employing a pump–push–probe scheme, where a pump pulse induces thermionic injection of electrons from a gold substrate into the conduction band of the semiconductor, and another delayed push pulse that excites direct transitions in the WS2, we can isolate the two processes experimentally and thus correlate the mutual interaction with its effect on the ultrafast dynamics in WS2. The fast decay time constants extracted from the experiments show a decrease with an increasing ratio between the injected and directly excited charge carriers, thus disclosing the impact of thermionic electron injection on the exciton formation dynamics. Our findings might offer a new vibrant direction for the integration of photonics and electronics, especially in active and photodetection devices, and, more in general, in upcoming all-optical nanotechnologies.

Influence of the cathode position on beam current characteristics in the thermionic electron gun

Influence of the cathode position on beam current characteristics in the thermionic electron gun

Beam based diagnostic and four dimensional transverse emittance measurement using solenoid scan for 100 keV thermionic electron gun at VECC, Kolkata

Solenoid in the low energy beam transport line used for focusing 100 keV electron beam has also been utilized as a beam diagnostic tool. 4D transverse emittance has been determined by analyzing the second moment beam distribution on a view screen placed downstream using solenoid scan technique which require no a-priori assumptions of the input beam. The measured transverse emittance is compared with the emittance tomographically reconstructed from the measured beam profiles corresponding to the same set of solenoidal fields in the solenoid scan.

Low-lying Dipole Resonances in FeCN−: A Viable Formation Pathway for FeCN− in Space

A low-lying resonance in FeCN− anion was identified through abrupt changes in the spectral dependence of the photoelectron angular distribution. Non-Franck–Condon transitions from the resonance to the neutral FeCN (4Δ), and the corresponding photoelectron angular distributions revealed that the resonance is a dipole scattering state. Significant thermionic electron emission was observed in the resonant photoelectron spectra, indicating a strong coupling of the resonance with the ground state of this triatomic anion and its competition over autodetachment. This low-lying resonance is identified to be an efficient pathway for the formation of FeCN− anion in the outer envelope of IRC+10216. The results in general reveal formation pathways in space for anions with low-lying resonances and large permanent dipole moment.

A Three‐Terminal Hybrid Thermionic‐Photovoltaic Energy Converter

AbstractHybrid thermionic‐photovoltaics (TIPV) are solid‐state thermal‐to‐electric energy converters that rely on the non‐isothermal transport of photons and electrons through a vacuum gap. In contrast to pure thermionic converters, the absorption of photons in a photovoltaic anode produces an electrochemical potential that can be delivered as electricity, ultimately boosting the power generation capacity of the device. In this work, the proof of concept of a three‐terminal TIPV converter where thermionic and photogenerated currents are collected independently is reported. Thermionic electrons are injected in the conduction band of a semiconducting anode (n‐type InP), from where they are directly extracted. Photogenerated electrons are also extracted from the conduction band of the anode, but they are then reinjected in the valence band through an independent hole‐selective contact (p‐type InGaAs). By using a low workfunction engineered anode (BaFx/InP) and cathode (ScxOy/W) a maximum power generation capacity of 125.6 and 0.35 mW cm−2 for PV and thermionic sub‐devices, respectively, is demonstrated, operating at 1400 °C. This proof of concept paves the way for the development of efficient hybrid thermionic and photovoltaic converters for the direct conversion of heat into electricity, and subsequently contributes to finding an efficient alternative to thermoelectric generators.

Focus on software, data acquisition, and instrumentation

Focus on software, data acquisition, and instrumentation

Numerical investigation of regimes of current transfer to anodes of high-pressure arc discharges

Unified 1D numerical modeling of high-pressure high-current arc discharges is revisited. Two regimes of current transfer to anodes are investigated. The “passive anode” regime occurs for low and moderate anode surface temperatures Ta. The energy flux from the plasma to the anode surface, qpl, depends on Ta rather weakly in this regime and may be conveniently expressed in terms of the local current density jc, and the so-called anode heating voltage Uh. Uh is independent of the arc length and the cathode surface temperature, although it weakly varies with jc between approximately 6 and 8.5 V for jc in the range from 105 to 108 A m−2. In the “active anode” regime, qpl is higher than in the passive anode regime and varies with Ta. The active anode regime may occur on hot refractory anodes, such as those of high-intensity discharge lamps, when Ta exceeds approximately 3000 K and the thermionic electron emission from the anode comes into play. The latter causes an increase in the electron density near the anode. One consequence is the increase in the electron energy transport from the bulk plasma to the near-anode layer by electron heat conduction. The other effect contributing to increase in qpl is the formation of a negative near-anode space-charge sheath with a positive voltage drop. In non-stationary simulations, the active regime occurs via the development of a thermal instability similar to that causing the appearance of spots on thermionic arc cathodes. The occurrence of the active regime is strongly affected by parameters, in particular, by the distance between the anode surface and the cooling fluid.

Analysis of electron behaviour around a spring-shape filament inside a low-energy electron gun

Electron behaviour inside a low-energy electron gun based on a spring-shape filament were studied with experiment and Particle-In-Cell simulation. The energy range of the electron beam which we expected was from 1 to 20 eV. The analysis told that smaller size of filament is more useful to improve electron density in front of an extraction hole in the gun to enhance beam current. Relation between a space potential distribution in the gun and electron transport was also studied. A heater voltage to drive a filament for thermionic electron emissions has another role to form a spatial potential distribution in the gun. The potential guides electrons, which are at a distant area from the beam extraction hole, toward the hole. It can be a significant help to realize efficient electron extraction by a beam extraction electric field induced near the hole.

Positively Charged Macroparticles in Low-Temperature Plasma

The evolution of a positively charged metallic macroparticle placed into the low-temperature plasma is considered. The influence of the value of the initial macroparticle charge on the dynamics of the electrical potential and temperature of the macroparticle, as well as the possibility of evaporation of a macroparticle due to its interaction with plasma particles are studied. The system of equations of the energy balance and the current balance based on the OML theory, that takes into account the changing of macroparticle potential and its temperature over the time is solved numerically. The solution of the system of equations shows the evolution of the macroparticle potential and temperature within the time interval from the moment when the macroparticle is placed in the plasma until the moment the macroparticle has charged to the floating potential. The positive charge of the macroparticle excludes the thermionic emission and secondary electron emission from the macroparticle surface, as well as the mechanisms of cooling of the macroparticle associated with these emission processes. Analytical expressions that determine the macroparticle potential, the electron current on the macroparticle, as well as the power transferred by plasma electrons in the case when the energy of attraction of electrons to the macroparticle strongly exceeds the energy of thermionic electrons, the energy of secondary electrons and the energy of plasma ions are obtained. A simplified system of equations of the energy balance and the current balance for a positively charged macroparticle is solved; the solution of the simplified equations matches with the solution of the general equations in the region of positive values of the macroparticle potential. Calculations show that during the charging of the macroparticle, its temperature increases up to the boiling point of the macroparticle substance. An equation that determines the conditions under which evaporation of macroparticles is possible has been obtained and solved numerically. The possibility of evaporation of macroparticles of a given size (critical value of the radius) due to initial charging to high positive values of potential is shown. The dependencies of the critical value of the radius on the initial value of the potential for tungsten and copper macroparticles that can be evaporated in a low-temperature plasma are obtained. These solutions bound the region of the parameters where evaporation of a macroparticle is possible and where it is not. The critical values of the potential for copper and tungsten particles with sizes of 0.1 and 1 μm are calculated. The dependence of the radius of a macroparticle on time during the process of vaporization is obtained.

Investigation of a novel ring-cusp magnetically confined plasma bridge neutralizer.

The plasma bridge neutralizer (PBN) based on a tungsten filament is a promising technique of a thermionic DC electron source where a hot filament is immersed in an inert gas flow and electrons are "bridged" from a small orifice to the ion beam. PBNs have been widely used in space propulsion and industrial applications due to their relatively simple structure and low power consumption. However, they have well-known disadvantages, namely, low emission current density and short lifetime. In this article, we propose a novel ring-cusp magnetically confined PBN (RCM-PBN) to address these issues. In the RCM-PBN, electrons are confined by a ring-cusp magnetic field, which improves the ionization efficiency and reduces the discharge chamber wall losses. Electrical insulation of the orifice plate from the chamber wall prevents a large number of electrons from being collected by the orifice plate, which greatly improves the extracted electron current. The effects of different operating parameters on the extracted electron current were studied through experiments. It was found that the increase in the extracted electron current with the extraction voltage was related to the anode spot formation. Analysis of the gas utilization factor and electron extraction cost shows that the optimal operating condition was obtained at an argon mass flow rate of 1.2 SCCM and a heater power of 45W. At its optimum, a stable electron current of 1.1A was extracted from the RCM-PBN with a gas utilization factor of 12.8 and an electron extraction cost of 143 W/A.

Pd Nanosheet-Decorated 2D/2D g-C3N4/WO3·H2O S-Scheme Photocatalyst for High Selective Photoreduction of CO2 to CO.

The development of the global economy in recent years, environmental problems, greenhouse effect, and so forth have been of concern for countries all over the world. The key for solving the greenhouse effect is the reduction of CO2. With the development of photocatalytic reduction of CO2, hybrid photocatalytic nanostructures composed of noble metals and plasmonic semiconductors are being widely studied. In this work, S-scheme photocatalysts with a g-C3N4/WO3·H2O/Pd heterostructure was constructed by introducing ultrathin Pd nanosheets into the optimized 2D/2D g-C3N4/WO3·H2O binary system. The S-scheme charge transfer generated by the matched band gap of g-C3N4 and WO3·H2O can effectually improve the electron transfer rate and the redox ability of photogenerated carriers. The introduction of Pd nanosheets can inject a large number of hot electrons into the semiconductor on the basis of the S-scheme heterojunction to participate in the reaction. The S-scheme electron transfer method is used to improve the utilization rate of thermionic electrons and achieve the effect of widening the near-infrared-light absorption area of the composite material. Moreover, the reaction was carried out in water without the addition of any sacrificial agent, which can better reflect the green environmental protection of the experiment. This investigation will promote the broad-spectrum application of new and environment-friendly thermoelectron-assisted S-scheme photocatalysts, and on this basis, the possible reaction mechanism is discussed.

Effect of MeV ion induced heating on secondary electron emission from Si surface and estimation of local temperature

We report the variation of secondary-electron yield and target temperature with the variation of ion beam intensity during 100 MeV N+5 bombardment on Si surfaces. Theoretical understanding of secondary-electron emission during ion bombardment predicts no variation of yield with ion beam intensity. We discovered that the variation of secondary-electron yield arises due to the rise of local temperature at ion impact sites of the target. The secondary-electron yield first decreases with the increase of ion beam current but subsequently, it increases with the further rise of beam current. The initial decrease is due to the temperature-dependent change of electrochemical potential of Si. The considerable increase of the secondary-electron yield with a further increase of ion beam current is caused by local surface heating and subsequent additional thermionic electron emission. We have estimated the surface temperature of ion-impact sites by separating the thermionic electron current from the collisional secondary electron current.

Beam dynamics simulation study for longitudinal bunch length measurement of RF modulated thermionic electron gun at VECC, Kolkata

A superconducting electron linac (e-linac) photo-fission driver is under construction at VECC for the upcoming rare isotope beam facility called ANURIB. Electron gun and straight low energy beam transport (LEBT) line has been installed and is operational at a test area, delivering electron beam of 100 keV. We discuss beam dynamics and analytical calculations leading to optimal parameters of analysing section of LEBT line and RF deflector cavity that will be used for measuring longitudinal characteristics for the low energy electron beam.

Key design and operation factors for high performance of C12A7:e-based cathodes

This work, based on an EU-funded project (NEMESIS), is summarising some of the results from the project activities on the research and development on electride-based cathode technology compatible with all kinds of electric propulsion (EP) systems requiring neutralization or electron emission. Further information describing in detail the performed tests and captured measurements can be found in the referenced documents of each section. Different cathode architectures and several emitter configurations with traditional and with alternative propellants are being developed and tested within the project, all of them using C12A7:e-electride material as thermionic electron source. Findings and conclusions derived from these multiple designs are allowing to figure out some of the key factors that determine the best performance of C12A7:e-electride based cathodes. In this work, a discussion of some of these key design and operation factors will be presented based both on the material characterization parameters, and on the performance tests carried out for the different cathode designs.

Electronic structures and emission properties of typical binary single crystal REB<sub>6</sub>

Binary rare earth hexaborides (REB<sub>6</sub>) have different rare earth elements with different valence electron distributions, which lead to different strange physical properties and different emission properties. However, in the electron emission properties, whether PrB<sub>6</sub>, NdB<sub>6</sub>, SmB<sub>6</sub> and GdB<sub>6</sub> all have excellent emission properties remains to be further studied, and the physical mechanism affecting their emission properties needs investigating. In this paper, the electronic structures, work functions of typical binary single crystal REB<sub>6</sub> (LaB<sub>6</sub>, CeB<sub>6</sub>, PrB<sub>6</sub>, NdB<sub>6</sub>, SmB<sub>6</sub>, GdB<sub>6</sub>) are studied by first principles calculations. The single crystal REB<sub>6</sub> are prepared by optical zone melting method, and their thermionic electron emission properties are tested experimentally. The theoretical calculation results show that the typical binary REB<sub>6</sub> have large densities of states near the Fermi level. The d-orbitals with broad distributions in conduction bands are beneficial to electron emission. The localized f-orbital electrons in valence bands are not conducive to their electron emission. The theoretical calculations of work functions of typical binary single crystal REB<sub>6</sub> (100) surface are consistent with the analyses of their electronic structures. The theoretical calculation values of work functions are ordered as GdB<sub>6</sub> (2.27 eV) < CeB<sub>6</sub> (2.36 eV) < LaB<sub>6</sub> (2.40 eV) < PrB<sub>6</sub> (2.58 eV) < SmB<sub>6</sub> (2.63 eV) < NdB<sub>6</sub> (2.91 eV). The experimental test results of thermionic electron emission of single crystal show that the experimental thermionic electron properties are consistent with the theoretical ones. The LaB<sub>6</sub> and CeB<sub>6</sub> both have good thermionic and field emission properties, and the GdB<sub>6</sub> has excellent field emission properties.

Suppression of instabilities in the transport of a high-power electron beam in the forevacuum pressure range using low-energy thermionic electrons

We report the results of our study on the effect of injection of low-energy thermionic electrons on the suppression of instabilities of the beam-plasma discharge (BPD) type in a beam plasma during the transport of a powerful continuous electron beam generated by a plasma–cathode electron source in the forevacuum range of pressure. As result of thermionic electron injection, the plasma electron temperature decreased to 0.3 eV and the plasma density decreased by an order of magnitude to 1015 m−3. The minimal thermoelectron current required for suppressing the BPD increases with increasing emission current and decreases with increase of the beam accelerating voltage.